Lower body positive pressure increases upper airway collapsibility in healthy subjects

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Abstract

We previously showed that rostral fluid displacement by lower body positive pressure (LBPP) narrows the upper airway (UA) and increases UA resistance, but effects on UA collapsibility remained unknown. To test if LBPP increases UA collapsibility, 13 healthy men were randomized into a control or LBPP arm then crossed over into the other arm with a 30-min washout in between. LBPP was applied by inflating anti-shock trousers wrapped around both legs to 40 mmHg. UA collapsibility was assessed by determining UA critical closing pressure (Pcrit) during application of different negative airway pressures. Pcrit and leg fluid volume were measured at baseline and after 5 min during both periods. LBPP caused a significant increase in Pcrit associated with a reduction in leg fluid volume. We conclude that rostral fluid displacement by LBPP increases UA collapsibility in healthy men, suggesting that fluid shift into the neck could increase UA collapsibility during sleep and thereby predispose patients with fluid overload states to obstructive sleep apnea.

Introduction

The upper airway (UA) of patients with obstructive sleep apnea (OSA) is narrower, more compliant and more collapsible than in subjects without OSA (Haponik et al., 1983, Brown et al., 1985, Gleadhill et al., 1991). While obesity and increased neck circumference contribute to increased UA collapsibility in patients with OSA (Watanabe et al., 2002), much of the variation in UA collapsibility has not been accounted for. Another factor that might contribute to UA collapsibility, but which has received little attention, is fluid accumulation in nuchal and peripharyngeal structures. Indeed, OSA is more common in subjects with fluid retaining states, such as heart failure and renal failure than in subjects without these conditions (Kimmel et al., 1989, Ferrier et al., 2005, Arzt et al., 2006).

We have previously shown that rostral fluid displacement from the legs by applying lower body positive pressure (LBPP) via medical anti-shock trousers in healthy, non-obese subjects, increases neck circumference and pharyngeal resistance (Chiu et al., 2006), and decreases pharyngeal cross-sectional area (Shiota et al., 2007). These observations suggest that rostral fluid shift when moving from the upright to the recumbent position at bedtime might predispose to pharyngeal collapse and OSA in susceptible individuals. Nevertheless, it remains uncertain whether UA narrowing due to fluid accumulation in the peripharyngeal tissues would render the UA more collapsible. For example, nuchal fluid shift might not increase UA collapsibility and predispose to OSA if increased tissue turgor made the pharynx stiffer despite luminal narrowing. However, Wasicko and colleagues showed in anaesthetized cats that systemic vasodilator infusion reduces pharyngeal cross-sectional area and increases pharyngeal collapsibility in response to application of external suction pressure, while systemic vasoconstriction does the opposite (Wasicko et al., 1990). These observations suggest that vasodilation in peripharyngeal tissues not only narrows the UA, but increases its collapsibility. These observations nevertheless leave unanswered the question as to whether nuchal fluid accumulation in response to the more physiological stimulus of caudal to rostral fluid shift causes a similar increase in UA collapsibility.

One means of assessing UA collapsibility in humans is by determining UA critical closing pressure (Pcrit) in response to application of external negative suction pressure (Schwartz et al., 1988). Using this technique, it has been shown that patients with OSA have a higher Pcrit than subjects without OSA both during wakefulness and sleep (Suratt et al., 1984, Suratt et al., 1985, Schwartz et al., 1988, Smith et al., 1988, Gleadhill et al., 1991). Therefore, to test the hypothesis that acute fluid displacement from the lower to the upper body increases UA collapsibility, we examined the effects of LBPP on Pcrit in healthy, non-obese subjects.

Section snippets

Subjects

Subjects were healthy, non-obese (body mass index <30 kg/m2) men without any history of cardiovascular, renal, respiratory or neurological disorders. Those with a history of habitual snoring, excessive daytime sleepiness or a history of UA surgery were excluded. The protocol was approved by the local research ethics board and written informed consent was obtained from subjects prior to participation.

Lower body positive pressure, leg fluid volume, and neck circumference

While lying supine, a pair of deflated anti-shock trousers (MAST III—AT; David Clark, Inc.,

Characteristics of the subjects

Thirteen healthy male subjects with a mean age of 33 ± 4 year completed this study. The mean body mass index was 24.2 ± 0.6 kg/m2 and the mean neck circumference was 38.6 ± 0.5 cm.

Leg fluid volume, neck circumference, and Pcrit

Compared to control, application of LBPP for 5 min resulted in a significant reduction in leg fluid volume (−188 ± 24 ml vs. 25 ± 11 ml, p < 0.001), and a significant increase in neck circumference (0.11 ± 0.04% vs. −0.02 ± 0.05%, p = 0.001).

Fig. 2 shows the airflow and pressure recordings in a representative subject. The V˙Imax during the

Discussion

In this study, we demonstrated that fluid shift from the lower to the upper body by application of LBPP increases UA critical closing pressure in healthy, non-obese adults. The change in Pcrit during the control and LBPP periods correlated inversely with the change in leg fluid volume. Since LBPP increased neck circumference and did not cause any significant effect on EELV, blood pressure or heart rate, the most likely cause of increased UA collapsibility was nuchal fluid accumulation due to

Acknowledgements

Supported by grants from the Toronto Rehabilitation Institute and the Canadian Institutes of Health Research. M.C. Su was supported by a research fellowship from Chang Gung Memorial Hospital—Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan, K.L. Chiu by a research fellowship from China Medical University, Taichung, Taiwan, and an unrestricted research fellowship from ResMed and Respironics Inc., P. Ruttanaumpawan by a research fellowship from Siriraj

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