DOI: 10.1148/rg.276075003
RadioGraphics 2007;27:1539-1565
© RSNA, 2007
Cardiac Conduction System: Anatomic Landmarks Relevant to Interventional Electrophysiologic Techniques Demonstrated with 64-Detector CT1
Farhood Saremi, MD and
Subramaniam Krishnan, MD
1 From the Department of Radiological Sciences and Cardiology, Division of Cardiothoracic Imaging (F.S.) and the Department of Medicine, Division of Cardiology (S.K.), UCI Medical Center, 101 City Dr South, Rte 140, Orange, CA 92868. Recipient of Magna Cum Laude and Excellence in Design awards for an education exhibit at the 2006 RSNA Annual Meeting. Received January 5, 2007; revision requested March 29 and received April 20; accepted May 17. S.K. is with the speakers’ bureau of Medtronic; F.S. has no financial relationships to disclose.
Address correspondence to F.S. (e-mail: fsaremi{at}uci.edu).
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Abstract
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The rapid development of clinical cardiac electrophysiology has triggered a renewed interest in the anatomy of the heart. A thorough knowledge of cardiac anatomy is a prerequisite for successful electrophysiologic procedures. Accurate description of the cardiac anatomy requires the use of a common language in describing this anatomy, as well as close interaction between radiologists, cardiologists, and surgeons. Given its capacity to provide relevant anatomic information in exquisite detail, multidetector computed tomography (CT) has the potential to allow faster and more accurate placement of intracardiac ablation catheters and pacemaker leads relative to the anatomy of interest. High-resolution reformatted images from 64-detector CT data provide accurate anatomic information for locating important landmarks relative to the cardiac conduction system or to current electrophysiologic interventions and cardiac resynchronization therapy.
© RSNA, 2007
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LEARNING OBJECTIVES FOR TEST 1
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After reading this article and taking the test, the reader will be able to:
- Identify the cardiac anatomic landmarks relevant to tachyarrhythmia.
- Describe reformation techniques for localizing the anatomy of interest.
- Discuss imaging findings that are pertinent to the electrophysiologist.
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Introduction
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The past 2 decades have witnessed a revolution in the treatment of patients with cardiac arrhythmias. Advances in this field have included the development of (a) techniques of catheter ablation that often require the precise destruction of minute amounts of arrhythmogenic tissues, and (b) techniques of resynchronization therapy that require the pacing of different parts of the atria as well as of the ventricular branches of the coronary venous system (1–3). These requirements underlie the increasing use of cardiac imaging procedures such as multidetector computed tomography (CT). With recent technologic advances in multidetector CT scanners, especially the introduction of high-resolution 64-detector scanners, it has become possible to perform a "virtual dissection" of the heart and cardiovascular system (4–7), which brings the radiologist’s role in the interpretation of cardiac images to a higher level. By providing the electrophysiologist with an anatomic "road map," the radiologist will make the ablation and pacing procedures much easier, and results as well as complications will be recognized immediately.
In this article, we describe the normal cardiac anatomy as well as anatomic landmarks of interest to electrophysiologists, discussing and illustrating these landmarks in terms of their anatomic localization in different planes, their relationships with other structures, and their anatomic variants. We also discuss the common arrhythmias and electrophysiologic procedures so that the radiologist can better understand these procedures.
Most of the images shown herein were obtained with a 64-detector CT scanner and a standard electrocardiographically gated coronary CT angiography protocol. Orally or intravenously administered metoprolol was used to achieve a target heart rate of less than 65 bpm as needed. A sublingual nitroglycerin tablet (0.4–0.8 mg) was given 1 minute before image acquisition unless contraindicated. Beta blockers are the drug of choice for treating the majority of supraventricular arrhythmias and are not contraindicated in patients with a history of these arrhythmias. Nitrates can also be used safely in this group.
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Diagnostic Electrophysiologic Study
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A diagnostic electrophysiologic study involves the insertion of multiple catheters via the venous or arterial system to study and record electrical activity from the atria, ventricles, and other parts of the conduction system, as well as to induce various arrhythmias (Fig 1). These studies are generally performed to obtain information on the specific type of rhythmic disturbance that may have occurred clinically and to provide the treating physician with more details concerning the underlying mechanism. Under fluoroscopic guidance, catheters are passed into the right atrium or right ventricle. Arteriovenous conduction is studied by positioning a separate catheter across the tricuspid annulus and obtaining a His bundle electrogram. To record activity from the left atrium and sometimes from the left ventricle, a catheter is guided into the coronary sinus. Left-sided procedures involving the left atrium and left ventricle are performed with a transseptal approach (Fig 2) or with a retrograde approach from the femoral artery. The general region of interest is usually determined first, after which more precise mapping is used to localize the arrhythmia focus or the reentrant circuit. Fluoroscopy is routinely used to localize anatomic landmarks for ablation. Multidetector CT can depict various cardiac structures that are difficult to visualize at fluoroscopy (eg, oval fossa, crista terminalis, eustachian ridge, coronary sinus, pulmonary vein ostia) and provides anatomic data for easier placement of intracardiac catheters.
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Figure 1a. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1b. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1c. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1d. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 2a. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Figure 2b. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Figure 2c. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Catheter-based Ablation
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Catheter-based ablation has become the standard therapy for many tachyarrhythmias, with its use surpassing that of pharmacologic and surgical methods (1–3,8). Ablation is the intentional destruction of arrhythmogenic myocardial tissue to control or cure arrhythmias. Radiofrequency (RF) current is the most commonly used ablative energy source. The principal method of tissue destruction with RF current is thermal in nature. Typical RF lesions are small, usually 3–6 mm in diameter and up to 3 mm deep when a standard catheter tip is used (1,8). Old RF ablation sites may be smaller than acute lesions.
Other energy sources that can be used include lasers and microwaves.
The success of catheter ablation varies depending on the type of arrhythmia and the clinical situation. High success rates (>90%) and low complication rates (<3%) are seen in ablation procedures for arrhythmias such as atrioventricular (AV) nodal reentry, accessory pathways, atrial flutter, idiopathic ventricular tachycardia, and AV node or junction ablation. Lower success rates (<90%) and higher complication rates (>3%) are seen in ablation procedures for atrial fibrillation (AF) and postinfarction ventricular tachycardia (1,9). Ablation in patients with underlying structural heart disease is usually less successful (10).
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Tachycardias and Anatomic Considerations for Treatment
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The cardiac conduction system consists of the sinoatrial node, the AV node, the His bundle, and the right and left bundle branches, as well as the fascicles and Purkinje fibers (11–14). Tachyarrhythmias are categorized according to the width of the QRS complex at electrocardiography (Fig 3). Narrow QRS complex tachyarrhythmias are generally supraventricular in origin. Supraventricular tachycardias are defined as those in which the atrium, including the AV node and the AV junctional portion, is critical to the perpetuation of the tachycardia. The three types of supraventricular tachycardias are AV node reentrant, AV reentry, and atrial tachycardia. Generally, atrial flutter and AF are considered to be distinct entities. A variety of tachyarrhythmias can be targeted for percutaneous catheter ablation (Fig 4).
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Figure 4. Diagram illustrates anatomic considerations in the treatment of supraventricular and ventricular arrhythmias. A variety of tachyarrhythmias can be targeted for percutaneous catheter ablation, including both atrial and ventricular arrhythmias that are either focal or make use of reentrant circuits. LV = left ventricular, MI = myocardial infarction, RV = right ventricular, VT = ventricular tachycardia.
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Paroxysmal Supraventricular Tachycardia
The most common type of supraventricular tachycardia is AV node reentrant tachycardia, followed by AV reentry tachycardia. Both types are due to a reentrant mechanism and require intact conduction over the AV node; hence the term AV node dependent (15,16). In AV node reentrant tachycardia, the anatomic substrate or abnormality is the presence of dual AV node pathways. The pathways are described as "slow" or "fast" depending on their relative conduction velocities (16). These pathways vary in anatomic location but are generally located within the Koch triangle. Catheter ablation of AV node reentrant tachycardia is performed by targeting the slow pathway. AV reentry tachycardia is a reentrant tachycardia that makes use of the AV node and an accessory pathway (16). The fundamental anatomic abnormality or substrate for AV reentry tachycardia is the presence of a second connection between the atria and ventricles (normally, the AV node is the only connection). This second connection is called an accessory pathway and is the abnormality seen in Wolff-Parkinson-White syndrome. Catheter ablation of both AV node reentrant tachycardia and AV reentry tachycardia is effective in over 95% of patients (17).
