Central and obstructive sleep apnoea during ascent to high altitude

Objective: The aim of the study was to investigate the relationship between central sleep apnoea (CSA) at high altitude and arterial blood gas tensions, and by inference, ventilatory responsiveness. Methodology: Fourteen normal adult volunteers were studied by polysomnography during sleep, and analysis of awake blood gases during ascent over 12 days from sealevel to 5050 m in the Nepal Himalayas. Results: Thirteen subjects developed CSA. Linear regression analysis showed tight negative correlations between mean CSA index and mean values for sleep SaO(2), PaCO2 and PaO2 over the six altitudes (r(2) greater than or equal to 0.74 for all, P < 0.03). Paradoxically there was poor correlation between the individual data for CSA index and those parameters at the highest altitude (5050-m) where CSA was worst ( r(2) < 0.12 for all, NS), possibly due to variation in degree of acclimatization between subjects. In addition, CSA replaced mild obstructive sleep apnoea during ascent. Obstructive sleep apnoea index fell from 5.5 +/- 6.9/h in rapid eye movement sleep at sealevel to 0.1 +/- 0.3/h at 5050 m (P < 0.001, analysis of variance), while CSA index rose from 0.1 +/- 0.3/h to 55.7 +/- 54.4/h (P < 0.001). Conclusion: There was a general relationship between decreasing PaCO2 and CSA, but there were significant effects from variations in acclimatization that would make hypoxic ventilatory response an unreliable predictor of CSA in individuals.