What can impulse oscillometry and pulmonary function testing tell us about obstructive sleep apnea: a case-control observational study?

Purpose: This study aims to determine whether functional residual capacity (FRC) in obese patients with obstructive sleep apnea (OSA) decreases more than in patients without OSA because of decreased outward recoil from chest wall mass loading as well as increased lung inward recoil.

Methods: Subjects who were overweight and obese to various degrees with normal spirometric values underwent overnight polysomnography to determine the presence or absence of OSA and were labeled as cases or controls. Lung volume and respiratory mechanical properties were measured by plethysmograph and impulse oscillometry, respectively.

Results: A total of 76 men and 31 women were diagnosed with OSA (cases); 64 men and 33 women without OSA were confirmed as controls. Expiratory reserve volume and FRC were significantly decreased in cases compared with controls. Respiratory impedance and resistance at 5 Hz were significantly higher in cases than in controls, although reactance at low frequencies was significantly lower in cases than in controls. Reactance at 5 Hz (Xrs5) was found to be independently highly correlated with the severity of OSA as defined by the Apnea-Hypopnea Index and was significantly correlated with FRC.

Conclusions: FRC is significantly decreased in overweight or obese patients with OSA compared with those without OSA, which may be attributed to an increase in lung elastic recoil. The stronger correlation between Xrs5 and OSA severity might indicate upper airway stenosis, and abnormally increased lung elastic recoil may contribute to OSA.