DOI: 10.1148/rg.254045151
RadioGraphics 2005;25:881-896
© RSNA, 2005
Coronary Artery Bypass Grafts: Assessment with Multidetector CT in the Early and Late Postoperative Settings1
Aletta Ann Frazier, MD,
Fauzia Qureshi, MD,
Katrina M. Read, DDR,
Robert C. Gilkeson, MD,
Robert S. Poston, MD and
Charles S. White, MD
1 From the Departments of Diagnostic Imaging (A.A.F., F.Q., K.M.R., C.S.W.) and Cardiac Surgery (R.S.P.), University of Maryland School of Medicine, 22 S Greene St, Baltimore, MD 21201; the Department of Radiologic Pathology, Armed Forces Institute of Pathology, Washington, DC (A.A.F.); Philips Medical Systems, Cleveland, Ohio (K.M.R.); and the Department of Radiology, University Hospitals of Cleveland, Case Western Reserve University School of Medicine, Cleveland, Ohio (R.C.G.). Recipient of a Certificate of Merit award for an education exhibit at the 2003 RSNA Annual Meeting. Received July 26, 2004; revision requested October 13 and received March 16, 2005; accepted March 17. K.M.R. is an employee of Philips Medical Systems; R.S.P. has received a grant for research on the aortic connector from St Jude Medical; C.S.W. receives grant funding from Philips Medical Systems; all other authors have no financial relationships to disclose.
Address correspondence to A.A.F. (e-mail: anniefrazier{at}mac.com).
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Abstract
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Coronary artery bypass graft (CABG) surgery is the standard of care in the treatment of advanced coronary artery disease. It is well known that the long-term clinical outcome after myocardial revascularization depends on the patency of the bypass grafts. In the past, invasive coronary angiography was used to assess the status of the grafts and check for graft occlusion. Recently, computed tomography (CT), particularly multidetector CT with electrocardiographic gating, has emerged as an important diagnostic tool for evaluation of CABGs in both the early (
1 month) and late (>1 month) postoperative settings. A variety of postoperative complications may manifest as dyspnea and chest pain, thereby mimicking recurrent angina secondary to graft occlusion. Owing to its improved spatial resolution compared with that of earlier-generation CT scanners and its ability to produce three-dimensional and multiplanar images, multidetector CT has assumed an integral role in characterization of graft patency while allowing investigation of alternative postoperative complications. In addition, the expanded capabilities of volumetric imaging may provide valuable information in preoperative planning for repeat CABG surgery.
© RSNA, 2005
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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:
- Discuss the basic principles and techniques of imaging CABGs with multidetector CT.
- Describe the anatomy of the native coronary arteries and the typical locations and CT appearances of venous and arterial CABGs.
- Identify the early and late complications of CABG surgery on multidetector CT images.
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Introduction
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Coronary artery bypass graft (CABG) surgery remains the standard of care in the treatment of advanced coronary artery disease. It is well recognized that the long-term clinical outcome after myocardial revascularization is dependent on the patency of the bypass grafts. Conventionally, invasive coronary angiography has been used to assess graft status and evaluate for graft occlusion.
The body of literature illustrating the value of computed tomography (CT) in the assessment of bypass grafts continues to grow with advances in CT technology. Multidetector CT scanners combine high spatial resolution with the ability to demonstrate anatomy through volume-rendered images, thus producing a more sensitive evaluation over conventional or spiral CT. The addition of electrocardiographic gating minimizes cardiac and coronary graft motion, further improving the sensitivity and specificity of multidetector CT evaluation for graft patency (1–6). These advances have also increased the ability to estimate the extent of intraluminal graft occlusion with noninvasive imaging techniques (Table).
With increased success in imaging grafts for patency, multidetector CT is being used more widely in the postoperative setting. Chest pain is common after CABG surgery and can have a variety of etiologies, including recurrent angina secondary to graft occlusion, sternal infection, pleural or pericardial effusion, and less common but potentially lethal complications such as pulmonary embolism or pseudoaneurysm formation. In this setting, multidetector CT can offer a rapid, convenient, and noninvasive means of discerning the correct underlying diagnosis. In addition, there are several recent reports on the merits of volume-rendered multidetector CT images in preoperative planning for repeat CABG surgery.
After detailing our current multidetector CT protocol, we review commonly used vascular bypass conduits, their typical CT appearances, and complications that may be evident at imaging after surgery. We also discuss briefly how multidetector CT may be useful in preoperative planning.
