Introduction: Fluid accumulation in the neck during recumbency might narrow the upper airway (UA) and thereby contribute to its collapse in patients with obstructive sleep apnea (OSA). We hypothesized that acute fluid shifts from the legs to the upper body in healthy subjects would increase neck circumference and reduce UA cross-sectional area (UA-XSA).
Methods: In 27 healthy, non-obese subjects (mean± SEM, 39±3 years, body mass index 23.2±0.6kg/m2), studied while supine, we measured leg fluid volume using bio-electrical impedance, neck circumference using a mercury strain gauge and mean UA-XSA between the velum and the glottis using acoustic pharyngometry at end-expiration. Measurements were made at baseline after which subjects were randomly assigned to a 5-minute time-control period or to a 5-minute application of LBPP at 40 mmHg by anti-shock trousers, separated by a 15-minute washout period. Subjects then crossed-over to the opposite arm of the study.
Results: Compared to control, application of LBPP significantly reduced leg fluid volume (p<0.001), increased neck circumference (p<0.001), both at 1-min and 5-min, and reduced UA-XSA after both 1-min (-0.15 cm2; 95% confidence interval [CI], -0.23 to -0.09 cm2, p<0.001) and 5 min (-0.20 cm2; 95% CI, -0.33 to -0.09 cm2, p<0.001).
Conclusion: In healthy subjects, displacement of fluid from the legs by LBPP causes distension of the neck and narrowing of the UA lumen. Therefore, fluid displacement from the lower to the upper body while recumbent may contribute to pharyngeal narrowing and obstruction to airflow in patients with OSA. This may have particular pathological significance in edematous states such as heart and renal failure.
- acoustic pharyngometry
- lower body positive pressure
- obstructive sleep apnea
- upper airway