Background: Obstructive sleep apnea (OSA) is a common disorder with multifactorial pathogenesis. It has been proposed that rostral fluid displacement contributes to the pathogenesis of OSA. We hypothesized that if fluid shift is an important factor in OSA, then it would be more severe during the second one-half of the night when there is maximally displaced fluid rostrally.
Methods: We prospectively enrolled 135 patients who underwent polysomnography. Neck and leg circumferences were measured to determine rostral fluid shift in recumbent position before and after sleep study. Breathing disturbance index (BDI) was derived for non-rapid eye movement sleep during the first and second one-half of the night in the same position.
Results: Sixty-five patients with demonstrable rostral fluid shift overnight were included in the analyses. Thirty-two (age, 52.5 ± 2.2 years) had OSA with a mean BDI of 27 ± 3/h (range, 5-80/h), whereas the other 33 with mainly upper airway resistance syndrome (age, 48.7 ± 2.4 years) had a BDI of 13 ± 2/h. Patients in both groups were obese. There was a statistically significant increase in neck circumference in patients with OSA (41.5 ± 0.8 to 42.6 ± 0.7 cm, P < .0001) and without OSA (38.6 ± 0.6 to 40.2 ± 0.6 cm, P < .0001) overnight, with concomitant decreases in leg circumferences. In spite of the fluid shift, BDI did not change (18 ± 3/h and 20 ± 3/h) between the first and second one-half of the night.
Conclusions: There is a significant fluid shift rostrally to the neck overnight in patients with and without OSA. However, this fluid shift is not associated with worsening of OSA, thus making it unlikely that fluid displacement is a contributing factor in the pathogenesis or severity of OSA.