The median Duke Treadmill Score was significantly different across all three workload levels (<7 METS: 4.3 7-9 METS: 7.0 ≥10 METS: 9.0, p<0.001 between each of the 3 groups). No other physiologic parameters varied by exercise workload. The small change in the percentage of the MAPHR achieved was not clinically-significant and was only statistically significant between <7 and ≥10 METS (93% vs. *BP=blood pressure ECG=electrocardiogram HR=heart rate METS=metabolic equivalents min=minutes max=maximum. This score, although misnamed by tradition, does provide a logical semi-quantitative measure, which combines both extent and severity of left ventricular inducible ischemia ( 11, 12). Finally, the “percent myocardial ischemia” was obtained by dividing the difference between summed stress and summed rest scores by the maximum possible difference. The five apical segments were weighted at 40% the value of non-apical segments to correct the standard 17 segment model so that each unit myocardial volume was given equal weight. The semi-quantitative summed stress, rest, and difference values were calculated from these segmental scores. Segmental scores were categorized by each reader who chose a score based on both the quantitative perfusion data and a qualitative visual assessment. In order to compare the results more easily with other published studies, each segment was categorized into normal, mild, moderate, severe defects, and absent tracer uptake (scores 0-4). Systolic and diastolic volumes and body surface area normalized volumes were also calculated ( 14). Reversibility was flagged by computer-based analysis of variance derived from the normal databases. Segments were flagged as normal or abnormal, based on normal databases. The UVA quantification program used provides continuous measurement of relative percent tracer uptake in each of 17 standard segments. All borderline or abnormal studies were reclassified by the consensus of two additional readers blinded to additional patient information. Myocardial perfusion studies were initially read clinically by experienced nuclear cardiology specialists using visual and quantitative image analysis ( 14). The third hypothesis was that patients reaching <85% of their MAPHR but ≥10 METS would still have a low prevalence of significant ischemia but greater than that seen in those attaining their target heart rate. A second hypothesis tested was that individuals achieving ≥85% of their MAPHR with lower workloads have a greater prevalence of ischemia. The hypothesis tested was that individuals reaching diagnostic heart rates (≥85% of their MAPHR) and ≥10 METS have a low prevalence of significant ischemia (≥10% of the left ventricle). It’s association with the prevalence of significant ischemia by quantitative SPECT, as compared to the Duke Treadmill Score, would be of interest ( 11, 12).Īccordingly, the primary objective of this study was to determine prospectively the relationship of cardiac workload attained to the prevalence and extent of myocardial abnormalities by gated SPECT in patients with known or an intermediate to high probability of CAD who achieved ≥85% of their maximum age-predicted heart rate (MAPHR). A cutpoint of 10 METS achieved predicts low mortality, even in the setting of significant coronary artery disease ( 9, 10). Higher workloads achieved during exercise stress predict improved survival rates, irrespective of age and gender ( 6- 8). The incremental value of stress myocardial perfusion imaging (MPI) is small for patients with a low-risk stress test, a low-risk Duke Treadmill Score, or a high rate-pressure product without ST-depression ( 3- 5).Įxercise capacity measured in metabolic equivalents (METS) alone is a powerful predictor of cardiovascular events ( 6). Improved pretest risk stratification is essential in order to utilize this expensive imaging modality in a cost-effective manner. In 2005 alone, 9.3 million nuclear myocardial perfusion studies were performed at significant cost to the healthcare system ( 2). Noninvasive diagnostic imaging assists with this process and consequently has grown more than any other physician service under Medicare reimbursement ( 1). Identifying those at highest risk of major adverse cardiac events is imperative for guiding therapy and maximizing the benefits of revascularization. The sequelae of coronary artery disease continue to cause significant morbidity and impose high economic costs.
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