Respiratory muscle strength and gas exchange in neuromuscular diseases: comparison with chronic pulmonary emphysema and idiopathic pulmonary fibrosis.

To examine whether or not the respiratory muscle weakness is correlated with decrease in arterial oxygen tension (PaO2), respiratory muscle and pulmonary functions in 14 patients with neuromuscular diseases (NMD) were studied and compared with those of 12 patients with chronic pulmonary emphysema (CPE) and 15 patients with idiopathic pulmonary fibrosis (IPF). Respiratory muscle strength was assessed by maximal static inspiratory and expiratory mouth pressure at three lung volumes (RV, FRC and TLC). Although mean pulmonary functions in NMD showed virtually normal function, respiratory muscle strength was significantly less than the corresponding values in CPE and IPF. In NMD, maximal inspiratory mouth pressure at RV level (PImax) correlated positively with %TLC and %VC (r = 0.652 and r = 0.536, respectively). Moreover, PImax was significantly correlated with PaO2 (r = 0.561), but not with PaCO2. Maximal expiratory mouth pressure at TLC (PEmax) correlated positively with %TLC and %VC. In CPE and IPF, respiratory muscle strength had no correlation with PaO2 and PaCO2. These findings suggest that inspiratory muscle dysfunction in NMD may be one of the factors responsible for determination of the level of hypoxemia and lung volume.