The purpose of the study is to investigate the effect of nasal continuous positive airway pressure in combination with acetazolamide as a treatment for sleep related breathing disturbances in patients with the obstructive sleep apnea syndrome living at low altitude during a sojourn at moderate 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