Patients with pulmonary hypertension have impaired respiratory muscle function. We investigated whether functional changes of inspiratory and expiratory muscles (diaphragm, external and internal intercostal muscles) are similar.
Six male Wistar rats (body weight 180g) were injected subcutaneously with 40 mg monocrotaline (MCT) per kilogram body weight. Another six rats were age-matched controls. The animals were sacrificed after 2 or 4 weeks. During anesthesia the intercostal muscles and diaphragm were dissected. Calibrated histochemical methods were used to determine muscle fiber cross-sectional area (CSA), maximum rate of oxygen consumption (VO2max), myoglobine (Mb) concentration and the number of capillaries per fiber. Statistical analyses were performed with the use of 2 way Analysis of Variance.
The diaphragm shows a significant decrease in VO2max x CSA ((P<0.0001) and a decrease in Mb x CSA (P<0.01) after 2 weeks. The number of capillaries per fiber was constant. The external intercostal muscle shows a significant decrease of VO2max x CSA (P<0.0001) and a decrease in Mb x CSA (P<0.0001) after 4 weeks. The number of capillaries per fiber was constant. The internal intercostal muscle shows, however, a significant increase of VO2max x CSA (P<0.0001), a decrease of Mb x CSA (P<0.01) and a decrease in number of capillaries per fiber (P<0.05).
The internal intercostal muscles show a training effect after 4 weeks of MCT, whereas there is reduced maximum power output in the diaphragm and external intercostal muscles. The results suggest that a reduction of lung compliance causes shortening of the fibers in the diaphragm and the external intercostal muscle and stretch of the internal intercostal muscles, which are known regulators of muscle atrophy and hypertrophy, respectively.
Functional changes of respiratory muscles in pulmonary hypertensive patients may be muscle-dependent.
Frances De Man, None.