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Fluid Shift by Lower Body Positive Pressure Increases Pharyngeal Resistance in Healthy Subjects

Sleep Research Laboratory, Toronto Rehabilitation Institute; Department of Medicine, Mount Sinai Hospital; Toronto General Hospital, University Health Network; Division of Otolaryngology, Department of Medicine, University of Toronto, Toronto, Ontario, Canada

Correspondence and requests for reprints should be addressed to T. Douglas Bradley, M.D., Toronto General Hospital/University Health Network, 9N-943, Toronto, ON, M5G 2C4 Canada. E-mail: douglas.bradley{at}utoronto.ca

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 1 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.

Key Words: fluid displacement • lower body positive pressure • obstructive sleep apnea • pharyngeal resistance

AT A GLANCE COMMENTARY Scientific Knowledge on the Subject Edematous patients have a high prevalence of obstructive sleep apnea, but the reason for this is unknown. One possibility is that fluid shift from the legs to the neck while the patient is recumbent increases pharyngeal obstruction. What This Study Adds to the Field We show that fluid shift to the neck created by applying lower body positive pressure increases neck size and pharyngeal resistance in healthy subjects. This suggests that rostral fluid shift may predispose individuals to pharyngeal obstruction in fluid overload states.

 

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