Diagnostic accuracy of coronary computed tomography angiography in patients post-coronary artery bypass grafting

Coronary artery disease (CAD) is the leading cause of mortality in this world with a rate expected to double by the year 2020.1 Of those diagnosed with CAD, approximately 800,000 patients worldwide undergo bypass surgery every year due to advanced coronary disease and the presence of medical co-morbidities.2 At 10-years after surgery, 50% of vein grafts and 10% of internal mammary grafts are occluded.3 As the prevalence of CAD is increasing worldwide, the number of patients who will be receiving bypass surgery will likely increase as well. Thus, the need for the accurate diagnosis and quantification of disease in bypass grafts as well as native vessels in coronary artery bypass graft (CABG) patients will continue to grow. Intracoronary angiography (ICA) remains the definitive diagnostic test for the diagnosis and quantification of bypass graft and native vessel stenoses, with non-invasive stress testing serving the role of localising perfusion defects.4 A newer, non-invasive modality being used to assess patency of vessels in CABG patients is multidetector-row computed tomography (MDCT).5 In most studies, the accuracy with which graft occlusion can be diagnosed by this method approaches 100%.6–21 The utility of MDCT in diagnosing disease in bypass grafts lies in the fact that venous grafts are typically larger than native coronary arteries (4–10 mm vs. 2–5 mm) and also subject to less cardiac motion, making them easier to visualise by computed tomography than native coronary vessels. With the newest generation of scanners, the negative predictive value (NPV) of ruling out occlusion has reached close to 100% with a sensitivity and specificity > 95%. A study of 52 patients using 16-detector MDCT demonstrated 99.4% accessibility of grafts, with a sensitivity and specificity of 100% for occlusion and 96% sensitivity and 100% specificity for detecting highgrade stenoses in patent grafts.6–21 A limitation of MDCT as the sole diagnostic test for the diagnosis of stenoses in CABG patients is that, often times, the native coronary vessels are heavily calcified, limiting visualisation.17,22 De novo coronary disease in these native vessels may be the cause of symptoms and therefore may be under-diagnosed. In addition, the post surgical patient may present with metallic clips and surgical wires which may render portions of the vessels unevaluable. The limitations of MDCT in all patients, irrespective of CABG status also remain and include difficulty in imaging patients with arrhythmias and the risks of contrast nephropathy and radiation exposure. Intracoronary angiography as the sole diagnostic tool, however, is not a viable option. This procedure also carries the risks of contrast nephropathy and radiation exposure. In addition, the risks of arrhythmia, stroke, native vessel or graft dissection, myocardial infarction, and embolic stroke in these patients portend a morbidity and mortality within the ranges of 0–2% and 0.14–0.28%, respectively.22 In spite of the few limitations of MDCT, due to its high NPV and paucity of complications from the imaging, this modality serves as a very useful role in the evaluation of the patency of bypass grafts in CABG patients with low pre-test probability of disease and those with little calcification of the native coronary vessels.