Atrial Flutter
The most common type of atrial flutter is isthmus-dependent atrial flutter, in which the reentrant circuit is confined to the tricuspid annulus with the wavefront progressing in either a counterclockwise or clockwise direction across the cavotricuspid isthmus (CTI) between the IVC and the tricuspid annulus (Fig 5) (18). This is a relatively narrow target but can easily be reached with ablation catheters introduced from the IVC. With new catheter techniques, the success rate for ablation of this form of atrial flutter is over 95% (19). Cardiac CT may be helpful in characterizing the CTI, including its size, its depth, and its anatomic relationship with the IVC, eustachian ridge, and coronary sinus ostium. Cardiac CT may also depict the pouches and recesses that are commonly present along the CTI and that sometimes make it difficult to create a complete line of block in the isthmus.
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Figure 5a. (a) Diagram shows the reentrant circuit (arrows) of the common variety of right atrial flutter (type I). The right atrium is shown in a right anterior oblique projection. The reentrant circuit is confined to the right atrium by the annulus of the tricuspid valve (TV) and barriers to conduction within the right atrium, and it circulates in a counterclockwise direction. Yellow hatched area between the tricuspid valve and the IVC indicates the critical isthmus of tissue that is targeted for ablation of this type of atrial flutter. CS = coronary sinus, CT = crista terminalis, ER = eustachian ridge, OF = oval fossa, RAA = right atrial appendage. (b) Endoscopic view of the right atrium demonstrates the spatial relationships between the coronary sinus (CS), IVC, and oval fossa (OF). NCS = noncoronary sinus, TV = tricuspid valve.
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Figure 5b. (a) Diagram shows the reentrant circuit (arrows) of the common variety of right atrial flutter (type I). The right atrium is shown in a right anterior oblique projection. The reentrant circuit is confined to the right atrium by the annulus of the tricuspid valve (TV) and barriers to conduction within the right atrium, and it circulates in a counterclockwise direction. Yellow hatched area between the tricuspid valve and the IVC indicates the critical isthmus of tissue that is targeted for ablation of this type of atrial flutter. CS = coronary sinus, CT = crista terminalis, ER = eustachian ridge, OF = oval fossa, RAA = right atrial appendage. (b) Endoscopic view of the right atrium demonstrates the spatial relationships between the coronary sinus (CS), IVC, and oval fossa (OF). NCS = noncoronary sinus, TV = tricuspid valve.
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Atrial Fibrillation
AF is a common rhythmic disturbance that is increasingly being recognized in the aging population. AF is now believed to be due to multiple reentrant wavelets or to a rapidly firing focus in the pulmonary vein (20,21). Ectopic activity of pulmonary vein sleeves has been identified as a major cause of paroxysmal AF in patients with structurally normal hearts (20,22). Typically, access to the pulmonary veins and left atrium is gained via a patent foramen ovale or, if the foramen ovale is not patent, by means of transseptal puncture (Fig 2). Pulmonary vein isolation procedures or circumferential left atrial ablation is increasingly being performed to cure AF. Circumferential left atrial ablation involves the creation of circumferential ablation lines around the left and right pulmonary veins 1–2 cm from the ostia of the veins. Ablation lines in the posterior left atrium and mitral isthmus are often created as well (Fig 6). The ablation success rate is about 80% for paroxysmal AF and 70% for chronic AF (21). Although the majority of foci triggering AF have been mapped to the pulmonary veins, AF may also be triggered by arrhythmogenic foci originating from the right atrium, left atrium, coronary sinus, SVC, or vein of Marshall (22–25). Preprocedural magnetic resonance (MR) imaging or CT allows excellent anatomic characterization of the veins that may help plan the procedure. At present, catheter ablation of AF is by far the most common electrophysiologic indication for cardiac multidetector CT. Postprocedural imaging can also be used to monitor the development of complications such as pulmonary vein stenosis (26,27).
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Figure 6a. AF ablation. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein. (a) Endoscopic image shows how two catheters are introduced into the left atrium through a transseptal puncture, including a deflectable circular mapping catheter (blue) and a deflectable ablation catheter (yellow). LAA = left atrial appendage, MV = mitral valve. (b) Posterior three-dimensional (3D) image of the left atrium and pulmonary veins demonstrates circumferential pulmonary vein ablation. Circumferential ablation lines (red) around two pulmonary vein lesions are connected by a roof ablation line (green). A mitral isthmus ablation line (blue) was created between the left inferior pulmonary vein and the lateral mitral annulus (arrows). AAo = ascending aorta, LA = left atrium, LCx = left circumflex artery, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein.
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Figure 6b. AF ablation. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein. (a) Endoscopic image shows how two catheters are introduced into the left atrium through a transseptal puncture, including a deflectable circular mapping catheter (blue) and a deflectable ablation catheter (yellow). LAA = left atrial appendage, MV = mitral valve. (b) Posterior three-dimensional (3D) image of the left atrium and pulmonary veins demonstrates circumferential pulmonary vein ablation. Circumferential ablation lines (red) around two pulmonary vein lesions are connected by a roof ablation line (green). A mitral isthmus ablation line (blue) was created between the left inferior pulmonary vein and the lateral mitral annulus (arrows). AAo = ascending aorta, LA = left atrium, LCx = left circumflex artery, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein.
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Anatomic Landmarks of the Right Atrium
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On 3D images of the heart obtained from above, the right atrium is positioned to the right and anterior, whereas the left atrium is situated to the left and mainly posterior (Fig 7a). The right atrium consists of three components: the appendage, the venous part, and the vestibule (11). A prominent muscular ridge known as the crista terminalis separates the smooth-walled venous part (sinus venosus) from the appendage. The vestibule is a smooth muscular rim that surrounds the tricuspid orifice. The terminal groove, or sulcus terminalis, is a fat-filled groove on the epicardial side and corresponds internally to the crista terminalis (Fig 7c). The sinus node and the terminal segment of the sinoatrial nodal artery are located in this groove, close to the cavoatrial junction.
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Figure 7a. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7b. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7c. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7d. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Sinus Venosus
The sinus venosus is located mainly in the posterolateral wall of the right atrium between the orifices of the SVC and IVC (Fig 7b). From the 3rd week after the development of the primitive heart tube, the primitive atria are separated from the sinus venosus by a structure known as the sinoatrial ring (28,29). The sinus venosus has two horns. The right horn gives rise to all the intercaval regions of the right atrium, including the crista terminalis, eustachian ridge, and thebesian valve. The left horn gives rise to the coronary sinus.
Crista Terminalis
The crista terminalis is a fibromuscular ridge formed by the junction of the sinus venosus and the primitive right atrium (10,11). Superiorly, it arches anterior to the orifice of the SVC, extends to the area of the anterior interatrial groove, and merges with the interatrial bundle, commonly known as the Bachmann bundle (Fig 8a). The inferior border of the crista terminalis near the IVC orifice is indistinct and merges with small trabeculations of the inferior portion of the CTI (30). The crista terminalis gives rise to a series of relatively thick bundles, the anterior pectinate muscles, which fan out anteriorly. The "septum spurium" is the most prominent anterior pectinate muscle arising from the crista terminalis (Fig 8c). The septum spurium is prominent in 80% of hearts, has a mean thickness of 4.5 mm, and should not be mistaken for intraatrial disease (30). The crista terminalis varies in size and extent among individuals (Fig 9a–9d). A large crista terminalis due to fatty infiltration has been reported in lipomatous hypertrophy of the atrial septum and may mimic a mass (31). Awareness of the variability in the size and extent of the crista terminalis and familiarity with the normal appearance of a prominent crista terminalis will minimize misdiagnosis of this structure.
