Overestimation of aerobic capacity with the bruce treadmill protocol in patients being assessed for suspected myocardial ischemia.

Introduction: Peak oxygen uptake (VO₂) is prognostic for morbidity and mortality. Estimating aerobic capacity during traditional exercise stress testing is common as it has been shown that total treadmill time on the Bruce protocol predicts peak VO₂. However, the potential to overestimate peak VO2 exists and may have clinical implications regarding the interpretation of exercise test data.

Methods: Subjects (N = 303) with symptoms suggestive of myocardial ischemia underwent a myocardial perfusion study and an exercise test with simultaneous ventilatory expired gas analysis. Estimated peak VO₂ from the Bruce treadmill protocol was compared with measured peak VO₂. The Duke Treadmill Score (DTS) was calculated with treadmill time (DTS(time)) and also with measured VO₂ (DTS(measured)),expressed as metabolic equivalents (METs), and converted to time.

Results: Peak measured METs was significantly lower than peak estimated METs in the entire cohort (6.5 ± 1.9 vs 8.8 ± 2.8, P < .001) as well as in female (5.7 ± 1.4 and 7.8 ± 2.1, P < .001) and male (7.3 ± 2.0 and 9.7 ± 3.1, P < .001) subgroups. Calculation of the DTS with measured METs resulted in a significantly lower score compared with its calculation with treadmill time (2.7 ± 3.5 vs 5.8 ± 4.6, P < .001). Receiver operating characteristic curve analysis revealed that DTS(measured) produce a statistically significant model for diagnosing a perfusion defect in both men and women (P < .05), whereas DTS(time) was diagnostic only in men (P < .05).

Discussion: This study demonstrates that estimates of aerobic capacity are significantly higher than measured values and this difference may result in a significant underestimation of morbidity/mortality risk.