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Multidetector CT Protocol
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Patients were scanned by using a 16-section multidetector CT scanner (Philips Medical Systems, Best, the Netherlands). Patients were positioned in the gantry supine and feet first with electrocardiographic leads placed on the anterior thorax to enable a retrospectively gated scan. Scan parameters were 140 kV, 0.4-second rotation speed, 400 mAs, and 16 x 0.75 detectors. Pitch, which was dependent on the heart rate, averaged 0.3. The CT system automatically recommended a pitch value to optimize the temporal resolution by the number of sectors reconstructed from each scan. The scanner used a "beat-2-beat" algorithm as part of the retrospectively gated reconstruction, which encompassed variations in heart rate throughout the scan. Scans were performed in the caudal to cephalic direction, with a scan range from the thoracic inlet through the lung bases. The proximal subclavian arteries were also included.
The beta-adrenergic blocking agent metoprolol tartrate injection (Lopressor IV 5 mL/5 mg; Novartis Pharmaceuticals, East Hanover, NJ) was administered intravenously (5–15 mL) to patients with a heart rate exceeding 70 beats per minute, unless underlying contraindications such as asthma were present. Iohexol 75.5% (Omnipaque [iodine, 350 mg/mL]; Nycomed, Princeton, NJ), a nonionic, iodinated, low-osmolar contrast medium, was injected intravenously in doses ranging from 120 to 150 mL, without direct variation with respect to patient weight, and no saline flush was used. The rate of bolus contrast material administration was 4 mL/sec. A single-head injector (Envision CT; Medrad, Indianola, Pa) was used. An automatic bolus tracking method (Bolus Pro; Philips Medical Systems) was used to optimize graft visualization. A region of interest was placed in the descending aorta by using a preset threshold of 150 HU; a 10-second delay followed before scanning was begun to ensure filling of the distal vessels with contrast material.
All scans were reconstructed by using retrospective gating (75% of the R-R interval), with 1-mm-thick images reconstructed every 0.4 mm. A multisector approach was used, which was automatically determined by the scanner software and chiefly determined by the patient’s heart rate. Axial images were automatically transferred to a freestanding workstation (MX View; Philips Medical Systems). In conjunction with axial source images, three-dimensional volume-rendered images, multiplanar reformation images, and less frequently maximum intensity projection images were generated. Volume rendering parameters were selected from preset protocols; within each protocol, there were specific Hounsfield unit ranges depicted by set colors. Although axial images remain an important part of the baseline evaluation, we found that multiplanar reformation and three-dimensional volumetric images often optimally demonstrated the relationships between graft anastomoses and individual grafts.
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Types of Grafts and Their Normal Radiologic Appearances
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The various conduits used for CABG surgery may be divided into arterial and venous grafts. Venous grafts have demonstrated a tendency to develop partial or complete occlusions with time, whereas arterial grafts have shown relative resistance to plaque formation and obstruction. However, arterial conduits are more limited in their availability and ease of procurement compared with venous grafts, specifically the saphenous vein. Therefore, saphenous vein grafts (SVGs) remain the most commonly used conduits.
Saphenous Vein Graft
A segment of the saphenous vein was used to perform the first CABG operation in 1962. Since then, the susceptibility of SVGs to occlusive failure in both early and late postoperative settings has been well documented and extensively investigated. Early graft occlusion is primarily due to vascular damage that can occur at surgery, whereas vessel wall changes resulting from exposure to systemic blood pressure may predispose to occlusion in later stages. A large angiographic study of CABGs (n = 5,065; 91% venous, 9% arterial) performed to assess graft disease found that 88% of grafts were patent perioperatively, 81% were patent at 1 year, and 75% were patent at 5 years, with a further decline to 50% at 15 years or later (7). Nevertheless, continued improvements in surgical techniques, combined with use of antiplatelet or anticoagulant agents and lipid-lowering drug therapy, have allowed SVGs to remain an important, convenient, and readily available choice for bypass grafting (8).
Saphenous vein conduits are harvested from the legs and grafted from the ascending aorta to the distal coronary artery beyond the obstructive coronary lesion (Fig 1). The vein is usually attached to the anterior aspect of the aorta. Left-sided grafts are typically anastomosed distally to the LAD artery, diagonal artery, circumflex artery, or the obtuse marginal branches of the circumflex artery (Figs 2–4). Right-sided grafts are usually connected to the distal right coronary artery or posterior descending artery. At our institution, intraoperative conduit blood flow is assessed with a transit-time flowmeter (Transonic Systems, Ithaca, NY), which quantifies blood flow and waveform within the graft once the proximal aortosaphenous anastomosis is complete (9). At postoperative multidetector CT, the proximal anastomosis of a graft is typically better visualized than its distal counterpart. Even if the distal anastomosis is not well visualized, if contrast material is demonstrated within the graft column at multidetector CT, we consider this evidence of graft patency.
