[Factors influencing severity variability in obstructive sleep apnea and the role of fluid shift]

Objective:The variability of the apnea-hypopnea index（AHI） measured in the first and second halves of the night is significant in patients with obstructive sleep apnea hypopnea syndrome（OSAHS）. This variation may be related to fluid redistribution caused by the supine position during sleep. Methods:Eighty-nine adult subjects were enrolled. Circumferences（neck, chest, waist, and calf） were measured before sleep onset and upon awakening. Polysomnography（PSG） was performed, and the night was divided into two halves based on the midpoint of total sleep time to calculate AHI for each half. The correlation between changes in AHI and changes in circumferences was analyzed. Results:Twenty simple snorers and sixty-nine OSAHS patients were included, with a median AHI of 22.6（11.8, 47.3） events/hour. Compared to pre-sleep measurements, there was no significant change in neck circumference upon awakening in the control group（P=0.073）, while reductions were observed in the other three measurements（P=0.006, P=0.038, P<0.001）. In the OSAHS group, neck circumference increased（P<0.001）, and reductions were noted in the other three measurements（P<0.001 for all）, with the most significant change observed in calf circumference 40.0（37.1, 42.0） cm to 38.0（35.8, 40.5） cm. Compared to the first half of the night, total AHI, supine AHI, and NREM AHI significantly decreased in the second half（P=0.010, P=0.031, P=0.001）, while no significant changes were observed in lateral AHI and REM AHI（P=0.988, P=0.530）. Further analysis revealed a significant relationship between increased chest circumference and decreases in NREM AHI, supine AHI, and supine NREM AHI（P=0.036, P=0.072, P=0.034）, as well as between decreased lateral position AHI and increased waist circumference（P=0.048）. Additionally, this study found a negative correlation between changes in calf circumference and changes in AHI（R=-0.24, P=0.048）, while neck circumference changes positively correlated with changes in AHI（R=0.26, P=0.03）. Conclusion:In OSAHS patients during the second half of sleep compared to before sleeping, chest circumference, waist circumference, and calf circumference decrease while neck circumference increases; total AHI, supine position AHI, and NREM period AHI decrease; increases in chest circumference are associated with decreases in NREM period AHI, supine position AHI, supine position NREM period AHI. There is nocturnal variability in AHI among OSAHS patients that may be associated with fluid shifts during sleep.