Pharyngeal critical pressure in patients with obstructive sleep apnea syndrome - Clinical implications

Current evidence suggests that patients with obstructive sleep apnea (OSA) may have greater pharyngeal critical pressure (Pcrit), which reflects the i ncrease in upper airway collapsibility. The contribution of Pcrit to the se verity of OSA and to the efficacious continuous positive pressure (nCPAP(ef f)) therapy has never been extensively described and no data are available about the interaction of Pcrit, age, and anthropometric variables. To deter mine the relationship between Pcrit, severity of the disease, nCPAP(eff), a nd anthropometric variables we measured Pcrit in a group of 106 patients wi th OSA. Pharyngeal critical pressure was derived from the relationship betw een maximal inspiratory flow and nasal pressure, Pcrit representing the ext rapolated pressure at zero flow. Upper airway resistance (Rus) was determin ed as the reciprocal of the slope (Delta Pn/Delta VImax cm H2O/L/s) in the regression equation. In a subgroup of 68 patients, during the diagnostic ni ght, we measured as indices of respiratory effort, the maximal inspiratory esophageal pressure (Pes) at the end of apnea (Pes(max)), the overall incre ase from the minimum to the maximum (Delta Pes), and the rate of increase o f Pes during apnea (RPes). As a group, the mean Pcrit was 2.09 +/- 0.1 cm H 2O (range, 0 to 4.5) and the mean Pus was 11.1 +/- 0.5 cm H2O/L/s. Although men have greater Pcrit, pharyngeal collapsibility was influenced neither b y neck size nor by body mass index (BMI). Although there was a significant relationship between Pcrit and apnea plus hypopnea index (AHI) (r = 0.23, p = 0.02), neck circumference was the stronger predictor of apnea frequency, with Pcrit contributing only to the 3% of the variance. In the group of pa tients as a whole, a model including AHI, BMI, Rus, and Pcrit explained the 36% of the variance in nCPAP(eff), with a greater contribution of AHI, Pcr it accounting for only 3% of the variation. In patients for whom the measur e of respiratory effort was obtained, 42% of the variance in nCPAP(eff) was explained by RPes (33%) and BMI. From these results we conclude that Pcrit alone does not yield a diagnostically accurate estimation of OSA severity and nCPAP(eff). Although individual collapsibility may predispose to pharyn geal collapse, upper airway occlusion may require the combination of severa l factors, including obesity, upper airway structure, and abnormalities in muscle control.