Six-minute walk test reveals delayed oxygen uptake kinetics in ischemic cardiomyopathy

Although correlated to peak oxygen uptake (VO2), the six-minute walk test (6MWT) alone cannot provide precise physiological insight regarding the specific cause(s) of exercise limitations. We aimed to analyze whether 6MWT is able to properly detect differences in the cardiorespiratory responses between patients with stable coronary artery disease (SCAD) and those with ischemic cardiomyopathy (IC) and determine whether the degree of abnormality in ejection fraction is related with impaired submaximal exercise capacity. Twenty-two subjects with SCAD and 19 subjects with IC underwent a 6MWT while simultaneously using a mobile telemetric cardiopulmonary monitor to assess cardiorespiratory responses. VO2 response at exercise onset was used to obtain VO2 on-kinetics, and the slope of ventilation vs carbon dioxide output (VE/VCO2) was calculated. IC subjects exhibited significantly delayed VO2 on-kinetics compared with the SCAD group (P<0.01) and higher VE/VCO2 slope (IC=40.45 [95%CI: 39.76 to 41.1] vs SCAD=34.36 [95%CI: 34.03 to 34.69], P=0.001). The left ventricular ejection fraction (LVEF) was moderately correlated with VO2 on-kinetics in the SCAD group, but no relationship was found in the IC group. Pulmonary function was correlated with the VE/VCO2 slope only in the IC group. Subjects with IC presented slower VO2 on-kinetics during the 6MWT than those with SCAD. Once reduction in left ventricular function is achieved, LVEF had no association with exercise capacity. Pulmonary function could help identify IC patients at risk of ventilatory inefficiency and may add diagnostic power to the 6MWT.