Sex differences in body fluid composition in humans with obstructive sleep apnea before and after CPAP therapy

Obstructive sleep apnea (OSA) is common in heart and kidney disease, both conditions prone to fluid retention. Nocturnal rostral fluid shift contributes to the pathogenesis of OSA in men more than women, suggesting a potential role for sex differences in body fluid composition in the pathogenesis of OSA, with men having a predisposition to more severe OSA due to an underlying volume expanded state. Continuous positive airway pressure (CPAP) increases intraluminal pressure in the upper airway and mitigates the rostral fluid shift; this, in turn, may prevent fluid redistribution from other parts of the body to the upper airway. We sought to determine the impact of CPAP on sex differences in body fluid composition. Twenty-nine (10 women, 19 men) incident, sodium replete, otherwise healthy participants who were referred with symptomatic OSA (oxygen desaturation index >15/h) were studied pre- and post-CPAP (>4 h/night × 4 weeks) using bioimpedance analysis. Bioimpedance parameters including fat-free mass (FFM, %body mass), total body water (TBW, %FFM), extracellular and intracellular water (ECW and ICW, %TBW), and phase angle (°) were measured and evaluated for sex differences before and after CPAP. Pre-CPAP, despite TBW being similar between sexes (74.6 ± 0.4 vs. 74.3 ± 0.2%FFM, p = 0.14; all values women vs. men), ECW (49.7 ± 0.7 vs. 44.0 ± 0.9%TBW, p < 0.001) was increased, while ICW (49.7 ± 0.5 vs. 55.8 ± 0.9%TBW, p < 0.001) and phase angle (6.7 ± 0.3 vs. 8.0 ± 0.3°, p = 0.005) were reduced in women compared to men. There were no sex differences in response to CPAP (∆TBW -1.0 ± 0.8 vs. 0.7 ± 0.7%FFM, p = 0.14; ∆ECW -0.1 ± 0.8 vs. -0.3 ± 1.0%TBW, p = 0.3; ∆ICW 0.7 ± 0.4 vs. 0.5 ± 1.0%TBW, p = 0.2; ∆Phase Angle 0.2 ± 0.3 vs. 0.0 ± 0.1°, p = 0.7). Women with OSA had baseline parameters favoring volume expansion (increased ECW, reduced phase angle) compared to men. Changes in body fluid composition parameters in response to CPAP did not differ by sex.