Performance characteristics of upper airway critical collapsing pressure measurements during sleep

Abstract

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.

Keywords: Pharyngeal collapsibility; neuromuscular compensation; obstructive sleep apnea; sleep disordered breathing; upper airway mechanics.

Figure 1

Study design. The sleep apnea status of subjects recruited from the clinic or community was confirmed with a baseline polysomnographic (PSG) study. Additionally, each subject underwent either 1 or 2 nights in the sleep laboratory for determination of passive and/or active upper airway collapsibility (P_CRIT) and up-stream resistance (R_US). During stable, NREM (stage 2) sleep, measurements were conducted during periods of relative hypotonia (“passive” state) or following extended periods of stable flow-limitation and upper airway muscle activity (“active” state). Repeated measurements were made either within-nights or between nights.

Figure 2

Within-night passive P_CRIT Bland-Altman plot. Bland-Altman plots displaying the difference of repeated measurements of (A) passive P_CRIT and (B) passive R_US plotted against the average of the repeated measurements. The mean difference (−0.1 ± 1.6 cm H₂O; solid line) was not different from zero (dashed line) and the limits of agreement (dotted lines) are represented as ± 2 standard deviations. The upper and lower limits of agreement were +3.0 and 3.2 cm H₂O for passive P_CRIT and +26.5 to −20.5 cm H₂O/L/s for R_US. The intraclass correlation coefficient (ICC) for between-night measurements during sleep was 0.90 for passive P_CRIT and 0.66 for R_US. The subject demographics of healthy control (open circles) and sleep apnea (closed circles) subjects are provided in Table 1A.

Figure 3

Difference between repeated passive P_CRIT measurements vs. time between the measurements. Scatter-plot showing the difference between repeated passive P_CRIT measurements (ΔP_CRIT) and the time in minutes between the measurements. The time between measurements did not systematically alter the magnitude of ΔP_CRIT (r² = 0.09, P < 0.07). The subject demographics of healthy control (open circles) and sleep apneic (closed circles) subjects are provided in Table 1A.

Figure 4

Within-night passive P_CRIT during hypnotic induced sleep. Bland-Altman plots displaying the difference of repeated measurements of passive P_CRIT plotted against the average of the repeat measurements. The mean difference (−0.1 ± 0.6 cm H₂O; solid line) was not different from zero (dashed line) and the limits of agreement (dotted lines) are represented as ± 2 standard deviations. The upper and lower limits of agreement were +1.1 and −1.2 cm H₂O for passive P_CRIT. The intraclass correlation coefficient (ICC) for between-night measurements during hypnotic-induced sleep was 0.99 for passive P_CRIT and 0.66 for R_US. The subject demographics of the subjects, none of whom had OSA (open circles) are provided in Table 1A.

Figure 5

Between-night passive P_CRIT and R_US measurement reproducibility. Bland-Altman plots displaying the difference between night 1 and night 2 measurements of (A) passive P_CRIT and (B) passive R_US plotted against the average of both measurements. The mean difference (0.1 ± 1.8 cm H₂O; solid line) is not different from zero and the limits of agreement (dashed lines) are represented as ± 2 standard deviations. The upper and lower limits of agreement for between-night passive P_CRIT were −3.3 and +3.5 cm H₂O and for R_US were −23.5 and +20.7 cm H₂O/L/s, respectively. The intraclass correlation coefficient (ICC) for between-night measurements was 0.87 for passive P_CRIT and 0.41 for R_US. The subject demographics of healthy control (open circles) and sleep apneic (closed circles) subjects are provided in Table 1B.

Figure 6

Between-night active P_CRIT and R_US measurement reproducibility. Bland-Altman plots displaying the difference between night 1 and night 2 measurements of (A) active P_CRIT and (B) active R_US plotted against the average of both measurements. The mean difference (solid line) shows a small systematic bias (−1.0 ± 1.6 cm H₂O) and the limits of agreement (dashed lines) are represented as ± 2 standard deviations. The upper and lower limits of agreement for between-night passive P_CRIT were –4.2 and +2.2 cm H₂O and for R_US were -7.4 and +13.3 cm H₂O/L/s, respectively. The intraclass correlation coefficient (ICC) for between-night measurements was 0.95 for passive P_CRIT and 0.87 for R_US. The subject demographics of healthy control (open circles) and sleep apneic (closed circles) subjects are provided in Table 1C.