Continuous Positive Airway Pressure and Acetazolamide to Treat Sleep Apnea Syndrome Patients at Altitude

Inclusion Criteria: * Obstructive sleep apnea syndrome, successfully on CPAP therapy * Residence at low altitude (\< 800 m) * Obstructive apnea/hypopnea index \>20/h and a complaint of excessive daytime sleepiness before introduction of CPAP therapy * \> 15 oxygen desaturations/h (\> 3% dips) during an ambulatory nocturnal pulse oximetry performed at the end of a 4-night period without CPAP Exclusion Criteria: * Sleep disorders other than OSA * More than mild cardiovascular disease, unstable cardiovascular disease * Any lung disease, pulmonary hypertension * Chronic rhinitis * Treatment with drugs that affect respiratory center drive (benzodiazepines or other sedatives or sleep inducing drugs, morphine or codeine derivates), stimulants (modafinil, methylphenidate, theophylline) * Internal, neurologic or psychiatric disease that interfere with sleep quality * Previous intolerance to moderate or low altitude (\< 2600 m) * Exposure to altitudes \> 1500m for \> 1 day within the last 4 weeks before the study