Lung volume and its correlation to nocturnal apnoea and desaturation

The cross-sectional area of the upper airway is known to be lung volume dependent. If, and to what extent, lung volume variables correlate to nocturnal obstructive apnoeas and oxygen desaturations independently of other factors known to affect lung volumes and sleep disordered breathing is still unclear. A total of 92 subjects were examined by ambulatory recording of nocturnal obstructive apnoeas and desaturations. Sixty-nine of the subjects had a history of snoring and 23 were healthy subjects without complaints of snoring and daytime sleepiness. All subjects performed static and dynamic spirometry for measurements of lung volumes. To evaluate the correlation between lung volume variables and apnoea index (AI) and oxygen desaturation index (ODI), simple and multiple regression analysis was performed. Expiratory reserve volume (ERV) was found to be lower in subjects with snoring and apnoeas (ERV = 1.01) than in non-snoring subjects (ERV = 1.71), (P<0.001). Forced expiratory volume in 1 sec (FEV1)/vital capacity (VC) was slightly, but significantly (P=0.031), lower in subjects with snoring and nocturnal apnoeas and desaturations. In the multiple regression analysis ERV was found to be independently correlated to both AI (R-2 = 0.13; P = 0.001) and ODI (R-2 = 0.11; P = 0.002). Multiple regression analysis also revealed that ERV, body mass index (BMI) and habitual smoking together accounted for 43% of the variation in AI and 48% of the variation in ODI. We find a significant independent association between ERV and nocturnal obstructive apnoea and oxygen desaturation frequency. Our results indicate that ERV is correlated to these events to a similar extent, as is obesity.