Diurnal change of respiratory muscle strength in patients with sleep-disordered breathing

Background: Obstructive sleep apnea is characterized by repetitive pharyngeal collapse, which increases inspiratory resistive load, and causes intermittent oxygen desaturation and frequent arousals during sleep. This could be damaging to respiratory muscles and result in their weakness. Therefore, we investigated respiratory muscle strength before and after nocturnal sleep in patients with sleep-disordered breathing (SDB). Methods: Forty eight male patients with SDB undergoing overnight polysomnography were enrolled. Maximal expiratory pressure (MEP) and maximal inspiratory pressure (MIP) were measured before and after the sleep study. Correlation between polysomnographic data and diurnal changes in maximal respiratory pressures were also assessed. Results: After nocturnal sleep, MEP increased from 94.6 ± 20.8 cm H₂O to 105.9 ± 24.1 cm H₂O (p < 0.001) and MIP increased from 72.9 ± 20.2 cm H₂O to 78.3 ± 21.7 cm H₂O (p = 0.004). There was no significant correlation between changes in MEP/MIP and apnea-hypopnea index, arousal index, and mean and minimal oxygen saturation. Conclusion: The respiratory muscle strength of patients with SDB was greater in the morning than at night, which may be contributed to by the restorative effect of nocturnal sleep. This diurnal difference was not correlated to the severity of SDB, which may suggest that respiratory muscles are less impaired by SDB.