Auto-trilevel ventilation, with a lower airway pressure at the beginning of expiration to prevent hypercapnia and a higher airway pressure at the end of expiration to prevent residual apnea events, should be able to provide a novel ventilation mode with a higher efficacy and lower average airway pressure for overlap syndrome. To prove the hypothesis, comparison is made between ordinary BiPAP and auto-Trilevel ventilation modes during treatment for patients with overlap syndrome.