NON-INVASIVE CARDIOVASCULAR IMAGING FOR EVALUATION OF CORONARY ARTERY BYPASS GRAFT DISEASE


Abdou Elhendy, M.D., Ph.D, Department of Cardiology, Marshfield Clinic, Marshfield, WI, USA

Coronary artery bypass graft (CABG) surgery is a widely used method of revascularization in patients with severe coronary artery disease (CAD). Various studies have shown improvement of survival and cardiac symtoms after CABG, particularly in patients with low ejection fraction and multivessel CAD. Myocardial perfusion can be compromised after CABG due to graft disease, progression of CAD distal to the graft or in remote CAD distribution. Although abnormalities can be silent, these can lead to adverse outcome with subsequent left ventricular dysfunction, myocardial infarction and death. Various non-invasive methods are available for evaluation of patients after CABG. Stress echocardiography is an effective method for evaluation of CAD after CABG. Advantages include wide availability, lack of irradiation and incremental prognostic information. Sensitivity is modest, particularly with submaximal heart rate. The addition of myocardial contrast allows evaluation of perfusion and enhances sensitivity. SPECT imaging is widely used and provides excellent tomographic evaluation of ischemia. Both stress echocardiography and SPECT predict prognosis. Patients with normal studies have excellent outcome, whereas the extent of stress abnormalities is associated with higher incidence of cardiac events. Stress imaging techniques cannot differentiate graft disease from stenosis distal to graft. In general, stress testing is not routinely indicated in asymptomatic patients within 5 years after CABG. Functional imaging with CT angiography and MRI allows visualization of bypass grafts with excellent accuracy to detect graft occlusion. High density artifacts caused by metal clips during CABG may locally disrupt graft assessment. Average sensitivity and specificity of cardiac CT are 96% and 97% respectively. MR angiography has a high sensitivity and specificity for diagnosis of CABG disease. MRI enables anatomic and functional analysis in a single acquisition without ionizing irradiation. In addition, both CT and MRI can evaluate perfusion. Disadvantages of MRI are exclusion of patients with defibrillators and pacemakers. Metallic clips may result in image degradation due to radiofrequency shielding. It is concluded that several non-invasive modalities provide good accuracy for evaluation of bypass grafts and native CAD after CABG and can predict prognosis. Understating the merits and limitations of each technique is a key for proper selection of the appropriate imaging modality in a given patient.