Between 3 and 19% of patients with clinical and polysomnographic features of obstructive sleep apnea syndrome (OSA) develop a high-frequency of central apneas and/or disruptive Cheyne-Stokes respiratory syndrome after application of continuous positive airway pressure (CPAP), a pattern called the complex sleep apnea syndrome (CompSAS). The trial seeks to determine the efficacy of the ASV vs. CPAP modes for the treatment of CompSAS over time. Baseline clinical and laboratory data will be collected, patients will be randomized to one of the two treatments. Overall study participation is approximately 3 months.