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CLINICAL RESEARCH: CARDIAC IMAGING

Achieving an Exercise Workload of 10 Metabolic Equivalents Predicts a Very Low Risk of Inducible Ischemia

Does Myocardial Perfusion Imaging Have a Role?

Cardiovascular Division and the Cardiovascular Imaging Center, Department of Internal Medicine, University of Virginia Health System, Charlottesville, Virginia

Manuscript received November 11, 2008; revised manuscript received April 8, 2009, accepted April 14, 2009.

^_* Reprint requests and correspondence: Dr. Jamieson M. Bourque, Cardiovascular Imaging Center, Cardiovascular Division, Department of Medicine, University of Virginia Health System, Box 800662, 1215 Lee Street, Charlottesville, Virginia 22908 (Email: jamieson2{at}gmail.com).

Objectives: We sought to identify prospectively the prevalence of significant ischemia (10% of the left ventricle [LV]) on exercise single-photon emission computed tomography (SPECT) imaging relative to workload achieved in consecutive patients referred for myocardial perfusion imaging (MPI).

Background: High exercise capacity is a strong predictor of a good prognosis, and the role of MPI in patients achieving high workloads is questionable.

Methods: Prospective analysis was performed on 1,056 consecutive patients who underwent quantitative exercise gated ^99mTc-SPECT MPI, of whom 974 attained 85% of their maximum age-predicted heart rate. These patients were further divided on the basis of attained exercise workload (<7, 7 to 9, or 10 metabolic equivalents [METs]) and were compared for exercise test and imaging outcomes, particularly the prevalence of 10% LV ischemia. Individuals reaching 10 METs but <85% maximum age-predicted heart rate were also assessed.

Results: Of these 974 subjects, 473 (48.6%) achieved 10 METs. This subgroup had a very low prevalence of significant ischemia (2 of 473, 0.4%). Those attaining <7 METs had an 18-fold higher prevalence (7.1%, p < 0.001). Of the 430 patients reaching 10 METs without exercise ST-segment depression, none had 10% LV ischemia. In contrast, the prevalence of 10% LV ischemia was highest in the patients achieving <10 METs with ST-segment depression (14 of 70, 19.4%).

Conclusions: In this referral cohort of patients with an intermediate-to-high clinical risk of coronary artery disease, achieving 10 METs with no ischemic ST-segment depression was associated with a 0% prevalence of significant ischemia. Elimination of MPI in such patients, who represented 31% (430 of 1,396) of all patients undergoing exercise SPECT in this laboratory, could provide substantial cost-savings.

Key Words: coronary artery disease • exercise capacity • radionuclide imaging • risk prediction

Abbreviations and Acronyms   CAD = coronary artery disease   ECG = electrocardiogram   LV = left ventricle/ventricular   LVEF = left ventricular ejection fraction   MAPHR = maximum age-predicted heart rate   MET = metabolic equivalent   MI = myocardial infarction   MPI = myocardial perfusion imaging   SPECT = single-photon emission computed tomography

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