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Figure 8a. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 8b. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 8c. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 9a. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9b. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9c. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9d. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9e. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9f. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9g. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9h. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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The crista terminalis is a significant structure in several forms of atrial tachyarrhythmias and occasionally is the target for catheter RF ablation. A study by Mizumaki et al (32) suggests that thickening of the crista terminalis may lead to the development of typical atrial flutter. Intracardiac echocardiographic studies have shown that two-thirds of focal right atrial tachycardias seen in the absence of structural heart disease arise along the crista terminalis (33).
Sinoatrial Node and Sinoatrial Nodal Artery
The sinoatrial node is the source of the cardiac impulse. It is composed histologically of cells that are slightly smaller than normal working cells (34). The sinoatrial node is a subepicardial spindle-shaped structure at the superior cavoatrial junction that extends from the SVC along the crista terminalis toward the IVC (30,34,35). It penetrates the musculature of the crista terminalis to lie in the subendocardium and is best seen on axial images (Fig 10a). The sinoatrial node varies in position and length (mean, 20 ± 3 mm) (28). Because of its proximity to the epicardial surface, it may be damaged at selected cardiac surgeries or by extensive pericardial diseases (36). The sinoatrial node surrounds the sinoatrial nodal artery, which can course centrally (70% of cases) or eccentrically within the node (30).
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Figure 10a. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Figure 10b. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Figure 10c. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Modification of the sinoatrial node with an RF ablation catheter has been established as a treatment for inappropriate sinus tachycardia (37). Because of the subepicardial location of the sinoatrial node, approaching from the endocardial surface requires more RF energy to ablate the node (30). Multidetector CT can be used to measure the thickness of the crista terminalis and demonstrate the approximate location of the sinoatrial nodal artery within the nodal tissue. A centrally located sinoatrial nodal artery may provide a cooling effect, reducing the extent of RF damage (30,38). Higher RF energy may not only injure the myocardium of the crista terminalis itself, but can also induce SVC stricture (39). RF ablation of the sinoatrial node is more effective with little risk if the ablation catheter is positioned at the site of the crista terminalis halfway between the caval veins, where the tail of the sinoatrial node is subendocardial (30). This distance can be measured with multidetector CT.
The sinoatrial nodal artery is usually a single branch that arises from the proximal right coronary artery (60% of cases) or LCx artery (40%) (Fig 10b, 10c) (40,41). Regardless of its artery of origin, the sinoatrial nodal artery usually courses along the anterior interatrial groove toward the superior cavoatrial junction. At the cavoatrial junction, the course of the sinoatrial nodal artery becomes variable, with the artery circling either anteriorly (precaval) or posteriorly (retrocaval) to enter the node.
Koch Triangle
Another area of significance to the electrophysiologist is the Koch triangle. The Koch triangle lies in the right atrium at the orifice of the coronary sinus. It is bordered posteriorly by a fibrous extension from the eustachian valve called the tendon of Todaro (42). The anterior border is demarcated by the attachment of the septal leaflet of the tricuspid valve. The apex of the Koch triangle corresponds to the central fibrous body of the heart, demarcating the site of penetration of the His bundle. The midportion of the triangle contains the compact AV node (fast pathway), and the base contains the slow pathway. 
The base of the triangle is bordered by the coronary sinus ostium and the "septal isthmus" (the area between the edge of the coronary sinus ostium and the attachment of the septal tricuspid valve) immediately anterior to it (Fig 11). The septal isthmus is often the target for ablation of the slow pathway in AV node reentrant tachycardia (43). The dimensions of the Koch triangle vary from one individual to another, a fact that is clinically relevant in catheter ablation procedures in this area, which are guided largely by anatomic landmarks. Multidetector CT can depict the boundaries of the triangle and its relationship to adjacent structures.
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Figure 11a. Localization of the Koch triangle. RA = right atrium, RV = right ventricle. (a) Endocardial view of the right AV junction shows the Koch triangle and the right atrial isthmus. The Koch triangle is demarcated by the tendon of Todaro–eustachian ridge (ER) posteriorly (white arrows), attachment of the septal tricuspid valve anteriorly (yellow arrows), coronary sinus (CS) inferiorly, and central fibrous body (CFB) at the apex (red arrow). The septal isthmus (small bracket), the area between the coronary sinus and septal tricuspid valve, is the target for ablation of AV node reentrant tachycardia. The CTI (large bracket) lies between the IVC orifice and the tricuspid valve. A = anterior, I = inferior, P = posterior, S = superior. (b–d) Coronal (b), sagittal (c), and axial (d) CT scans show the Koch triangle (yellow lines) from different perspectives. Arrow in d indicates the central fibrous body at the triangle apex.
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Figure 11b. Localization of the Koch triangle. RA = right atrium, RV = right ventricle. (a) Endocardial view of the right AV junction shows the Koch triangle and the right atrial isthmus. The Koch triangle is demarcated by the tendon of Todaro–eustachian ridge (ER) posteriorly (white arrows), attachment of the septal tricuspid valve anteriorly (yellow arrows), coronary sinus (CS) inferiorly, and central fibrous body (CFB) at the apex (red arrow). The septal isthmus (small bracket), the area between the coronary sinus and septal tricuspid valve, is the target for ablation of AV node reentrant tachycardia. The CTI (large bracket) lies between the IVC orifice and the tricuspid valve. A = anterior, I = inferior, P = posterior, S = superior. (b–d) Coronal (b), sagittal (c), and axial (d) CT scans show the Koch triangle (yellow lines) from different perspectives. Arrow in d indicates the central fibrous body at the triangle apex.
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Figure 11c. Localization of the Koch triangle. RA = right atrium, RV = right ventricle. (a) Endocardial view of the right AV junction shows the Koch triangle and the right atrial isthmus. The Koch triangle is demarcated by the tendon of Todaro–eustachian ridge (ER) posteriorly (white arrows), attachment of the septal tricuspid valve anteriorly (yellow arrows), coronary sinus (CS) inferiorly, and central fibrous body (CFB) at the apex (red arrow). The septal isthmus (small bracket), the area between the coronary sinus and septal tricuspid valve, is the target for ablation of AV node reentrant tachycardia. The CTI (large bracket) lies between the IVC orifice and the tricuspid valve. A = anterior, I = inferior, P = posterior, S = superior. (b–d) Coronal (b), sagittal (c), and axial (d) CT scans show the Koch triangle (yellow lines) from different perspectives. Arrow in d indicates the central fibrous body at the triangle apex.
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Figure 11d. Localization of the Koch triangle. RA = right atrium, RV = right ventricle. (a) Endocardial view of the right AV junction shows the Koch triangle and the right atrial isthmus. The Koch triangle is demarcated by the tendon of Todaro–eustachian ridge (ER) posteriorly (white arrows), attachment of the septal tricuspid valve anteriorly (yellow arrows), coronary sinus (CS) inferiorly, and central fibrous body (CFB) at the apex (red arrow). The septal isthmus (small bracket), the area between the coronary sinus and septal tricuspid valve, is the target for ablation of AV node reentrant tachycardia. The CTI (large bracket) lies between the IVC orifice and the tricuspid valve. A = anterior, I = inferior, P = posterior, S = superior. (b–d) Coronal (b), sagittal (c), and axial (d) CT scans show the Koch triangle (yellow lines) from different perspectives. Arrow in d indicates the central fibrous body at the triangle apex.
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AV Node and AV Nodal Artery
The AV node lies in the Koch triangle. It consists of a compact portion and an area of transitional cells (43). The AV node continues distally with the penetrating His bundle. The His bundle starts to be surrounded by the connective tissue of the central fibrous body, thus becoming a conducting tract that takes information to the ventricles (43).
