Oral airway resistance during wakefulness in eucapnic and hypercapnic sleep apnea syndrome.

The purpose of this study was to evaluate whether there was an abnormal increase of upper airway resistance in the sitting and supine positions in hypercapnic obstructive sleep apnea syndrome (OSAS) patients compared with eucapnic OSAS or normal controls as measured by impulse oscillometry (IOS) while awake. Twenty subjects without OSAS served as controls (group I), and 20 patients with moderate or severe eucapnic OSAS (group II) and another eight hypercapnic severe OSAS patients (group III) were studied. Group II was further divided into two subgroups. Group IIa consisted of 14 subjects whose BMI was less than 35 and group IIb of six subjects whose BMI was greater than 35. All subjects also had an overnight sleep study. Oral airway resistance (AR) (including impedance (Zrs), resistance (R) and reactance (X)) was measured by impulse oscillometry (IOS) (MasterScreen IOS, VIASYS Healthcare GmbH, Germany) in the upright (seated) position and then in the supine position while awake. The results demonstrated that in both group I and group II, Zrs was normal in the sitting position. However, there was a high Zrs in the supine position for group II patients. In contrast, in group III patients, there was a high Zrs in both the sitting and supine positions. In conclusion, upper airway resistance was increased both sitting and supine in the hypercapnic OSAS patients; this would presumably increase the work of breathing and might explain why these subjects were hypercapnic while awake, while eucapnic OSAS patients and normal controls were not. Secondly, the increased upper airway resistance in the supine position in the eucapnic OSAS patients may contribute to their OSAS.