Fluid shift by lower body positive pressure increases pharyngeal resistance in healthy subjects

Introduction: Fluid displacement into nuchal and peripharyngeal soft tissues while recumbent may contribute to narrowing and increased airflow resistance of the pharynx (Rph), and predispose to pharyngeal collapse in patients at risk for obstructive sleep apnea. Objectives: To determine whether displacement of fluid from the lower body to the neck will increase both neck circumference and Rph in healthy subjects. Methods: In 11 healthy, nonobese subjects, studied while awake and supine, leg fluid volume, neck circumference, and Rph were measured at baseline. Subjects were then randomized to a control period or to application of lower body positive pressure (LBPP) of 40 mm Hg via antishock trousers to displace fluid from the legs, after which they crossed over to the other arm. Baseline measurements were repeated at I and 5 min during the control and LBPP periods. Results: Compared with the control period, application of LBPP caused a significant reduction in leg fluid volume (p < 0.001) and a significant increase in neck circumference (p = 0.004). Rph remained stable during the control period, but increased significantly from baseline after 1 and 5 min of LBPP (from 0.43 +/- 0.10 to 0.60 +/- 0.11 cm H2O/L/s, p = 0.034, and to 0.87 +/- 0.19 cm H2O/L/s, p < 0.001, compared with baseline, respectively). Conclusions: Fluid displacement from the legs by LBPP increases neck circumference and Rph in healthy subjects. These findings suggest the hypothesis that fluid displacement to the upper body during recumbency may predispose to pharyngeal obstruction during sleep, especially in fluid overload states, such as heart and renal failure.