The AV nodal artery originates from the apex of the U-turn of the distal right coronary artery and penetrates the base of the posterior interatrial septum at the level of the crux of the heart in 80%–87% of patients (44–46). In the remaining patients, it originates from the terminal portion of the LCx artery (8%–13% of cases) or, less commonly, from both the right coronary artery and the LCx artery (2%–10%) (Fig 12). The artery provides branches to the posterior interventricular septum, interatrial septum, AV node, and penetrating His bundle (45). In some patients, at the level of the Koch triangle, the AV nodal artery courses just beneath the endocardium near the ostium of the coronary sinus and the septal isthmus. This fact may explain the higher risk for AV nodal artery coagulation during RF ablation in the slow pathway region, although complete AV block is commonly a direct result of tissue injury to the AV node (47,48). This relationship can easily be demonstrated with multidetector CT (Fig 13b).
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Figure 12a. Short-axis CT scans obtained at the level of the inferior pyramidal space show the right AV nodal artery (arrow) arising from the right coronary artery (RCA) (a) and the LCx artery (b). AAo = ascending aorta, RA = right atrium.
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Figure 12b. Short-axis CT scans obtained at the level of the inferior pyramidal space show the right AV nodal artery (arrow) arising from the right coronary artery (RCA) (a) and the LCx artery (b). AAo = ascending aorta, RA = right atrium.
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Figure 13a. CT evaluation of the CTI and the base of the Koch triangle. AO = aorta, AVN = AV node (yellow arrow), AVNa = AV nodal artery, CS = coronary sinus (green double-headed arrows), ER = eustachian ridge, LV = left ventricle, RA = right atrium, SI = septal isthmus (blue double-headed arrows), STV = septal leaflet of the tricuspid valve, TT = tendon of Todaro. (a) By using a short-axis CT scan (center) obtained at the level of the coronary sinus, long-axis views (left, right) are obtained through the isthmus and demonstrate anatomic landmarks of this region. Red double-headed arrows indicate the CTI. (b) CT scans (right image is a magnified view of the boxed area at left) demonstrate the septal isthmus. Note the prominent eustachian ridge and the relatively large tendon of Todaro within its musculature. Note also the proximity of the AV nodal artery to the septal isthmus and coronary sinus. The septal isthmus and the coronary sinus ostium represent the base of the Koch triangle.
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Figure 13b. CT evaluation of the CTI and the base of the Koch triangle. AO = aorta, AVN = AV node (yellow arrow), AVNa = AV nodal artery, CS = coronary sinus (green double-headed arrows), ER = eustachian ridge, LV = left ventricle, RA = right atrium, SI = septal isthmus (blue double-headed arrows), STV = septal leaflet of the tricuspid valve, TT = tendon of Todaro. (a) By using a short-axis CT scan (center) obtained at the level of the coronary sinus, long-axis views (left, right) are obtained through the isthmus and demonstrate anatomic landmarks of this region. Red double-headed arrows indicate the CTI. (b) CT scans (right image is a magnified view of the boxed area at left) demonstrate the septal isthmus. Note the prominent eustachian ridge and the relatively large tendon of Todaro within its musculature. Note also the proximity of the AV nodal artery to the septal isthmus and coronary sinus. The septal isthmus and the coronary sinus ostium represent the base of the Koch triangle.
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Eustachian Valve and Ridge
The eustachian valve, which guards the entrance to the IVC, is variably developed. It usually inserts medially onto the eustachian ridge, which is the border between the oval fossa and the coronary sinus. The free border of the eustachian valve continues as the tendon of Todaro, which runs in the musculature of the eustachian ridge (43). In some cases, the eustachian valve is large and muscular, posing an obstacle to catheter passage (Fig 13b).
Cavotricuspid Isthmus
The right atrial CTI, the area between the IVC and the tricuspid valve, is the target of catheter ablation techniques that have become the treatment of choice for isthmus-dependent atrial flutter (49–51). Autopsies and angiographic findings have shown a highly variable isthmian anatomy, which can make ablation difficult (50,52). Obstacles such as a large eustachian ridge, aneurysmal pouches, or even a concave deformation of the entire isthmus may lead to more difficult ablation sessions (53,54). Therefore, adaptation of the ablation approach to the anatomic variants of this region may contribute to successful ablation. Cardiac multidetector CT provides the anatomic information necessary for successful RF catheter ablation, including the size and anatomic variants of the CTI, coronary sinus, and eustachian ridge (Fig 14).
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Figure 14a. Cavotricuspid isthmus. (a, b) On a two-chamber view of the right side of the heart (b) through the central portion of the CTI as indicated by the red double-headed arrow in a (posterior 3D image), the IVC is closest to the AV groove (yellow arrow in b). The length and depth of the central isthmus (white double-headed arrow) can be measured on this view. AO = aorta, CS = coronary sinus, LA = left atrium, MPA = main pulmonary artery, RA = right atrium, RV = right ventricle. (c) Three-dimensional images obtained in middiastolic phase (70% of R-R interval) demonstrate how the length of the right atrial isthmus (double-headed arrow) varies among individuals. Knowledge of these anatomic variants prior to catheter ablation will save time and increase the success rate.
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Figure 14b. Cavotricuspid isthmus. (a, b) On a two-chamber view of the right side of the heart (b) through the central portion of the CTI as indicated by the red double-headed arrow in a (posterior 3D image), the IVC is closest to the AV groove (yellow arrow in b). The length and depth of the central isthmus (white double-headed arrow) can be measured on this view. AO = aorta, CS = coronary sinus, LA = left atrium, MPA = main pulmonary artery, RA = right atrium, RV = right ventricle. (c) Three-dimensional images obtained in middiastolic phase (70% of R-R interval) demonstrate how the length of the right atrial isthmus (double-headed arrow) varies among individuals. Knowledge of these anatomic variants prior to catheter ablation will save time and increase the success rate.
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Figure 14c. Cavotricuspid isthmus. (a, b) On a two-chamber view of the right side of the heart (b) through the central portion of the CTI as indicated by the red double-headed arrow in a (posterior 3D image), the IVC is closest to the AV groove (yellow arrow in b). The length and depth of the central isthmus (white double-headed arrow) can be measured on this view. AO = aorta, CS = coronary sinus, LA = left atrium, MPA = main pulmonary artery, RA = right atrium, RV = right ventricle. (c) Three-dimensional images obtained in middiastolic phase (70% of R-R interval) demonstrate how the length of the right atrial isthmus (double-headed arrow) varies among individuals. Knowledge of these anatomic variants prior to catheter ablation will save time and increase the success rate.
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Subthebesian Pouch
The atrial wall inferior to the orifice of the coronary sinus is usually pouchlike (Fig 15). It is often described as the sinus of Keith, or subeustachian sinus (11,30). When seen in attitudinally correct orientation, it is anterior to the orifice of the IVC and is subthebesian rather than subeustachian (51). Because of its special arrangement of muscle fibers, it can be the substrate for the reentrant circuit during atrial flutter. In an angiographic study of the right isthmus, a deep subthebesian pouch was observed in 47% of cases, with a mean depth of 4.3 ± 2.1 mm (range, 1.5–9.4 mm) (52). This peculiar anatomy is recognized as one of the main sources of procedural difficulty.
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Figure 15a. Subthebesian pouch. (a) Three-dimensional images and short-axis CT scan obtained at the level of the coronary sinus (CS) show a relatively large subthebesian pouch (STP) (arrows) inferior to the thebesian valve. The subthebesian pouch is a diverticular extension of the CTI under the coronary sinus, is anterior to the orifice of the IVC, and is subthebesian rather than subeustachian. (b) Short-axis (top) and axial (bottom) CT scans show the view angles for the endoscopic image obtained in the corresponding projection. These angles are shown automatically at virtual endoscopy. Arrow indicates the AV node. (c) Endocardial image displays the internal aspects of the isthmic region of the right atrium and the spatial relationship of the subthebesian pouch (STP) to the coronary sinus (CS) and IVC. A large subthebesian pouch may hinder access to the coronary sinus or impair local RF delivery during ablation of isthmus-dependent flutter. A = anterior, AVN = AV node, ER = eustachian ridge (green arrows), EV = eustachian valve (red arrows), I = inferior, NCS = non-coronary sinus, OF = oval fossa, P = posterior, RAA = right atrial appendage, RV = right ventricle, S = superior, ThV = thebesian valve (blue arrow), TV = tricuspid valve (yellow arrows).