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Figure 1. Three-dimensional volume-rendered image shows the typical appearances of right (arrow) and left (solid arrowhead) SVGs sutured to the anterior aorta. The left SVG is attached to the diagonal artery distally; the distal anastomosis of the right SVG to the posterior descending artery is not seen. There is also a left internal mammary artery (IMA) graft (open arrowhead), which is connected to the left anterior descending (LAD) artery.
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Figure 2. Drawing shows the normal anatomy of the coronary arteries. a = left coronary artery, b = LAD artery, c = circumflex artery, d = diagonal artery, e = obtuse marginal branch of the circumflex artery, f = right coronary artery, g = posterior descending artery, and h = acute marginal branch of the right coronary artery. The intra-cardiac locations of the circumflex artery and its obtuse marginal branch are indicated by broken lines.
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Figure 3. Drawing shows examples of CABGs. A right SVG (a) is attached to the anterior aorta proximally and to the posterior descending artery distally. A left SVG (b) has an altered appearance because it is attached to the aorta with an aortic connector device (arrow); the origin of this SVG is moved laterally to prevent kinking. A typical left IMA graft (c) is left intact at its origin and grafted to the LAD artery distally.
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Figure 4. Left lateral slab maximum intensity projection image shows the entire length of an SVG from its proximal aortic origin (white arrow) to its distal anastomosis with the LAD artery (arrowhead). A clip artifact (black arrow) overlies part of the distal graft.
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Suturing the graft to the aorta and the coronary artery is a critical part of the CABG procedure and requires application of a clamp on the aorta, thereby increasing the risk of stroke and other embolic complications. The recent development of a mechanical aortovenous connector allows quicker attachment of venous grafts to the aorta without use of aortic clamps or sutures (Fig 5). This device, called the Symmetry Bypass System aortic connector (St Jude Medical, St Paul, Minn), alters the normal appearance of the bypass graft (Figs 3, 6). The aortic connector requires a 90° angle between the graft vessel and the aorta; therefore, the usual site of the proximal graft anastomosis at the anterior aspect of the aorta is altered to support the course of the emerging graft (Figs 7, 8). Right coronary artery grafts are instead attached to the right side of the aorta, or lower on the anterior aorta, so that the graft lies in the right atrioventricular groove. Grafts for the left coronary artery are brought from the left side of the aorta so as to be supported by their course over the pulmonary artery (10). Use of the aortic connector is currently controversial due to anecdotal reports of early restenosis and aortic dissection (9,11–13).
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Figure 6. Three-dimensional volume-rendered image shows the typical appearances of right (long arrow) and left (short arrow) SVGs attached to the ascending aorta with aortic connectors. Note the raised-star footprint at the aortic attachment site of each venous graft (open arrowhead). A left IMA graft is also present (solid arrowhead).
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Figure 7. Three-dimensional volume-rendered image shows a right SVG positioned at a 90° angle relative to the aorta (long arrow) and coursing along the right atrioventricular groove. A left SVG (short arrow) has a course over the pulmonary artery (*) to protect the graft from kinking. A left IMA graft is also present (arrowhead).
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Figure 8. Three-dimensional volume-rendered image shows the typical appearance of multiple CABGs. The patient received three left SVGs (long arrow), a right SVG (short arrow), and a left IMA graft (arrowhead). Note the raised-star footprints and lateral placement of the SVGs, an appearance characteristic of the use of aortic connectors.
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IMA Graft
Although the IMA was also used for bypass grafting in the early years of CABG surgery, its resistance to thrombosis became recognized only decades later. The advantages of IMA conduits over SVGs are now well documented and include decreased postoperative mortality, improved cardiac event–free survival rates, and long-term patency rates well above 90% at 10 years (8,14). Thus, the IMA has become the preferred bypass graft. Typically, the left IMA is used as the graft and the right IMA is left in place.
Because of its location near the LAD artery, the left IMA is most often used to revascularize the LAD artery in order to supply the greatest territory of the heart (Fig 3). The left IMA is typically separated from the chest wall. Its origin at the subclavian artery remains intact and the distal end is connected to the target vessel, distal to the site of occlusion (Fig 9).
