Are metabolic equivalents (METS) an accurate method for estimating change in peak oxygen consumption after cardiac rehabilitation?

Background: Maximal cardiopulmonary exercise testing (CPET) is the “gold standard” method of determining Vo2peak. When CPET is unavailable, VO2peak and metabolic equivalents (METs) are estimated from treadmill or cycle ergometer workloads. UK cardiac rehabilitation programmes (CR) use estimated METs to report changes in cardiorespiratory fitness (CRF). However, the accuracy of determining changes in VO2peak based on changes in estimated METs is not known. Methods: 27 patients with coronary heart disease (88.9% male; age 59.5 ± 10.0 years, body mass index 29.6 ± 3.8 kg.m-2) performed maximal CPET before and after an exercise based CR intervention. VO2peak was directly determined using ventilatory gas exchange data and was also estimated using the American College of Sports Medicine (ACSM) leg cycling equation for METs. Agreement between changes in directly determined VO2peak and VO2peak estimated from METs was tested using Bland-Altman limits of agreement (LoA), and intraclass correlation coefficients. Results: Directly determined VO2peak did not increase significantly following CR (0.5 ml.kg-1.min-1 (2.7%); p=0.332). In contrast, estimated VO2peak increased significantly (0.4 METs; 1.4 ml.kg-1.min-1; 6.7%; p=0.006). The mean bias for estimated VO2peak versus directly-determined VO2peak was 0.7 ml.kg-1.min-1 (LoA -4.7 to 5.9 ml.kg-1.min-1). Aerobic efficiency, (ΔVO2/ΔWR slope) was significantly associated with estimated VO2peak measurement error. Conclusion: Changes in estimated VO2peak determined using the ACSM equation for leg cycling are not accurate surrogates for directly determined changes in VO2peak. Reporting mean CRF changes using estimated METs may over-estimate the efficacy of CR and lead to a different interpretation of study findings compared to directly determined VO2peak.