Respiratory Mechanics in Untreated Myasthenia Gravis With Special Reference to the Respiratory Forces

The respiratory mechanics have been studied in nine patients with myasthenia gravis in the untreated state. Comparisons have been made with the findings in healthy controls. Although the patients mainly had moderate myasthenia gravis the maximum respiratory pressures as expressions of the respiratory muscle strength were significantly decreased. Vital capacity, functional residual capacity, total lung capacity and the dynamic lung volumes were also diminished. No mean changes in lung resistance or lung compliance were shown to exist in the group. The relative mean value of maximum expiratory pressure (P E max) (the observed value in per cent of the predicted normal mean value) was lower than any other relative lung function variable. Thus the maximum inspiratory force (P 1 max), of paramount importance for the ventilatory capacity, was systematically less decreased than P E max. Even after consideration of the effect of decreased respiratory muscle strength vital capacity, total lung capacity and functional residual capacity were still diminished. It is suggested that decreased physical activity might diminish these static lung volumes via a low thoracic compliance. The results also suggested another volume‐pressure relationship between vital capacity and P 1 max, as is found in control cases with normal muscular strength. Otherwise similar functional relationships were at hand. The decreased values of dynamic lung volumes in comparison with those predicted only from anthropometric variables could only be explained to a minor extent by the direct influence of decreased respiratory muscle strength. The differences were mainly mediated via changes in the vital capacity, which is dependent on the respiratory force available.