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Figure 9a. Left IMA grafts. (a) Three-dimensional volume-rendered image shows a left IMA graft (arrow) from its origin at the left subclavian artery to its anastomosis with the LAD artery. There is also a right SVG (arrowhead), which is attached to the posterior descending artery. Note the smaller diameter of the arterial graft compared with that of the venous graft. (b) Volume-rendered image (lateral cut plane) shows a left IMA graft (arrow), which is anastomosed to an obtuse marginal branch of the circumflex artery. There is also a right IMA graft (arrowhead), the distal aspect of which is attached to the LAD artery.
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Figure 9b. Left IMA grafts. (a) Three-dimensional volume-rendered image shows a left IMA graft (arrow) from its origin at the left subclavian artery to its anastomosis with the LAD artery. There is also a right SVG (arrowhead), which is attached to the posterior descending artery. Note the smaller diameter of the arterial graft compared with that of the venous graft. (b) Volume-rendered image (lateral cut plane) shows a left IMA graft (arrow), which is anastomosed to an obtuse marginal branch of the circumflex artery. There is also a right IMA graft (arrowhead), the distal aspect of which is attached to the LAD artery.
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The right IMA may also be used in a variety of ways. As an in situ graft, the right IMA remains attached to the right subclavian artery proximally and anastomoses with the target coronary artery distally (Fig 10). When grafted to the left-sided coronary system, the right IMA will often pass through the transverse sinus, since this has been shown to result in higher patency rates (15). Alternatively, the right IMA may be removed from the subclavian artery and used as a free or composite graft. The proximal end of the free graft is typically attached to the ascending aorta. When used as a composite arterial graft, the right IMA is attached proximally to a left IMA graft so that the left IMA inflow supplies both grafted vessels. This configuration can be used to accomplish total arterial myocardial revascularization instead of using a left IMA and an additional SVG when two-vessel bypass is necessary. Studies have shown that total arterial myocardial revascularization has the advantages of decreased recurrent angina and superior patency rates at 1 year when compared with those of conventional coronary artery bypass surgery in which a left IMA graft is coupled with an SVG (16).
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Figure 10a. Right IMA graft. (a) Curved multiplanar reformation image shows a patent right IMA graft (arrows) within the anterior mediastinum. The full extent of the graft is seen from its origin to its distal anastomosis with the LAD artery. (b) Three-dimensional volume-rendered image shows the right IMA graft from its origin (arrow) to its distal anastomosis with the LAD artery (arrowhead).
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Figure 10b. Right IMA graft. (a) Curved multiplanar reformation image shows a patent right IMA graft (arrows) within the anterior mediastinum. The full extent of the graft is seen from its origin to its distal anastomosis with the LAD artery. (b) Three-dimensional volume-rendered image shows the right IMA graft from its origin (arrow) to its distal anastomosis with the LAD artery (arrowhead).
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Other Arterial Grafts
Owing to the success of IMA grafts, there has been an initiative to develop other arterial bypass grafts. The radial artery was selected because of its ease of procurement from the forearm. Initial attempts at using radial artery grafts were complicated by frequent graft closure. However, with recent improvements in harvesting, surgical techniques that avoid endothelial disruption, and use of calcium channel blockers postoperatively to overcome graft spasm, the radial artery is currently regarded as a valid arterial graft option. Patency rates are now similar to those seen with IMA grafts, ranging from 95.7% at 12 weeks to 91.6% at 10 years (8,17). As with a right IMA graft, the radial artery may be used as a free or composite graft, with the goal of providing greater arterial revascularization (Fig 11).
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Figure 11a. Radial artery grafts. (a) Axial multidetector CT image shows a patent radial artery graft (arrow). Its patency is confirmed by the presence of intraluminal contrast material. An SVG (arrowhead) is seen anterior to the main pulmonary artery. (b) Axial multidetector CT image shows the origin of the SVG (arrow). Note the difference in caliber between the arterial graft and the SVG. (c) Three-dimensional volume-rendered image shows multiple CABGs in another patient. The radial artery (long arrow) is smaller in caliber than the SVG (short arrow) and similar in size to the IMA graft (arrowhead).
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Figure 11b. Radial artery grafts. (a) Axial multidetector CT image shows a patent radial artery graft (arrow). Its patency is confirmed by the presence of intraluminal contrast material. An SVG (arrowhead) is seen anterior to the main pulmonary artery. (b) Axial multidetector CT image shows the origin of the SVG (arrow). Note the difference in caliber between the arterial graft and the SVG. (c) Three-dimensional volume-rendered image shows multiple CABGs in another patient. The radial artery (long arrow) is smaller in caliber than the SVG (short arrow) and similar in size to the IMA graft (arrowhead).
