Eighteen patients with OSA living at < 600 m discontinued CPAP therapy during studies in Zurich ( 490 m ) and during two sojourns of 3 days in the Swiss Alps ( 2 days at 1,860 m and 1 day at 2,590 m ) separated by a 2-week washout period at < 600 m. Patients received acetazolamide ( 2 250 mg/d ) or placebo at altitude in a randomized , double-blind , crossover design .