Relationship between critical pressure and volume exhaled during negative pressure in awake subjects with sleep-disordered breathing.

Background: Critical pressure (Pcrit) is considered a reliable parameter to evaluate the mechanical properties of the passive upper airway (UA) and is signifi cantly increased in patients with obstructive sleep apnea-hypopnea (OSAH) compared with normal subjects. The volume exhaled in the fi rst 0.5 s after application at the mouth of 5 cm H 2 O negative pressure at the onset of expiration (V,NEP 0.5 ) during wakefulness has been used as a marker of UA collapsibility. The aim of this study was to investigate if there is a signifi cant relationship between V,NEP 0.5 and Pcrit in normal subjects, snorers, and patients with OSAH. Methods: Thirty men, 10 with OSAH (aged 64 6 9.1 years, BMI 32 6 4.9 kg/m 2 , apnea-hypopnea index [AHI] 43.8 6 24, neck circumference 46.6 6 3.7 cm), 10 snorers (aged 68 6 11 years, BMI 26.6 6 4.6 kg/m 2 , AHI 3.5 6 0.8, snoring time 30% of sleep time, neck circumference 42.2 6 3.9 cm), and 10 controls (aged 67 6 12 years, BMI 25.4 6 2.2 kg/m 2 , AHI 1.9 6 1.2, neck circumference 41.2 6 2.2 cm) underwent V,NEP 0.5 measurement in supine position while awake and Pcrit measurement during sleep. Correlation between V,NEP 0.5 and Pcrit was performed in all subjects. Results: Controls had V,NEP 0.5 of 456 6 82 mL and Pcrit of 2 1.38 6 0.6 cm H 2 O, snorers had V,NEP 0.5 of 321 6 33 mL and Pcrit 2 0.55 6 0.3 cm H 2 O, and patients with OSAH showed V,NEP 0.5 of 295 6 67 mL and Pcrit of 0.99 6 1 cm H 2 O ( P , .001 vs normal subjects). A strong correlation was found between V,NEP 0.5 and Pcrit ( r 2 5 0.61, P , .0001). Conclusions: In males with neck circumference . 37 cm, V,NEP 0.5 during wakefulness strongly refl ects Pcrit in a wide range of values. Our fi ndings suggest that V,NEP 0.5 can be used as valuable substitute for Pcrit to assess UA collapsibility for clinical and research purposes in these subjects.