Upper airway obstruction commonly occurs after induction of general anesthesia. The aim of this study is to determine if head rotation improves the efficiency of mask ventilation of anesthetized apneic adults.

When apnea is achieved after induction, face mask ventilation is started in either a neutral head position or a head position axially rotated 45 degrees to the right. Mask ventilation will be carried out for 1 minute (Step 1) with pressure control ventilation of an anesthesia machine at peak inspiratory pressure 15 cmH2O, 10 breaths per minute, inspiration time to expiration time ratio 1:2 and no positive end-expiratory pressure. Then, mask ventilation will continue but head position will be crossed over (Step 2) and Step 1 will be repeated (Step 3). Expiratory tidal volume (VTE) will be measured by using respiratory inductive plethysmograph.

Inclusion Criteria:

• Patients with body mass index in the range of 18.5 to 35.0 kg/m2, who meet American society of anesthesiologists physical status classification I to III and require general anesthesia with tracheal intubation.

Exclusion Criteria:

• Patients with limited head rotation/extension, gastro-esophageal reflux or a full stomach, known sleep apnea in continuous positive airway pressure therapy and any anticipated difficult airway likely requiring awake intubation.