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 ₂O 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 H₂O and was reduced to ∼ ± 1 cm H ₂O 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 H₂O 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.