Influence of upper airway size on volume exhaled under negative pressure during evaluation of upper airway collapsibility

It has been shown that volume exhaled in the first 0.5 s after application at the mouth of 5 cmH(2)O negative pressure (V,NEP0.5) during wakefulness strongly reflects critical pressure (Pcrit) during sleep but only in males with neck circumference (NC) > 37 cm. The aim of this study was to establish the relationship between upper airway (UA) size and V,NEP0.5, to obtain V,NEP0.5 values as percent predicted and then correlate them with Pcrit obtained in the same subjects. In 20 (8 women) normal subjects (age, 39 +/- 16 years; BMI, 22.5 +/- 3.0 kg/m(2); AHI, 0.8 +/- 1.0), NC, mean pharyngeal cross-sectional area (APmean) by acoustic pharyngometry and V,NEP0.5 in the supine position were measured. Correlations between APmean, NC and V,NEP0.5 were performed. A strong relationship was found between APmean and NC, and the predicted V,NEP0.5 values were obtained using the equation derived from the relationship between V,NEP0.5 and NC. Subsequently, nine normal subjects (age, 26.3 +/- 2.5 yrs, BMI 23.9 +/- 3.2 kg/m(2), AHI 2.3 +/- 0.5), ten snorers (age, 68 +/- 11 years; BMI, 26.6 +/- 4.6 kg/m(2); AHI, 3.5 +/- 0.8) and ten OSAH patients (age, 64 +/- 9 years; BMI, 32 +/- 4.9 kg/m(2); AHI, 43.8 +/- 24.4) underwent measurement of V,NEP0.5 in the supine position while awake and Pcrit during sleep. Correlations between Pcrit and both V,NEP0.5 and V,NEP0.5 expressed as percent predicted were performed in all subjects. Controls had V,NEP0.5 of 387 +/- 103 mL (100.1 +/- 13% predicted) and Pcrit of -3.7 +/- 2.0 cmH(2)O, snorers had V,NEP0.5 of 320 +/- 33 mL (62 +/- 12% predicted) and Pcrit of -0.6 +/- 0.3 cmH(2)O while OSAH patients had V,NEP0.5 of 295 +/- 67 mL (48 +/- 12% predicted) and Pcrit of 1.0 +/- 1.0 cmH(2)O. The linear regression analysis showed a close and highly significant correlation between V,NEP0.5 percent predicted and Pcrit (r (2) = 0.79, p < 0.001). V,NEP0.5 expressed as percent predicted according to NC strongly reflects Pcrit in a wide range of values and can be used as a surrogate of Pcrit to assess UA collapsibility independently from UA size and sex.