In spontaneously breathing subjects, intrathoracic expiratory flow limitation can be detected by applying a negative expiratory pressure (NEP) at the mouth during tidal expiration. To assess whether NEP might increase upper airway resistance per se, the interrupter resistance of the respiratory system (Rint,rs) was computed with and without NEP by using the flow interruption technique in 12 awake healthy subjects, 6 nonsnorers (NS), and 6 nonapneic snorers (S). Expiratory flow (V˙) and Rint,rs were measured under control conditions with V˙ increased voluntarily and during random application of brief (0.2-s) NEP pulses from −1 to −7 cmH2O, in both the seated and supine position. In NS, Rint,rs with spontaneous increase inV˙ and with NEP was similar [3.10 ± 0.19 and 3.30 ± 0.18 cmH2O ⋅ l−1 ⋅ s at spontaneous V˙ of 1.0 ± 0.01 l/s and atV˙ of 1.1 ± 0.07 l/s with NEP (−5 cmH2O), respectively]. In S, a marked increase in Rint,rs was found at all levels of NEP ( P < 0.05). Rint,rs was 3.50 ± 0.44 and 8.97 ± 3.16 cmH2O ⋅ l−1 ⋅ s at spontaneous V˙ of 0.81 ± 0.02 l/s and atV˙ of 0.80 ± 0.17 l/s with NEP (−5 cmH2O), respectively ( P < 0.05). With NEP, Rint,rs was markedly higher in S than in NS both seated ( F = 8.77; P < 0.01) and supine ( F = 9.43; P < 0.01). In S,V˙ increased much less with NEP than in NS and was sometimes lower than without NEP, especially in the supine position. This study indicates that during wakefulness nonapneic S have more collapsible upper airways than do NS, as reflected by the marked increase in Rint,rs with NEP. The latter leads occasionally to an actual decrease in V˙ such as to invalidate the NEP method for detection of intrathoracic expiratory flow limitation.
Effect of expiratory flow limitation on respiratory mechanical impedance: a model study