Prolonged Relaxation Rate of Inspiratory Muscles in Patients with Sleep Apnea

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Abstract
To evaluate whether inspiratory muscle function is impaired in patients with sleep apnea, we measured inspiratory muscle strength and relaxation rate before and after sleep in 13 patients. The sleep apnea group was composed of eight patients with severe obstructive sleep apnea, and the non-apnea group was composed of five patients without significant sleep apnea. We chose the time constant of relaxation (TauR) as an index of impaired inspiratory muscle contractility, and in subsets of each group, we measured the inspiratory pressure-time index as an indicator of a fatiguing breathing pattern. In patients with sleep apnea, presleep TauR was 79 .+-. 22 ms (SD), longer than that of normal subjects (normal, 59 .+-. 7 ms) (p < 0.05). TauR increased by 21 .+-. 16 ms during sleep (p < 0.01). In patients without apnea, presleep TauR was 67 .+-. 7 ms and it did not change after sleep. Maximal inspiratory and expiratory pressures were unchanged after sleep. We conclude that patients with sleep apnea do not develop overt inspiratory muscle failure but do have impaired contractility. We speculate that hypoxemia as well as increased work load was responsible.