Oxygen Consumption of Respiratory Muscles in Patients With COPD

We measured the oxygen consumption ( V˙o2) of respiratory muscles in 8 COPD patients and 12 age-matched healthy subjects using a closed circuit device which allows a continuous increase in external dead space and is equipped with a 9-L Collins spirometer. Furthermore, we measured simultaneously mouth occlusion pressure at 0.1 s of inspiration (P0.1), minute ventilation (VE), and other ventilatory parameters during the measurement of total V˙o2 ( V˙o2tot). We found that the logarithm of V˙o2tot (log V˙o2tot) had a good correlation with V˙E in both groups. The mean slope of the regression line of log V˙o2tot and V˙E (Δlog V˙o2tot/Δ V˙E) of COPD patients was significantly higher than that of normal subjects (p < 0.001). However, the mean Y-intercept (metabolic V˙o2[ V˙o2met]) of the regression lines did not differ between the two groups. The P0.1 in COPD patients was higher than that in normal subjects at the corresponding dead space loading. However, the VE did not differ between the two groups except for at rest and the first 1 min after dead space loading. These results suggest that the V˙o2 of respiratory muscles in patients with COPD is higher at given ventilation compared with that in age-matched normal subjects and that this increased V˙o2 partly may be due to an augmented ventilatory drive.