Effects of fluid shift on upper airway patency and neck circumference in normal-weight subjects

Fluid shift from the lower body into the neck could narrow the upper airway (UA) and increase the propensity of its obstruction in people with obstructive sleep apnea. Although studies have demonstrated an increase in the neck circumference (NC) due to fluid shift, it remains unclear as to whether a large increase in NC means a large reduction in the cross-sectional area of the UA (CSA-UA). This study tested a hypothesis that a significant UA narrowing due to fluid redistribution is not necessarily linked to an apparent increase in NC, and vice versa.

Magnetic resonance imaging (MRI) studies were performed on 30 male and 20 female normal-weight subjects. Fluid shift was achieved by raising their legs by >50°. The coordinates of the neck and UA boundaries were extracted from the MRI to calculate the NC and CSA-UA.

After elevating the legs for 8 min, the CSA-UA was reduced by 27.6% while the NC increased by 1.5% in 50 subjects (p < 0.001). In 10% of the males and 10% of the females, NC was almost unchanged, while the reduction in CSA-UA was large. In 7% of the males and 20% of the females, the change in NC was large, while that in CSA-UA was small. The correlation coefficient between the changes in NC and CSA-UA was -0.211, which was consistent with the hypothesis.

The amount of NC increase should not be used to judge the degree of reduction in CSA-UA. Clinically, those people who have little NC increase after body position changes have more risk of CSA-UA reduction at bedtime.