A Simplified Approach to Select Exercise Endurance Intensity for Interventional Studies in COPD

Time to exercise limitation (Tlim) in response to constant work rate (CWR) is sensitive to interventions in chronic obstructive pulmonary disease (COPD). This is particularly true when the pre-intervention test lasts between 3 and 8 min (Tlim_3'-8'). There is, however, no simple method to select a work rate which is consistently associated with Tlim_3'-8' across the spectrum of COPD severity. We assessed 59 GOLD stages II-IV patients who initially cycled to Tlim at 75% peak. In case of short (<3 min, low-endurance) or long (>8 min, high-endurance) tests, patients exercised after 60 min at 50% or 90%, respectively (CWR_{50%⇐75%⇒90%}). Critical mechanical constraints and limiting dyspnea at 75% were reached within the desired timeframe in 27 "mid-endurance" patients (46%). Increasing work rate intensity to 90% hastened the mechanical-ventilatory responses leading to Tlim_3'-8' in 23/26 (88%) "high-endurance" patients; conversely, decreasing exercise intensity to 50% slowed those responses leading to Tlim_3'-8' in 5/6 (83%) "high-endurance" patients. Repeating the tests at higher (60%) or lower (80%) intensities fail to consistently produce Tlim_3'-8' in "low-" and "high-endurance", respectively (p > 0.05). Compared to a fixed work rate at 75%, CWR_{50%⇐75%⇒90%} significantly decreased Tlim's coefficient of variation; consequently, the required N to detect 100 s or 33% improvement in Tlim decreased from 82 to 26 and 41 to 14, respectively. This simplified approach to individualized work rate adjustment (CWR_{50%⇐75%⇒90%}) might allow greater sensitivity in evaluating interventional efficacy in improving respiratory mechanics and exercise tolerance while simultaneously reducing sample size requirements in patients with COPD.