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Figure 11c. Radial artery grafts. (a) Axial multidetector CT image shows a patent radial artery graft (arrow). Its patency is confirmed by the presence of intraluminal contrast material. An SVG (arrowhead) is seen anterior to the main pulmonary artery. (b) Axial multidetector CT image shows the origin of the SVG (arrow). Note the difference in caliber between the arterial graft and the SVG. (c) Three-dimensional volume-rendered image shows multiple CABGs in another patient. The radial artery (long arrow) is smaller in caliber than the SVG (short arrow) and similar in size to the IMA graft (arrowhead).
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The gastroepiploic artery has also been used as a bypass graft. After the median sternotomy is extended toward the umbilicus, the gastroepiploic artery is dissected from the greater curvature of the stomach. It can be grafted to the right or circumflex coronary artery by directing it in a retrograde fashion, or grafted to the LAD artery in an antegrade fashion. This is a technically difficult operation to perform, and the gastroepiploic artery has not become a popular bypass graft.
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Early Complications
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Thrombosis
Within the first postoperative month, the primary mechanism for graft failure is thrombosis (Fig 12) resulting from a combination of endothelial and medial damage during surgical retrieval and attachment. Graft closure from thrombosis at 1 month is a recognized complication in 10%–15% of cases (7). Perioperative venous graft failure after off-pump CABG procedures is chiefly determined by the two factors of graft endothelial damage and patient hypercoagulability (including resistance to antiplatelet therapies) (9) (Figs 13, 14). High-pressure distention of venous grafts and their inherently weaker antithrombotic properties contribute to increased rates of early venous graft attrition. Specifically, too short of a graft may result in stretching of the vessel and damage to the endothelium, thereby initiating the cascade of thrombus formation.
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Figure 12. Volume-rendered image obtained 5 days after CABG surgery shows aortic connectors (arrows), which mark the proximal attachments of two SVGs to the ascending aorta. The grafts are not seen due to acute thrombosis. An intact left IMA graft is seen (arrowhead).
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Figure 13a. Thrombosis of an SVG. (a) Axial multidetector CT image shows an SVG with an intraluminal thrombus (arrow) near its proximal anastomosis. (b) Axial multidetector CT image obtained slightly caudad shows that the graft is patent (arrow) with no evidence of thrombosis. (c) Curved axial multiplanar reformation image of the SVG shows a stent (arrowhead) and a thrombosed segment partly occluding the lumen (arrow).
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Figure 13b. Thrombosis of an SVG. (a) Axial multidetector CT image shows an SVG with an intraluminal thrombus (arrow) near its proximal anastomosis. (b) Axial multidetector CT image obtained slightly caudad shows that the graft is patent (arrow) with no evidence of thrombosis. (c) Curved axial multiplanar reformation image of the SVG shows a stent (arrowhead) and a thrombosed segment partly occluding the lumen (arrow).
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Figure 13c. Thrombosis of an SVG. (a) Axial multidetector CT image shows an SVG with an intraluminal thrombus (arrow) near its proximal anastomosis. (b) Axial multidetector CT image obtained slightly caudad shows that the graft is patent (arrow) with no evidence of thrombosis. (c) Curved axial multiplanar reformation image of the SVG shows a stent (arrowhead) and a thrombosed segment partly occluding the lumen (arrow).
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Figure 14. Left lateral oblique slab maximum intensity projection image shows contrast material within only a short proximal segment of an SVG (arrow). This appearance represents complete occlusion of the SVG.
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Graft Malposition or Kinking
Malposition or kinking of the graft can also result in early graft occlusion (18), particularly in longer grafts (Fig 15). Technical factors associated with use of an aortic connector may predispose venous grafts to kinking (19). As mentioned earlier, the angle of the attachment is critical; if the aortic connector is malpositioned without adequate support for the graft, the vessel may kink as it emerges from the proximal anastomosis (Fig 16). In addition, unlike conventional techniques, use of the connector requires that the proximal anastomosis be created first. This may make selecting the proper length of the vein more challenging, potentially resulting in either a stretched short segment of vein or kinking of an overly long segment of the graft.
