Role of lung volumes in sleep apnoea-related oxygen desaturation.

We studied the influence of lung volumes on apnoea-induced desaturation in ten subjects with sleep apnoea syndrome. Lung volumes were measured by helium dilution in the sitting and supine position and closing volume with the single-breath nitrogen washout test. To characterize the severity of apnoea-induced desaturations, we determined a desaturation curve for each patient. This curve was obtained by plotting the fall in arterial oxygen saturation (SaO2) reached at the end of each apnoea against the apnoea duration. From this curve we selected two indices: 1) the SaO2 fall following 30 s of obstructive apnoea (delta SaO2 30); and 2) the desaturation surface between 10-30 s of obstructive apnoea (DS 10-30). Both the delta SaO2 30 and the DS 10-30 were significantly correlated with the expiratory reserve volume (ERV), measured in the sitting (r = 0.77 and 0.65, respectively; p less than 0.05) and the supine positions (r = 0.96 and 0.87; p less than 0.005). A strong correlation was also observed with the difference between the supine ERV and the seated closing volume CV) (r = 0.99 with delta SaO2 30 and 0.89 with DS 10-30; p less than 0.005). Obesity influenced sitting and supine ERV values. We conclude that, among lung volumes, supine ERV and supine ERV-seated CV are the best indicators of the severity of apnoea-induced desaturation.