Abstract 5959: High Cardiac Workload Identifies a Cohort at Low Risk of Myocardial Ischemia: Is SPECT Imaging Necessary?
Introduction: Improved risk stratification of patients with suspected myocardial ischemia is essential to reduce the morbidity and cost of unnecessary testing. Exercise capacity during stress testing is a powerful prognostic indicator and may assist in risk assessment. We sought to identify the prevalence of ischemia stratified by exercise capacity and the role of exercise ST-depression to identify a cohort with low likelihood of significant ischemia on myocardial perfusion imaging (MPI).
Methods: We analyzed 972 consecutive patients over one year who achieved ≥85% of their MAPHR and underwent 99mTc SPECT MPI. Images were quantitatively and visually analyzed by 3 readers using a 17-segment model to assess the extent and severity of LV ischemia. We divided the cohort into 3 groups based on cardiac workload, which we compared for clinical characteristics, exercise ST-depression, cardiac volumes, LVEF, and %LV ischemia through T-tests and chi-square analysis.
Results: The 972 subjects (median age 57, 56.9% male) showed increased hypertension, diabetes, and obesity. The %LV ischemia decreased with higher attained workloads. The 473 patients (48.7%) achieving ≥10 Mets had a very low prevalence of ischemia, while those with <7 Mets had an 18-fold higher prevalence of ≥10% LV ischemia, considered by many the threshold for revascularization consideration. The sensitivity and positive predictive value of ≥1mm ST-depression for ≥5% ischemia were 50% (3/6) and 7% (3/43) respectively in the ≥10 Mets subgroup.
Conclusions: Exercise ST-depression is a poor predictor of significant ischemia. Moreover, high cardiac workload is a better predictor of a low-risk stress myocardial perfusion study than achieving target heart rate. Patients reaching 85% of their MAPHR and ≥10 Mets have a markedly low prevalence of ≥10% LV ischemia. Thus, in this cohort, an imaging procedure may not be cost-effective. The number of these patients referred to an imaging laboratory is substantial.