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.