Effect of obesity on respiratory mechanics during rest and exercise in COPD

The presence of obesity in COPD appears not to be a disadvantage with respect to dyspnea and weight-supported cycle exercise performance. We hypothesized that one explanation for this might be that the volume-reducing effects of obesity convey mechanical and respiratory muscle function advantages. Twelve obese chronic obstructive pulmonary disease (COPD) (OB) [forced expiratory volume in 1 s (FEV(1)) = 60%predicted; body mass index (BMI) = 32 ± 1 kg/m(2); mean ± SD] and 12 age-matched, normal-weight COPD (NW) (FEV(1) = 59%predicted; BMI = 23 ± 2 kg/m(2)) subjects were compared at rest and during symptom-limited constant-work-rate exercise at 75% of their maximum. Measurements included pulmonary function tests, operating lung volumes, esophageal pressure, and gastric pressure. OB vs. NW had a reduced total lung capacity (109 vs. 124%predicted; P < 0.05) and resting end-expiratory lung volume (130 vs. 158%predicted; P < 0.05). At rest, there was no difference in respiratory muscle strength but OB had greater (P < 0.05) static recoil and intra-abdominal pressures than NW. Peak ventilation, oxygen consumption, and exercise endurance times were similar in OB and NW. Pulmonary resistance fell (P < 0.05) at the onset of exercise in OB but not in NW. Resting inspiratory capacity, dyspnea/ventilation plots, and the ratio of respiratory muscle effort to tidal volume displacement were similar, as was the dynamic performance of the respiratory muscles including the diaphragm. In conclusion, the lack of increase in dyspnea and exercise intolerance in OB vs. NW could not be attributed to improvement in respiratory muscle function. Potential contributory factors included alterations in the elastic properties of the lungs, raised intra-abdominal pressures, reduced lung hyperinflation, and preserved inspiratory capacity.