Decrease of respiratory muscle strength and static lung volumes in obese asthmatics.

Ten obese patients with steroid-dependent bronchial asthma were studied. All were female nonsmokers and all had normal chest roentgenograms. The patients were divided into 2 groups according to their total lung capacity (TLC), i.e., those with decreased TLC (N = 4) and those with normal TLC (n = 6). It was found that patients with low TLC had decreased respiratory muscle strength, as measured by maximal inspiratory esophageal pressure, and that the pressure-volume (P-V) curve of the lung was shifted downward and to the right. There were no differences between the 2 groups in degree of airflow limitation, in per cent increase in body weight, and in duration and dose of steroid therapy. All had normal values of diffusion, corrected for alveolar volume. It is concluded that respiratory muscle strength is an important factor in maintaining normal static lung volumes. The data suggest that the combination of obesity and steroid therapy can cause a decrease in respiratory muscle strength in certain obese patients with bronchial asthma. The change in P-V curve seen in the patients with decreased TLC may be secondary to the decrease in static lung volumes.