Performance characteristics of upper airway critical collapsing pressure measurements during sleep.

OBJECTIVE: The critical pressure (P(CRIT)), a measurement of upper airway collapsibility, is a determinant of the severity of upper airway obstruction during sleep. We examined the performance characteristics of the passive and active P(CRIT) by examining both within-night and between-night variability in the measurements. METHODS: We studied 54 sleep apnea patients (39 men, 15 women) and 34 normal subjects (20 men, 14 women) on either 1 or 2 nights during sleep. The P(CRIT) was measured during relative hypotonia ("passive" state) or during periods of sustained upper airway obstruction used to recruit upper airway neuromuscular responses ("active" state) within- and between-nights. In a subgroup of 10 normal subjects, we performed repeated measurements during hypnotic-induced sleep. Bland-Altman analyses were used to determine the within-night and between-night reliability of the P(CRIT) measurements. RESULTS: There were no significant within-night or between-night differences for the mean passive P(CRIT). The active P(CRIT) was ∼1 cm H(2)O more collapsible on the second night than on the first night. The limits of agreement, which bound the passive and active P(CRIT), was ∼ ± 3 cm H(2)O and was reduced to ∼ ± 1 cm H(2)O for the passive P(CRIT) with hypnotic-induced sleep. CONCLUSION: Passive and active P(CRIT) measurements are reasonably reliable within and between nights. An approximately 3 cm H(2)O change in passive or active P(CRIT) appears to represent the minimally significant change in P(CRIT) necessary to assess the effect of an intervention (e.g., positional therapy, surgical interventions, oral appliance effects, and pharmacotherapy) on upper airway mechanical loads or neuromuscular responses.

Effect of end-expiratory lung volume on upper airway collapsibility in sleeping men and women.

The relationship between changes in absolute end-expiratory lung volume (EELV) and collapsibility has not been rigorously quantified. We hypothesized that pharyngeal collapsibility varies inversely with absolute lung volume in sleeping humans during 1) conventional and 2) isovolume measurements of passive critical pressure (Pcrit). Eighteen healthy subjects (11 male, 7 female) slept in a negative pressure ventilator for measurements of pharyngeal collapsibility (Pcrit) during non-rapid eye movement sleep. EELV was 1) allowed to vary with changes in nasal pressure for conventional Pcrit measurements and 2) controlled by maintaining a fixed pressure difference across the respiratory system (P(RS)) from the nose to the body surface for isovolume Pcrit measurements at elevated EELV (P(RS) = +10 cmH(2)O), reduced EELV (P(RS) = -5 cmH(2)O), and functional residual capacity (FRC; P(RS) = 0 cmH(2)O). In each condition, the absolute EELV was determined and the corresponding Pcrit was derived from upper airway pressure-flow relationships. In the entire group, Pcrit varied inversely with EELV (P < 0.001). Pcrit decreased as EELV increased from the conventional to the FRC isovolume condition by -3.5 ± 1.0 cmH(2)O/l (P < 0.003). Subjects with a conventional Pcrit below -2 cmH(2)O exhibited greater reductions in EELV and correspondingly greater decreases in the FRC isovolume compared with the conventional Pcrit (P < 0.001). The overall response, ΔPcrit/ΔEELV, was -2.0 ± 0.2 cmH(2)O/l (P < 0.001) and did not differ between men and women (P = 0.16). Nevertheless, men and women differed significantly in FRC (2.63 ± 0.16 vs. 1.88 ± 0.13 liters, P <0.05) and FRC isovolume Pcrit (-2.3 ± 0.8 vs. -7.2 ± 1.2 cmH(2)O, P < 0.05), implying that the men had larger lungs and more collapsible airways than the women. The ΔPcrit/ΔEELV response was independent of sex, conventional Pcrit, body mass index, and neck, waist, and hip circumferences. We conclude that Pcrit varies inversely with absolute EELV, which may lead to 1) an underestimation of the magnitude of quantitative differences in Pcrit across the spectrum from health (negative Pcrit) to disease (positive Pcrit) and 2) increases in sleep apnea susceptibility in obesity.