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Figure 15b. Subthebesian pouch. (a) Three-dimensional images and short-axis CT scan obtained at the level of the coronary sinus (CS) show a relatively large subthebesian pouch (STP) (arrows) inferior to the thebesian valve. The subthebesian pouch is a diverticular extension of the CTI under the coronary sinus, is anterior to the orifice of the IVC, and is subthebesian rather than subeustachian. (b) Short-axis (top) and axial (bottom) CT scans show the view angles for the endoscopic image obtained in the corresponding projection. These angles are shown automatically at virtual endoscopy. Arrow indicates the AV node. (c) Endocardial image displays the internal aspects of the isthmic region of the right atrium and the spatial relationship of the subthebesian pouch (STP) to the coronary sinus (CS) and IVC. A large subthebesian pouch may hinder access to the coronary sinus or impair local RF delivery during ablation of isthmus-dependent flutter. A = anterior, AVN = AV node, ER = eustachian ridge (green arrows), EV = eustachian valve (red arrows), I = inferior, NCS = non-coronary sinus, OF = oval fossa, P = posterior, RAA = right atrial appendage, RV = right ventricle, S = superior, ThV = thebesian valve (blue arrow), TV = tricuspid valve (yellow arrows).
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Figure 15c. Subthebesian pouch. (a) Three-dimensional images and short-axis CT scan obtained at the level of the coronary sinus (CS) show a relatively large subthebesian pouch (STP) (arrows) inferior to the thebesian valve. The subthebesian pouch is a diverticular extension of the CTI under the coronary sinus, is anterior to the orifice of the IVC, and is subthebesian rather than subeustachian. (b) Short-axis (top) and axial (bottom) CT scans show the view angles for the endoscopic image obtained in the corresponding projection. These angles are shown automatically at virtual endoscopy. Arrow indicates the AV node. (c) Endocardial image displays the internal aspects of the isthmic region of the right atrium and the spatial relationship of the subthebesian pouch (STP) to the coronary sinus (CS) and IVC. A large subthebesian pouch may hinder access to the coronary sinus or impair local RF delivery during ablation of isthmus-dependent flutter. A = anterior, AVN = AV node, ER = eustachian ridge (green arrows), EV = eustachian valve (red arrows), I = inferior, NCS = non-coronary sinus, OF = oval fossa, P = posterior, RAA = right atrial appendage, RV = right ventricle, S = superior, ThV = thebesian valve (blue arrow), TV = tricuspid valve (yellow arrows).
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Interatrial Septum
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Atrial septation is a complex process involving several tissue components (55). A septum is best defined as a wall that can be removed without exposing the heart cavity to extracardiac structures (11). The true atrial septum is made up of the flap valve of the foramen ovale (septum primum) and part of its anteroinferior margin. The superior rim of the fossa, often referred to as the septum secundum, is the infolded wall between the SVC and the right pulmonary veins known as the interatrial groove and is not a true septum (Fig 16).
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Figure 16a. Components of the interatrial septum. LA = left atrium, LV = left ventricle. Four-chamber view (a) and a magnified view of the boxed area in a (b) show accumulation of extracardiac fat in nonseptal areas, including the superior interatrial groove (IAG), eustachian ridge (ER), and muscular AV septum (blue double-headed arrow). The only true septum between the two atria is confined to the area of the oval fossa (red double-headed arrow) and a small portion of the inferior rim (yellow double-headed arrow). Green double-headed arrow indicates the superior rim. CT is one of the best modalities for depicting the anatomic boundary of the oval fossa. CT = crista terminalis, MV = mitral valve, RA = right atrium, STV = septal leaflet of the tricuspid valve.
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Figure 16b. Components of the interatrial septum. LA = left atrium, LV = left ventricle. Four-chamber view (a) and a magnified view of the boxed area in a (b) show accumulation of extracardiac fat in nonseptal areas, including the superior interatrial groove (IAG), eustachian ridge (ER), and muscular AV septum (blue double-headed arrow). The only true septum between the two atria is confined to the area of the oval fossa (red double-headed arrow) and a small portion of the inferior rim (yellow double-headed arrow). Green double-headed arrow indicates the superior rim. CT is one of the best modalities for depicting the anatomic boundary of the oval fossa. CT = crista terminalis, MV = mitral valve, RA = right atrium, STV = septal leaflet of the tricuspid valve.
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The flap of the foramen ovale closes against the atrial septum, with fusion usually occurring within the first 2 years of life (55). Fusion is incomplete in about 25% of the population, resulting in probe patent defect, or patent foramen ovale. The patent foramen ovale is usually less than 5 mm in diameter (55). Preprocedural anatomic knowledge of the atrial septum can minimize complications of transseptal approaches. Multidetector CT is a powerful tool that provides detailed anatomic information regarding the size, morphologic features, and location of the foramen ovale and allows detection of associated findings (Figs 17, 18) (56). A patent foramen ovale is often associated with atrial septal aneurysm and Chiari network (57–59). An atrial septal aneurysm is defined as a localized bulge (>15 mm) of the interatrial septum that protrudes into the right or left atrium (Fig 18) (57). Atrial septal aneurysms are increasingly being recognized as closely associated with cryptogenic strokes and migraine headaches (57).
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Figure 17a. Foramen ovale. (a) Left atrial view shows a probe in a patent foramen ovale. (b, c) Short-axis CT angiographic images show a flap valve with no patent foramen ovale (arrow in b) and a flap valve with a patent foramen ovale (black arrow in c). Note the jet of contrast material (white arrows in c) moving from the left atrium (LA) to the right atrium.
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Figure 17b. Foramen ovale. (a) Left atrial view shows a probe in a patent foramen ovale. (b, c) Short-axis CT angiographic images show a flap valve with no patent foramen ovale (arrow in b) and a flap valve with a patent foramen ovale (black arrow in c). Note the jet of contrast material (white arrows in c) moving from the left atrium (LA) to the right atrium.
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Figure 17c. Foramen ovale. (a) Left atrial view shows a probe in a patent foramen ovale. (b, c) Short-axis CT angiographic images show a flap valve with no patent foramen ovale (arrow in b) and a flap valve with a patent foramen ovale (black arrow in c). Note the jet of contrast material (white arrows in c) moving from the left atrium (LA) to the right atrium.
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Figure 18a. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Figure 18b. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Figure 18c. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Figure 18d. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Figure 18e. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Figure 18f. Anatomic variants of the oval fossa and atrial septum. CT scans show a flap valve (arrows in a), atrial septal aneurysm (arrows in b), lipomatose hypertrophy of the septum (arrow in c), patent foramen ovale (arrow in d), atrial septal defect (septum secundum) (arrows in e), and atrial septal defect (sinus venosus) (arrows in f). All of these findings were discovered incidentally in patients who had been referred for coronary CT angiography.
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Septal Components of the AV Junction
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The septal components of the AV junction are important because they conduct the cardiac impulse from the atria to the ventricles (55). With multidetector CT, detailed anatomic study of this region is feasible (Fig 19). The central fibrous body (apex of the Koch triangle) lies superior and anterior to the muscular AV septum. The central fibrous body consists in part of the right fibrous trigone and in part of the membranous septum and fuses together the aortic, mitral, and tricuspid valves.
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Figure 19a. CT evaluation of the septal components of the AV junction. From a short-axis CT scan (b) obtained at the level of the membranous septum, four-chamber views (a) are reconstructed at the levels of the internal cardiac crux (white arrow), muscular AV septum (blue arrow), membranous AV septum (red arrow), and membranous interventricular (IV) septum (green arrow). Note the offset between the septal attachment of the tricuspid valve and the mitral valve at the muscular AV septum. The membranous septum is divided into two parts by the septal leaflet of the tricuspid valve, namely, the AV and interventricular components.
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Figure 19b. CT evaluation of the septal components of the AV junction. From a short-axis CT scan (b) obtained at the level of the membranous septum, four-chamber views (a) are reconstructed at the levels of the internal cardiac crux (white arrow), muscular AV septum (blue arrow), membranous AV septum (red arrow), and membranous interventricular (IV) septum (green arrow). Note the offset between the septal attachment of the tricuspid valve and the mitral valve at the muscular AV septum. The membranous septum is divided into two parts by the septal leaflet of the tricuspid valve, namely, the AV and interventricular components.
