2001 - Mechanisms and Assessment of Exercise Limitation in Patients with Chronic Obstructive Pulmonary Disease
Dr. WK Lam, North District Hospital
Abstract
Breathing reserve (BR) and breathing reserve index (BRI) are commonly measured in cardiopulmonary exercise test (CPET) to identify ventilatory limitation at maximal exercise. A new index of ventilatory limitation- the breathing reserve index at lactate threshold (BRILT)- was evaluate during anaerobic threshold(AT)as a surrogate for lactate threshold in 30 COPD patients undergoing our pulmonary rehabilitation programme (PRP). Patients' mean FEV1 was 0.80L. Mean BR was low at 9L/min. Mean BRI was high at 0.82+/-0.04. BRIAT was calculated for 18 patients who attained AT during exercise test. Mean BRIAT for these patients was 0.54 (95%CI 0.47-0.60). Previous literature suggested a BRILT>=0.42 predicted ventilatory limitation with a sensitivity of 97% and specificity of 95%. Among BR, BRI and BRIAT, BRIAT appeared to have the strongest correlation (r=-0.60, p=0.014) with FEV1. The three indices consistently suggested that our patients were ventilatorily limited at maximal exercise. Mechanisms of exercise limitation in COPD patients, potential uses and limitations of various methods in identifying ventilatory limitation at maximal exercise are discussed.
Aerobic capacity of COPD patients has been shown to improve after exercise training. Mechanisms include increase in endurance of ventilatory muscles and oxidative capacity of skeletal muscles. Our patients showed a small overall increase in VO2max after PRP (+0.08L/min, p=0.016). No differences in patient characteristics were found in patients with improved Vo2max and patients without improvement. CPET remains necessary to objectively assess whether an individual has shown physiological improvement after PRP.
