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 (Tlim3'-8'). There is, however, no simple method to select a work rate which is consistently associated with Tlim3'-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 (CWR50%⇐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 Tlim3'-8' in 23/26 (88%) "high-endurance" patients; conversely, decreasing exercise intensity to 50% slowed those responses leading to Tlim3'-8' in 5/6 (83%) "high-endurance" patients. Repeating the tests at higher (60%) or lower (80%) intensities fail to consistently produce Tlim3'-8' in "low-" and "high-endurance", respectively (p > 0.05). Compared to a fixed work rate at 75%, CWR50%⇐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 (CWR50%⇐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.
Keywords: COPD; dyspnoea; endurance; exercise; lung function.