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Left Atrium
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The left atrium is a very important structure for interventional cardiologists (60). Like the right atrium, the left atrium consists of a venous component, a vestibule, and an appendage (10,11). The venous component, with the pulmonary vein orifices at each corner, is located posteriorly (Figs 6, 7). The vestibular component surrounds the mitral orifice. The greater part of the left atrium, including the venous component, the vestibule, and the septal component, is smooth walled. The LAA is derived from the primitive atrium and has a rough, trabeculated surface. It is a potential site for deposition of thrombus owing to its narrow neck with the left atrium (Fig 7b). The anterior wall of the left atrium just behind the aorta is usually thin and vulnerable to being torn. The superior wall, or dome, is thickest, measuring 3.5–6.5 mm (11).
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Role of Multidetector CT in Catheter Ablation Procedures for AF
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It has been shown that the pulmonary veins may be the dominant source of triggers initiating AF, and that catheter ablation may successfully eliminate these triggers (22). Different ablation strategies have been proposed. Both ostial segmental isolation of the pulmonary vein (21,22) and anatomically based circumferential ablation (61,62) are commonly used and have proved to be effective in the elimination of AF (Fig 6). Regardless of the strategy used, knowledge of the pulmonary vein anatomy and of the exact location of the junction of the left atrium and pulmonary vein is mandatory for successful ablation. Multidetector CT can be used to guide the electrophysiologist, providing anatomic details noninvasively.With multidetector CT, information regarding the size, number, location, and anatomic variants of the pulmonary veins can easily be obtained and used to select the size of the catheters before performing the procedure (63–66). Preprocedural multi-detector CT has been shown to reduce RF ablation time. MR imaging is also frequently used for pulmonary vein evaluation (66).
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What Electrophysiologists Need to Know before Performing AF Ablation
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Electrophysiologists should be aware of the following entities before performing AF ablation:
- Normal anatomy and anatomic variants of the pulmonary veins (Fig 20).
- Ostial diameters of each vein and the distance to the first-order branch.
- Presence of accessory or supernumerary pulmonary veins.
- Dimensions of the left atrium and the presence of LAA thrombus.
- Anatomic course of the esophagus relative to the posterior left atrial wall and pulmonary veins.
- Presence of major anomalies, such as a common ostium to the superior and inferior veins, persistent left SVC, vein of Marshall, or anomalous pulmonary venous return.
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Figure 20a. Anatomic variants of the pulmonary veins. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein. Three-dimensional images show conjoined ostia (a), a common variant seen in up to 25% of cases; a supernumerary branch (b), which in some cases has an aberrant insertion perpendicular to the posterior left atrial wall; "early branching" of the pulmonary veins (ostial branch) (c), which is also common; and a small right middle accessory pulmonary vein (d) draining the right middle lobe (RML), which represents the most common variant. It is important to report the ostial cross-sectional diameters and the trunk length (the distance from the ostium to the first-order branch) (green double-headed arrows in d), as well as anatomic variants of the pulmonary veins. The superior pulmonary veins have a larger ostium and a longer trunk than do the inferior pulmonary veins.
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Figure 20b. Anatomic variants of the pulmonary veins. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein. Three-dimensional images show conjoined ostia (a), a common variant seen in up to 25% of cases; a supernumerary branch (b), which in some cases has an aberrant insertion perpendicular to the posterior left atrial wall; "early branching" of the pulmonary veins (ostial branch) (c), which is also common; and a small right middle accessory pulmonary vein (d) draining the right middle lobe (RML), which represents the most common variant. It is important to report the ostial cross-sectional diameters and the trunk length (the distance from the ostium to the first-order branch) (green double-headed arrows in d), as well as anatomic variants of the pulmonary veins. The superior pulmonary veins have a larger ostium and a longer trunk than do the inferior pulmonary veins.
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Figure 20c. Anatomic variants of the pulmonary veins. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein. Three-dimensional images show conjoined ostia (a), a common variant seen in up to 25% of cases; a supernumerary branch (b), which in some cases has an aberrant insertion perpendicular to the posterior left atrial wall; "early branching" of the pulmonary veins (ostial branch) (c), which is also common; and a small right middle accessory pulmonary vein (d) draining the right middle lobe (RML), which represents the most common variant. It is important to report the ostial cross-sectional diameters and the trunk length (the distance from the ostium to the first-order branch) (green double-headed arrows in d), as well as anatomic variants of the pulmonary veins. The superior pulmonary veins have a larger ostium and a longer trunk than do the inferior pulmonary veins.
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Figure 20d. Anatomic variants of the pulmonary veins. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein. Three-dimensional images show conjoined ostia (a), a common variant seen in up to 25% of cases; a supernumerary branch (b), which in some cases has an aberrant insertion perpendicular to the posterior left atrial wall; "early branching" of the pulmonary veins (ostial branch) (c), which is also common; and a small right middle accessory pulmonary vein (d) draining the right middle lobe (RML), which represents the most common variant. It is important to report the ostial cross-sectional diameters and the trunk length (the distance from the ostium to the first-order branch) (green double-headed arrows in d), as well as anatomic variants of the pulmonary veins. The superior pulmonary veins have a larger ostium and a longer trunk than do the inferior pulmonary veins.
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Pulmonary Venous Anatomy and Anatomic Variants
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Most of the information regarding pulmonary vein location and ostia can be obtained from axial and multiplanar reformatted images (Fig 20). Three-dimensional and endoluminal views of the left atrium and pulmonary veins are also helpful in guiding electrophysiologists prior to AF ablation (Fig 6). The normal pulmonary venous anatomy consists of two right pulmonary veins and two left pulmonary veins with separate ostia. However, it has been demonstrated that anatomic variants of the left atrium and pulmonary veins are common (63–65). The pulmonary venous trunk is defined as the distance from the ostium to the first-order branch. It is important to report the trunk length and the ostial diameters of each vein, which influence the selection of catheter diameter. However, the pulmonary veins and their ostia are usually oval. With use of multiplanar reformation, two orthogonal pulmonary vein measurements can be obtained in a plane perpendicular to the long axis of the vessel. The superior pulmonary vein ostia are larger (19–20 mm) than the inferior pulmonary vein ostia (16–17 mm) (27,64), and the superior pulmonary veins tend to have a longer trunk (21.6 ± 7.5 mm) than do the inferior pulmonary veins (14.0 ± 6.2 mm) (64).
Common anomalies include a conjoined (common) left or right pulmonary vein, which is seen in 25% of individuals (27,64). Conjoined pulmonary vein is more frequently seen on the left side than on the right (65). Supernumerary veins are also frequently seen, the most common of which is a separate right middle pulmonary vein that drains the middle lobe of the lung (66). The ostial diameter of a right middle pulmonary vein (mean, 9.9 ± 1.9 mm) is smaller than that of other veins. In some patients, the supernumerary pulmonary vein has an aberrant insertion with a perpendicular orientation to the posterior left atrial wall (Fig 20). Finally, early branching of the pulmonary vein (<1 cm) is also frequently observed. In addition to depicting the pulmonary venous anatomy, multidetector CT can be used to evaluate for the presence of (a) LAA thrombus prior to ablation and (b) pulmonary vein stenosis or thrombosis after ablation (67,68). Left atrial ablation should not be performed in the presence of known atrial thrombus. Unmixed blood and contrast material in the LAA can mimic a thrombus or mass at multidetector CT (Fig 21). A rare complication of RF ablation for AF is a fistula between the esophagus and the posterior left atrial wall (69). Multidetector CT is valuable for defining the relationship of the esophagus to the posterior left atrial wall prior to ablation (70). Recently, image integration systems for catheter ablation procedures have been introduced (71–73). With this new technology, 3D multidetector CT scans acquired prior to ablation will be fused with electroanatomic mapping data at the time of the procedure with an accuracy of 2 mm distance between corresponding points on the two images (72). This fusion allows accurate positioning of the catheter in the anatomic area of interest, thereby facilitating ablation procedures for AF.
