Upper airway obstruction (UAO) is common complication during induction of general anesthesia. The mechanism of UAO during anesthesia has not been well understood. Posterior displacement of soft palate are believed to be the primary contributing factors. The mechanism of UAO during anesthesia share many similarities with obstructive sleep apnea (OSA). Since nasal continuous positive airway pressure (nCPAP) can maintain the airway patent in patients with OSA, the investigators hypothesize that nCPAP during induction of anesthesia will reduce the incidence and severity of UAO.