Tidal Volume Vital Capacity ratio VT/VC

To investigate the influence of resting pulmonary mechanics on the pattern of breathing during exercise in chronic obstructive pulmonary disease (COPD), we studied 29 patients with moderate to severe COPD (FEV1 50 +/- 20 percent predicted), and 10 normal subjects. Lung mechanics were studied using esophageal balloon technique and body-box. Incremental exercise testing was performed to exhaustion. As minute ventilation (VE) increases, COPD patients with the highest pulmonary resistance (RI) or lowest elastic recoil pressure (PL), used a greater tidal volume/vital capacity ratio (VT/VC) than the COPD patients with more normal RL or lowest PL. To describe the breathing pattern during exercise, an exponential constant (K) describes the rates of increase in VT/VC ratio with increasing VE, calculated according to the equation VT = VC(1-e-KVE). The K values achieved by COPD patients were higher than in normal subjects. In addition, K value correlated negatively with the resting FEV1 and FVC of COPD patients. When COPD patients were grouped according to their K values, it was revealed that patients with high K values generated greater VT/VC ratio and also have the most abnormal resting lung mechanics. These results suggest that the exercise breathing pattern in COPD patients is significantly influenced by the degree of impairment of resting lung mechanics.