In this prospective cohort study of unselected patients returning for a coronary angiogram at one-year following CABG, SPECT-MPI was shown to have a good sensitivity, specificity, and accuracy for detecting bypass graft disease with an adequate exercise or pharmacological stress test. However, in those individuals who failed to achieve >80% of their maximal predictive heart rate, the sensitivity, specificity and accuracy decreased significantly. We also found that the accuracy of SPECT-MPI did not vary between different myocardial territories. To our knowledge, these are the first prospective data on the accuracy of SPECT-MPI in an unselected cohort of patients after CABG.
There are limited data on the accuracy of SPECT imaging in asymptomatic patients with prior CABG. However, in previous retrospective studies of symptomatic patients who underwent coronary angiography, the sensitivity and specificity of detecting graft disease as judged by exercise thallium-201 SPECT was 80% and 87% respectively [5
]. Further, sensitivity of SPECT-MPI was significantly higher than that of the exercise electrocardiogram stress test in patients with typical recurrent angina (84% vs. 24%), as well as in those with atypical symptoms (70% vs. 50%). Moreover, among symptomatic patients who underwent coronary angiography, comparable results were shown with adenosine thallium-201 SPECT, with a sensitivity and specificity for detecting graft disease of 96% and 60%, respectively [14
]. A low specificity was attributed to several factors, including perfusion abnormalities in the distribution of non-bypassed native vessels, and partial volume effects as a result of regional wall motion and conduction abnormalities [14
]. Because the pretest probability of bypass graft disease in these symptomatic patients was elevated, the accuracy of the SPECT-MPI found in these studies may not necessarily be extrapolated to unselected patient cohorts. Thus the present investigation fills this gap in the literature by assessing the utility of SPECT-MPI in an unselected study cohort decreasing the chances of a selection bias.
We have found that with optimal stress conditions, SPECT-MPI has a sensitivity and a negative predictive value of >75% with either adequate exercise or pharmacological stress testing. The sensitivity is slightly lower than prior studies because of differences in symptoms at the time of presentation. At 5
years following bypass surgery, it is expected that 25% of vein grafts have significant disease and a substantial proportion of patients have symptoms, increasing the pretest probability. Further, exclusion of patients with a negative stress test in previous studies might have introduced a selection bias in the sensitivity and specificity calculations [15
]. Indeed, exclusion of selected patients may curtail the number of true-negative results and consequently raises the sensitivity of a test [16
In the present study, SPECT-MPI was more sensitive and specific for detecting bypass graft disease in patients with an adequate exercise heart rate response compared with those who did not achieve an adequate heart rate. The sensitivity of adequate exercise SPECT-MPI was 77%, in comparison to
50% in patients with an inadequate exercise heart rate response. These data underscore the importance of continuing exercise stress testing until MPHR is achieved or opting for adenosine protocol in patients who are unlikely to achieve adequate exercise.
Until now invasive bypass graft angiography remains the gold standard for detecting graft disease [17
]. As shown in this study SPECT-MPI is useful to detect graft disease accurately. Recently, it has been shown that combining perfusion CT imaging and cardiac CT angiography is feasible, and CT perfusion adds incremental value to cardiac CT angiography in the detection of significant coronary artery disease [19
], but these studies were not performed specifically in patients with bypass grafts.
In our study 38 of the 79 (48%) patients had significant graft disease (≥70% stenosis, including total occlusion) involving 56 of 251 (22%) potential grafts, which is slightly higher than previously reported. It is known that approximately 15% of saphenous venous grafts occlude in the first year after CABG surgery and that the majority of these occlusions are clinically silent [1
]. Potential explanations for these differences are considered. First, prior studies have reported more significant stenosis or complete occlusion to define graft disease, whereas we reported any stenosis
70%. Of note a complete graft occlusion was present in 33 (13%) of 251 grafts, which is similar to previous reports. Second in a study with a small sample size like ours, a few patients can alter the proportion with significant lesions. Third, the fact that 99% of patients were male and 28% of them were active smokers might have contributed to the slight increase in graft disease.
The most notable strength of this investigation is its prospective study design. Indeed, all of the previous information in this area has originated from retrospective analyses in which potential bias has been created in referral to either the nuclear stress lab and/or coronary angiography. In addition to the prospective nature and the all inclusiveness of the participants, an additional strength of the present study is that the cardiac catheterization and the nuclear imaging tests were performed within 24 hours of each other and both tests were interpreted without knowledge of the results of the other. In previous studies, these examinations were interpreted weeks and sometimes months apart, during which changes in coronary anatomy or perfusion might have occurred. This study also has some limitations. First, 42 out of 68 patients who performed an exercise stress SPECT did not achieve the MPHR. Chronic beta-blocker use and high prevalence of non-cardiac co-morbidities such as COPD are the most likely explanation for this finding. This observation raises the issue of whether a pharmacological stress should be considered the preferred modality in this patient population. Second, it might be difficult to assess the perfusion defects based on a cut of stenosis severity focally due to distal circulation status and collateral vessels. It is possible that some perfusion defects were due to preexisting distal vessel disease. Third, the sample size is small in statistical terms. Fourth, nearly all patients were male with high prevalence of COPD. Caution should be exercised in extrapolating these results to women and the study findings require confirmation in other populations with a lower prevalence of COPD.