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Figure 16. Three-dimensional volume-rendered image shows right and left SVGs attached to the aorta with aortic connectors. The right graft (long arrow) does not appear to be positioned at the recommended 90° angle relative to the aorta. Ongoing studies are investigating whether such malpositioning may predispose to postoperative thrombosis and graft failure. The left graft (short arrow) demonstrates the required angulation and is better supported by the pulmonary artery. A left IMA graft is also present (arrowhead).
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Graft Spasm
Radial artery grafts are notably prone to postoperative vasospasm, a complication that may mimic fixed graft stenosis. Cardiothoracic surgeons employ various techniques to minimize this tendency, including intraoperative pretreatment of the graft with topical 
-adrenergic antagonist solutions (such as phenoxybenzamine) or pharmacologic prophylaxis with calcium channel blockers (20–22). The latest generation of multidetector CT scanners shows promise in the demonstration of luminal narrowing in radial artery spasm (Fig 17) (23).
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Figure 17. Curved multiplanar reformation image shows a radial artery graft with postoperative vasospasm. Note that the more proximal aspect of the graft (black arrow) is narrower than the distal aspect (white arrow).
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Iatrogenic Complications
Iatrogenic causes may directly or indirectly lead to graft occlusion. Damage to the vascular endothelium from any source can result in clot formation. There are reports in the literature of iatrogenic vessel dissection leading to early acute occlusion (24). We recently encountered a less common iatrogenic cause of graft occlusion: a retained clip associated with a new graft in a patient who had just undergone aortocoronary venous grafting (Fig 18).
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Figure 18a. Iatrogenic early graft occlusion due to a retained surgical clip. (a) Three-dimensional volume-rendered image (coronal cut plane) shows a faint shadow along the course of a right SVG (long arrow). The shadow, which corresponds to a thrombus, leads to an atraumatic spring clip (Novare, Cupertino, Calif) distally (short arrow). A normal-appearing left SVG is evident (arrowhead). (b) Axial multidetector CT image shows the retained clip (arrow) lateral to the right atrium.
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Figure 18b. Iatrogenic early graft occlusion due to a retained surgical clip. (a) Three-dimensional volume-rendered image (coronal cut plane) shows a faint shadow along the course of a right SVG (long arrow). The shadow, which corresponds to a thrombus, leads to an atraumatic spring clip (Novare, Cupertino, Calif) distally (short arrow). A normal-appearing left SVG is evident (arrowhead). (b) Axial multidetector CT image shows the retained clip (arrow) lateral to the right atrium.
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Pericardial Effusion
Pericardial effusions are common after coronary artery bypass, occurring with a reported prevalence of 22%–85% (25,26). Important risk factors include postoperative anticoagulant therapy or coagulation abnormalities that are often related to the use of cardiopulmonary bypass. Despite their frequency, postoperative pericardial effusions rarely progress to become hemodynamically significant. Resultant cardiac tamponade has been reported in 0.8%–6% of patients (27). If this develops within the first 24 hours, it is termed early cardiac tamponade and is treated surgically to identify the source of bleeding. Late cardiac tamponade occurs at least 5–7 days postoperatively and may be related to excessive mediastinal drainage (28).
Pleural Effusion
Most patients who undergo coronary artery bypass grafting develop pleural effusions; the prevalence is approximately 90% within the first week after surgery (29,30). These tend to be small, unilateral, and left sided with no relationship to an enlarged cardiac silhouette, atelectasis, or placement of a chest tube (31). Patients are generally asymptomatic, and the effusion usually resolves spontaneously over several weeks (31). Only 1%–4% of CABG surgery patients proceed to develop clinically significant effusions that manifest with chest pain and dyspnea and require thoracentesis. The pathophysiology of pleural effusion after CABG is unknown, but several etiologies have been postulated such as pericardial inflammation or intraoperative pleural injury, which may lead to lymphatic drainage or increased fluid production (29,31–35).
Sternal Infection
Approximately 2%–20% of CABGs are complicated by a surgical site infection (36). Infections can be categorized as involving the presternal, sternal, or retrosternal compartments (37). Much of the literature on surgical site infections after cardiothoracic surgical procedures has focused on retrosternal, deep chest infections, particularly mediastinitis (Fig 19). Although deep sternal infection occurs infrequently after CABG surgery (reported in 1%–4% of patients), it carries a significant mortality rate of nearly 25% (38,39). Risk factors that have been specifically linked to the development of sternal wound infections include obesity, diabetes mellitus, current cigarette smoking, and steroid therapy. Surgical risk factors linked to sternal site infections are numerous and include the following: previous sternotomy, complexity of the surgery, type of bone saw used, type of sternal closure, blood transfusions, and early reexploration to control hemorrhage. The potential for increased infection risk after bilateral IMA grafting remains controversial.
