Perceived inspiratory difficulty in neuromuscular patients with primary muscle disorders.

Intensity of perceived inspiratory difficulty was investigated in 17 patients with severe respiratory insufficiency due to muscle disease, compared with healthy matched controls. Subjects breathed through a threshold valve generating a constant inspiratory negative pressure proportional to their maximal inspiratory pressure. Four load levels ranging from 10 to 40% of the maximal inspiratory pressure were applied in random order. Patients had significantly less perceived inspiratory difficulty than controls at each load level expressed as a percentage of maximal inspiratory pressure P < 0.001. However, when the load was expressed as the absolute value, the slope of the Borg scale score versus mouth pressure was similar in the two groups (P = 0.11). The ventilatory pattern remained unchanged in each group as the load increased. We conclude that in patients with myopathy, loads leading to respiratory muscle fatigue (40% of maximal inspiratory pressure) may fail to produce perceived inspiratory difficulty.