Critical Power and Respiratory Compensation Point Are Not Equivalent in Patients with COPD

Abstract

Introduction Several studies report that pulmonary oxygen uptake (VO2) at the respiratory compensation point (RCP) is equivalent to the VO2 at critical power (CP), suggesting that the variables can be used interchangeably to demarcate the threshold between heavy and severe intensity domains. However, if RCP is a valid surrogate for CP, their values should correspond even when assessed in patients with chronic obstructive pulmonary disease (COPD) in whom the "normal"mechanisms linking CP and RCP are impeded. The aim of this study was to compare VO2 at CP with VO2 at RCP in patients with COPD. Methods Twenty-two COPD patients (14 male/8 female; forced expiratory volume in 1 s, 46% ± 17% pred) performed ramp-incremental cycle ergometry to intolerance (5-10 W·min-1) for the determination of gas exchange threshold (GET) and RCP. CP was calculated from the asymptote of the hyperbolic power-duration relationship from 3-5 constant-power exercise tests to intolerance. CP was validated with a 20-min constant-power ride. Results GET was identified in 20 of 22 patients at a VO2 of 0.93 ± 0.18 L·min-1 (75% ± 13% VO2peak), whereas RCP was identified in just 3 of 22 patients at a VO2 of 1.40 ± 0.39 L·min-1 (85% ± 2% VO2peak). All patients completed constant-power trials with no difference in peak physiological responses relative to ramp-incremental exercise (P > 0.05). CP was 46 ± 22 W, which elicited a VO2 of 1.04 ± 0.29 L·min-1 (90% ± 9% VO2peak) during the validation ride. The difference in VO2 at 15 and 20 min of the validation ride was 0.00 ± 0.04 L, which was not different from a hypothesized mean of 0 (P = 0.856), thereby indicating a VO2 steady state. Conclusions In COPD patients, who present with cardiopulmonary and/or respiratory-mechanical dysfunction, CP can be determined in the absence of RCP. Accordingly, CP and RCP are not equivalent in this group.