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Figure 19a. Deep sternal infection after CABG surgery. (a) Axial multidetector CT image shows sternal dehiscence (short arrow) with a large fluid collection (long arrow) in the anterior mediastinum. The graft vessel is also identified (arrowhead). (b) Axial multidetector CT image shows the patent bypass graft (arrowhead) coursing through the posterior aspect of the fluid collection. The infection was confirmed by means of positive bacterial cultures.
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Figure 19b. Deep sternal infection after CABG surgery. (a) Axial multidetector CT image shows sternal dehiscence (short arrow) with a large fluid collection (long arrow) in the anterior mediastinum. The graft vessel is also identified (arrowhead). (b) Axial multidetector CT image shows the patent bypass graft (arrowhead) coursing through the posterior aspect of the fluid collection. The infection was confirmed by means of positive bacterial cultures.
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Pulmonary Embolism
Clinical diagnosis of deep vein thrombosis and pulmonary embolism may be especially challenging after CABG surgery. Complaints of chest pain and dyspnea are common after cardiac bypass. Postoperative atelectasis, pleural effusion, or fluid overload may all contribute to the development of hypoxemia. Similarly, lower extremity edema with pain and swelling at the site of the saphenous vein harvesting are typical findings early after CABG surgery. For all these reasons, it is not uncommon for deep vein thrombosis and pulmonary embolism to remain unrecognized in the early postoperative setting (40).
A recent review of the literature regarding pulmonary embolism in the post–CABG surgery population showed an overall prevalence of 23% for deep vein thrombosis by 1 week after surgery, with less than 2% of these cases identified clinically (40). Although it is uncommon for pulmonary embolism to occur in this population (reported in 0.4%–9.5% of cases), because it is unsuspected, it may manifest as an unexpected and devastating clinical event (40). This diagnostic dilemma underscores the value of multidetector CT in this setting. We have encountered one patient in whom unsuspected pulmonary embolism was identified at postoperative multidetector CT performed to assess bypass graft patency (Fig 20).
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Figure 20. Three-dimensional volume-rendered image (axial cut plane) shows an unsuspected pulmonary embolism after CABG surgery. A ribbonlike thrombus (arrow) is present within the central pulmonary arteries; the patent graft (arrowhead) is seen anterior to the pulmonary trunk.
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Late Stenosis and Occlusion
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Occlusion after the first month following CABG surgery is primarily due to thrombosis resulting from progressive pathologic changes related to exposure of the SVG to systemic blood pressure (Fig 21). After surgery, the vein graft undergoes a process of arterialization. This results from progressive thickening of the media and neointimal formation, which begins within days of implantation and continues over months to years. These changes form a foundation for eventual atherosclerotic narrowing, which may ultimately lead to late graft occlusion.
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Figure 21. Coronal multidetector CT images show an SVG with a long segment of aneurysmal dilatation and secondary thrombosis (open arrowhead), which extend to its insertion at the LAD artery (solid arrowhead). A patent second SVG is identified adjacent to the abnormal segment (arrow).
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In contradistinction, IMA grafts are strikingly resistant to atheroma formation, resulting in higher long-term patency rates than for SVGs. Late IMA graft failure more commonly occurs from progressive atherosclerotic disease of the grafted native vessel distal to the anastomosis. Infrequently, stenosis can occur in the IMA graft from intimal hyperplasia, technical errors at the anastomotic site, or rarely atheroma formation (8).
The use of the aortic connector may prove to be an important factor in inducing thrombosis at the aortovenous anastomosis. Investigations of graft patency after use of the aortic connector have shown mixed results. Although initial limited studies demonstrated satisfactory graft patency (41), recent reports have documented the development of significant stenoses (of 95% or more) and occlusion in 13.7%–15.5% of vein grafts attached with the aortic connector (19,42). It is hypothesized that the connector elicits an intimal hyperplastic reaction similar to that seen with coronary stents, leading to severe ostial stenosis and symptoms of unstable angina, typically within 6 months of the CABG procedure.
