Respiratory Muscle Strength and Control of Ventilation in Patients with Neuromuscular Disease

To assess the relationship between respiratory mechanics and muscle strength and control of ventilation in patients with neuromuscular disease (NMD), we compared PImax and PEmax at RV, FRC and TLC, total respiratory elastance (Ers) with VT, TI, TT, VE, VT/TI, TI/TT, P.01, and P.01/(VT/TI) effective impedance in 21 patients with NMD and 21 healthy control (C) subjects, in seated position breathing room air. Ers in NMD patients was 79 percent higher than in the C subjects. While TI, TT, and VT in NMD were approximately half the corresponding C values, P.01 was 66 percent greater than in the C subjects (both p less than 0.001). NMD PImax and PEmax ranged from 37 to 52 percent of corresponding C values, respectively. Despite significant respiratory muscle weakness, only 7 of 16 patients demonstrated a PaCo2 greater than 45 mm Hg. Ventilatory output in NMD was modulated by respiratory mechanics as indicated by the increased P.01. In spite of muscle weakness, central drive in patients with NMD is not decreased, and in fact, is often increased. VE is not an accurate measure of central drive because of abnormal intrinsic respiratory mechanics and the effects of conscious responses or reflexes.