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Figure 21a. Unmixed blood and contrast material mimicking a thrombus in the LAA. (a) Contrast material–enhanced CT scan obtained in a patient with a history of AF shows a filling defect in the LAA (arrows), a finding that is consistent with thrombus. MPa = main pulmonary artery. (Courtesy of Jeroen J. Bax, MD, Leiden University Medical Center, Leiden, the Netherlands.) (b) CT scan obtained in a different patient shortly after contrast material administration shows triangular nonenhancing blood layering (arrow) above the contrast material in the LAA. AAo = ascending aorta, LA = left atrium. (c) On a CT scan obtained 1 minute after b, the blood layering has completely disappeared.
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Figure 21b. Unmixed blood and contrast material mimicking a thrombus in the LAA. (a) Contrast material–enhanced CT scan obtained in a patient with a history of AF shows a filling defect in the LAA (arrows), a finding that is consistent with thrombus. MPa = main pulmonary artery. (Courtesy of Jeroen J. Bax, MD, Leiden University Medical Center, Leiden, the Netherlands.) (b) CT scan obtained in a different patient shortly after contrast material administration shows triangular nonenhancing blood layering (arrow) above the contrast material in the LAA. AAo = ascending aorta, LA = left atrium. (c) On a CT scan obtained 1 minute after b, the blood layering has completely disappeared.
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Figure 21c. Unmixed blood and contrast material mimicking a thrombus in the LAA. (a) Contrast material–enhanced CT scan obtained in a patient with a history of AF shows a filling defect in the LAA (arrows), a finding that is consistent with thrombus. MPa = main pulmonary artery. (Courtesy of Jeroen J. Bax, MD, Leiden University Medical Center, Leiden, the Netherlands.) (b) CT scan obtained in a different patient shortly after contrast material administration shows triangular nonenhancing blood layering (arrow) above the contrast material in the LAA. AAo = ascending aorta, LA = left atrium. (c) On a CT scan obtained 1 minute after b, the blood layering has completely disappeared.
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Left Atrial Isthmus and AF
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In patients with AF, the success rate for catheter ablation depends on achieving adequate electrical isolation by performing linear ablation of lesions (Fig 6). However, recurrence of atrial arrhythmias is not uncommon, particularly in the left atrial isthmus (the area between the orifice of the left inferior pulmonary vein and the posteroinferior mitral annulus) (74,75). The length of the left atrial isthmus is highly variable, having been reported by Becker (76) as ranging from 17 to 51 mm. It has been shown that the isthmus is longer in patients with AF (77). Multidetector CT can accurately demonstrate the boundaries of this area, including the exact location of the mitral valve ring, coronary sinus, and great cardiac vein as well as their anatomic variants (Fig 22) (77). A serious complication of left atrial isthmus ablation is injury to the adjacent vessels, including the LCx artery (78). Multidetector CT can demonstrate the relationships between the coronary sinus, LCx artery, and left atrial wall, thereby providing a safer approach in this type of ablation (Fig 22).
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Figure 22a. Left atrial isthmus. CT scan (a), endoscopic image (b), and 3D image (c) show the left atrial isthmus (double-headed arrow), the area between the orifice of the left inferior pulmonary vein (LIPV) and the posteroinferior margin of the mitral annulus. The left atrial isthmus may be the source of recurrence after circumferential pulmonary vein catheter ablation for AF. Preablation CT can easily help evaluate the length, depth, and morphologic variants of this region. LSPV = left superior pulmonary vein, LV = left ventricle, MV = mitral valve.
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Figure 22b. Left atrial isthmus. CT scan (a), endoscopic image (b), and 3D image (c) show the left atrial isthmus (double-headed arrow), the area between the orifice of the left inferior pulmonary vein (LIPV) and the posteroinferior margin of the mitral annulus. The left atrial isthmus may be the source of recurrence after circumferential pulmonary vein catheter ablation for AF. Preablation CT can easily help evaluate the length, depth, and morphologic variants of this region. LSPV = left superior pulmonary vein, LV = left ventricle, MV = mitral valve.
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Figure 22c. Left atrial isthmus. CT scan (a), endoscopic image (b), and 3D image (c) show the left atrial isthmus (double-headed arrow), the area between the orifice of the left inferior pulmonary vein (LIPV) and the posteroinferior margin of the mitral annulus. The left atrial isthmus may be the source of recurrence after circumferential pulmonary vein catheter ablation for AF. Preablation CT can easily help evaluate the length, depth, and morphologic variants of this region. LSPV = left superior pulmonary vein, LV = left ventricle, MV = mitral valve.
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AF Originating from the Ligament of Marshall and Persistent Left SVC
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AF may arise not only in the pulmonary veins but also from the left or right SVC, ligament of Marshall, free posterior left atrial wall, crista terminalis, coronary sinus ostium, and interatrial septum (23,79). The vein of Marshall, which drains into the coronary sinus, is located within a vestigial fold of pericardium known as the ligament of Marshall, which is the developmental remnant of the embryonic left SVC. The ligament of Marshall is mostly obliterated in the majority of individuals. It remains patent as an isolated malformation, persistent left SVC, and drains into the coronary sinus in 0.3% of the population (Fig 23) (80). The ligament of Marshall and persistent left SVC have been implicated in the initiation and maintenance of AF (79,80).
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Anomalous Pulmonary and Systemic Connections
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Unlike a total anomalous pulmonary venous connection, a partial anomalous pulmonary venous connection is an acyanotic lesion and may be detected incidentally at imaging studies (Fig 24) (81,82). A partial anomalous pulmonary venous connection has classically been described as manifesting in childhood, most frequently on the right side and in association with a sinus venosus atrial septal defect (83,84). In a partial anomalous pulmonary venous connection of the left upper lobe, intraparenchymal left upper lobe pulmonary veins join to form an anomalous vein described as the vertical vein, which drains into the left brachiocephalic vein. In one recent review of 1825 chest CT scans, the prevalence of partial anomalous pulmonary venous connection was 0.2% (85). In this study, 79% of patients had an anomalous left upper lobe vein connecting to a persistent left vertical vein and 17% percent had an anomalous right upper lobe vein draining into the SVC, with 3% having an anomalous right lower lobe vein draining into the suprahepatic IVC (Fig 24) (85).
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Figure 24a. Partial anomalous pulmonary venous connections discovered incidentally in three different patients. (a) CT scan shows partial drainage of the right superior pulmonary vein (arrows) directly into the SVC. (b) CT scans show anomalous drainage of the left superior pulmonary vein (arrows) into the left brachiocephalic vein via the vertical vein. (c) CT scans show anomalous drainage of the right inferior pulmonary vein (arrows) into the suprahepatic segment of the IVC (scimitar sign).
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Figure 24b. Partial anomalous pulmonary venous connections discovered incidentally in three different patients. (a) CT scan shows partial drainage of the right superior pulmonary vein (arrows) directly into the SVC. (b) CT scans show anomalous drainage of the left superior pulmonary vein (arrows) into the left brachiocephalic vein via the vertical vein. (c) CT scans show anomalous drainage of the right inferior pulmonary vein (arrows) into the suprahepatic segment of the IVC (scimitar sign).
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Figure 24c. Partial anomalous pulmonary venous connections discovered incidentally in three different patients. (a) CT scan shows partial drainage of the right superior pulmonary vein (arrows) directly into the SVC. (b) CT scans show anomalous drainage of the left superior pulmonary vein (arrows) into the left brachiocephalic vein via the vertical vein. (c) CT scans show anomalous drainage of the right inferior pulmonary vein (arrows) into the suprahepatic segment of the IVC (scimitar sign).