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Graft Aneurysms
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Aneurysmal dilatation of a bypass graft requiring surgical correction is generally regarded as exceeding 2 cm (43). True aneurysms typically arise more than 5 years after bypass and occur in the body of the graft. The dominant mechanism is related to accelerated atherosclerosis (44,45). Pseudoaneurysms more commonly occur within 6 months after surgery, although they may also arise several years later. Pseudoaneurysms arise at either proximal or distal anastomotic sites (Fig 22). Earlier-onset cases may be related to wound infection or to tension at the anastomosis that leads to suture rupture; the pathogenesis of later pseudoaneurysms most likely involves progressive atherosclerosis (43,44,46). Less common graft body pseudoaneurysms have been reported secondary to host vessel degeneration and technical factors involved in harvesting the SVG (44).
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Figure 22a. Pseudoaneurysm of an SVG. (a) Axial multidetector CT image shows a thrombosed pseudoaneurysm of a left SVG (arrowhead). (b) Left lateral volume-rendered image shows the relationship between the proximal SVG (arrowhead) and the pseudoaneurysm (arrow), which arises from the distal anastomosis.
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Figure 22b. Pseudoaneurysm of an SVG. (a) Axial multidetector CT image shows a thrombosed pseudoaneurysm of a left SVG (arrowhead). (b) Left lateral volume-rendered image shows the relationship between the proximal SVG (arrowhead) and the pseudoaneurysm (arrow), which arises from the distal anastomosis.
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Complications of graft aneurysmal disease include thrombosis or thromboembolism, fistula formation to the right atrium or ventricle, aneurysmal rupture with secondary hemorrhage, and myocardial infarction (43,47). Graft aneurysms are often first identified as a mediastinal or hilar mass at chest radiography. Contrast-enhanced CT and magnetic resonance imaging usually demonstrate the vascular nature of the structure, as well as its extent and local mass effect (43,46).
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Planning Repeat CABG Surgery
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As medical and surgical treatments for coronary artery disease have improved, patients are living longer. Consequently, second CABG operations have become more common. Injury to a preexisting left IMA graft at sternal reentry is a well-recognized risk in this setting, and there has been extensive investigation into ways to prevent this potentially devastating complication (48,49). Multidetector CT is emerging as a useful means of mapping the course of a left IMA graft before repeat surgery (50–52). Three-dimensional volume-rendered images are the result of actively rotating anatomic structures with computer software in order to better delineate relationships between the sternum, ribs, and bypass grafts, thereby minimizing the risk of injury to the graft vessel during surgical reentry (Fig 23). Assessing the remainder of the heart and great vessels can also have important surgical implications. For example, the presence of an aortic aneurysm may displace an attached SVG toward the sternum. Understanding the sternal proximity of preexisting bypass grafts, as well as normal structures including the aorta, pulmonary artery, and native coronary arteries, allows the surgeon to plan an appropriate surgical approach.
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Figure 23a. Use of volume-rendered images in preoperative assessment of an existing left IMA graft. (a) Left lateral image shows the course of a left IMA graft (arrow) within the anterior mediastinum. (b) Left lateral image of another patient shows a left IMA graft tethered to the sternum (arrowhead).
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Figure 23b. Use of volume-rendered images in preoperative assessment of an existing left IMA graft. (a) Left lateral image shows the course of a left IMA graft (arrow) within the anterior mediastinum. (b) Left lateral image of another patient shows a left IMA graft tethered to the sternum (arrowhead).
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Conclusions
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Patency of the CABG is often the most pressing clinical question in evaluation of the CABG patient after surgery. The advancing technology of electrocardiographically gated multidetector CT now allows the radiologist to address this clinical concern in a rapid, convenient, and noninvasive manner. In addition, multidetector CT has the added advantage over traditional angiographic evaluation of simultaneously allowing evaluation for alternate postoperative complications that may also manifest with chest pain and dyspnea, thereby mimicking recurrent angina. Improvements in spatial resolution and the ability to generate three-dimensional and multiplanar images also permit greater application of multidetector CT in preoperative planning before repeat CABG surgery in order to minimize the risk of injury to a graft vessel during reentry. The increasing capability of multidetector CT evaluation of the CABG patient after surgery suggests that its role is likely to increase in the near future.
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Footnotes
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Abbreviations: CABG = coronary artery bypass graft, IMA = internal mammary artery, LAD = left anterior descending, SVG = saphenous vein graft
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References
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- Poston RS, Prastein D, Gu J, Lee A, Pierson R, Griffith B. Virchow’s triad, but not use of an aortic connector device, predicts vein graft thrombosis after off-pump bypass. Heart Surg Forum
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- Mack MJ, Emery RW, Ley LR, et al. Initial experience with proximal anastomoses performed with a mechanical connector. Ann Thorac Surg
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