Published ahead of print on September 22, 2006, doi:10.1164/rccm.200607-927OC Am. J. Respir. Crit. Care Med., Volume 174, Number 12, December 2006, 1378-1383 A more recent version of this article appeared on December 15, 2006
Submitted on July 10, 2006 Fluid Shift by Lower Body Positive Pressure Increases Pharyngeal Resistance in Healthy SubjectsKuo-Liang Chiu1,1 Sleep Research Laboratory, Toronto Rehabilitation Institute, Toronto, Ontario, Canada, 2 Sleep Research Laboratory, Toronto Rehabilitation Institute, Toronto, Ontario, Canada; Department of Medicine, Mount Sinai Hospital and Toronto General Hospital of the University Health Network, Toronto, Ontario, Canada, 3 Division of Otolaryngology, University of Toronto, Toronto, Ontario, Canada, 4 Department of Medicine, Mount Sinai Hospital and Toronto General Hospital of the University Health Network, Toronto, Ontario, Canada; Department of Medicine, University of Toronto, Toronto, Ontario, Canada, 5 Sleep Research Laboratory, Toronto Rehabilitation Institute, Toronto, Ontario, Canada; Department of Medicine, Mount Sinai Hospital and Toronto General Hospital of the University Health Network, Toronto, Ontario, Canada; Department of Medicine, University of Toronto, Toronto, Ontario, Canada * To whom correspondence should be addressed. 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 (OSA). 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, non-obese 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 mmHg via anti-shock 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 to 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 cmH2O/L/s to 0.60±0.11, p=0.034; and to 0.87±0.19, p<0.001 compared to 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: pharyngeal resistance, fluid displacement, lower body positive pressure, obstructive sleep apnea
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