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Cardiac Venous System: Implications for CRT with Biventricular Pacing
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In approximately 30% of patients with chronic heart failure, the disease process affects the conduction pathways by causing a delay in the onset of right or left ventricular systole due to bundle branch block (86). Intraventricular conduction delay may further impair the ability of the failing heart to eject blood. Cardiac resynchronization therapy (CRT) with biventricular pacing can improve the synchrony of contractions of the two ventricles, leading to improved hemodynamics and ejection fraction. Pacing of the left ventricle is accomplished via the coronary sinus. However, the presence of a suitable branch of the coronary sinus is crucial for positioning the left ventricular lead with a transvenous approach. Multidetector CT has been used to evaluate the coronary sinus and coronary veins prior to lead placement (87–90). Major components of the cardiac venous system include the coronary sinus, great cardiac vein, and anterior and posterior interventricular veins. The great cardiac vein courses alongside the LCx artery and subsequently drains into the coronary sinus (88). It receives two main branches, namely, the left marginal vein (LMV), which courses along the lateral border of the left ventricle, and the posterior vein or posterolateral vein. The LMV and posterolateral vein are often the target veins for pacemaker lead placement in CRT (Fig 25). Ideally, the anatomy of the cardiac venous system should be assessed noninvasively on an outpatient basis before referring the patient for CRT implantation.
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Figure 25a. (a) Three-dimensional image depicts the cardiac venous anatomy. The great cardiac vein (GCV) receives two main branches: the LMV, along the lateral border of the left ventricle, and the posterior vein or posterolateral vein (PLV). Implantation of the coronary sinus (CS) lead usually involves the lateral and posterior branches, which are quite variable in number, tortuosity, dimensions, and angulation with the great cardiac vein. PIV = posterior interventricular vein. (b) Three-dimensional image shows how left ventricular pacing for CRT involves cannulation of the coronary sinus (CS), which is easier to perform via the SVC. Green arrows indicate the path of the catheter, which is usually placed in the periphery of the LMV. Circles indicate the anatomic areas of interest and angle locations. AO = aorta, RA = right atrium.
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Figure 25b. (a) Three-dimensional image depicts the cardiac venous anatomy. The great cardiac vein (GCV) receives two main branches: the LMV, along the lateral border of the left ventricle, and the posterior vein or posterolateral vein (PLV). Implantation of the coronary sinus (CS) lead usually involves the lateral and posterior branches, which are quite variable in number, tortuosity, dimensions, and angulation with the great cardiac vein. PIV = posterior interventricular vein. (b) Three-dimensional image shows how left ventricular pacing for CRT involves cannulation of the coronary sinus (CS), which is easier to perform via the SVC. Green arrows indicate the path of the catheter, which is usually placed in the periphery of the LMV. Circles indicate the anatomic areas of interest and angle locations. AO = aorta, RA = right atrium.
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Important Considerations in Pre-CRT Multidetector CT
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Pre-CRT multidetector CT should be performed for the following reasons:
- Comprehensive evaluation of the coronary venous anatomy.
- Measurement of the coronary sinus orifice and the target veins.
- Evaluation for the presence of anatomic barriers to accessing the coronary sinus (thebesian and Vieussens valves [valves at the ostium or within the coronary sinus], subthebesian pouch, unusual coronary sinus anatomy, coronary sinus diverticulum, vein of Marshall, luminal narrowing due to a crossing artery [LCx artery]).
- Clarification of the relationship of the left phrenic neurovascular bundle to the target vein.
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Coronary Sinus and Its Anatomic Variants
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The coronary sinus is approximately 45 mm long and 10–12 mm in diameter, with highly variable morphologic features (91–93). The beginning of the coronary sinus is marked by either an outer constriction, the opening of the oblique vein of Marshall, or the Vieussens valve internally (92). The coronary sinus ostium is guarded by the thebesian valve, which usually consists of a thin semilunar fold in the anteroinferior rim of the ostium. The coronary sinus is used as a conduit for catheter treatment of arrhythmias as well as left ventricular pacing (93–95). It is well known that these procedures have technical limitations due to the variability of the course and valves of the coronary sinus and its relationship to adjacent structures (Fig 26). To record electrical activity from the left atrium and left ventricle, a catheter is guided into the coronary sinus. Because of the sharp angle between the coronary sinus and the left atrium, catheterization of this structure is easier via the SVC (Fig 25b). Careful evaluation of anatomic barriers is important for treatment success (96). The Vieussens valve is usually rudimentary and incomplete but may be a major source of difficulty in cannulation of the coronary sinus (97). The great cardiac vein may kink at the level where it is crossed by the deep-seated muscular LCx artery, resulting in luminal obstruction (92). Congenital coronary sinus anomalies do exist, including diverticulum, stenosis, ectasia, unroofed sinus, ostial atresia, agenesis, and duplication (98). The majority of coronary sinus diverticula are located along its inferior aspect, usually at its junction with the middle cardiac vein (Fig 26) (96). A coronary sinus diverticulum differs from a subthebesian pouch, which is a recess of the right atrial CTI extending below the orifice of the coronary sinus. A coronary sinus diverticulum may form the anatomic basis of posteroseptal or left posterior accessory pathways. In such cases, endodiverticular ablation is necessary (95,99). It has been shown that the proximal coronary sinus in patients with AV junctional reentry tachycardia is significantly larger than in healthy patients and resembles a wind sock (100).
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Left Phrenic Nerve and Its Relation to Target Coronary Veins Relevant to Left Ventricular Pacing
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Left phrenic nerve stimulation after CRT is a well-recognized complication (101). The left phrenic nerve passes at a distance of less than 3 mm from the LMV in 43% of cadaveric hearts (102). Given the anatomic variability of the target coronary veins for CRT and the proximity of the left phrenic nerve to these structures, it is important to understand their relationship so as to avoid the phrenic nerve during left ventricular lead placement. Coronary multidetector CT angiography has the potential to help detect the left phrenic nerve in its neurovascular bundle as it passes over the left ventricular pericardium (Fig 27) (103).
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Figure 27a. Anatomic course of the left phrenic neurovascular bundle. Axial CT scans (a–c) and left lateral 3D multidetector CT scan (d) show the left phrenic nerve running outside the pericardium over the tip of the LAA (arrow in a, top arrow in d) and in proximity to the obtuse marginal artery and left LMV (arrow in b, middle arrow in d). Arrow in c and bottom arrow in d indicate the left phrenic neurovascular bundle.
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Figure 27b. Anatomic course of the left phrenic neurovascular bundle. Axial CT scans (a–c) and left lateral 3D multidetector CT scan (d) show the left phrenic nerve running outside the pericardium over the tip of the LAA (arrow in a, top arrow in d) and in proximity to the obtuse marginal artery and left LMV (arrow in b, middle arrow in d). Arrow in c and bottom arrow in d indicate the left phrenic neurovascular bundle.
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Figure 27c. Anatomic course of the left phrenic neurovascular bundle. Axial CT scans (a–c) and left lateral 3D multidetector CT scan (d) show the left phrenic nerve running outside the pericardium over the tip of the LAA (arrow in a, top arrow in d) and in proximity to the obtuse marginal artery and left LMV (arrow in b, middle arrow in d). Arrow in c and bottom arrow in d indicate the left phrenic neurovascular bundle.
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View larger version (106K):
[in this window]
[in a new window]
[Download PPT slide]
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Figure 27d. Anatomic course of the left phrenic neurovascular bundle. Axial CT scans (a–c) and left lateral 3D multidetector CT scan (d) show the left phrenic nerve running outside the pericardium over the tip of the LAA (arrow in a, top arrow in d) and in proximity to the obtuse marginal artery and left LMV (arrow in b, middle arrow in d). Arrow in c and bottom arrow in d indicate the left phrenic neurovascular bundle.
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Conclusions
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Accurate anatomic description of the heart requires the use of a common language in describing cardiac anatomy as well as close interaction between radiologists, cardiologists, and surgeons. High-resolution reformatted images from 64-detector CT data provide accurate anatomic information for locating important landmarks relative to the cardiac conduction system or to current electrophysiologic interventions and CRT.
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Footnotes
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Abbreviations: AF = atrial fibrillation, AV = atrioventricular, CRT = cardiac resynchronization therapy, CTI = cavotricuspid isthmus, IVC = inferior vena cava, LAA = left atrial appendage, LCx = left circumflex, LMV = left marginal vein, RF = radiofrequency, SVC = superior vena cava
See the commentary by Rademaker, Saremi & Krishnan following this article.
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Abstract
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