Ventilation Strategy for the Elderly Patients in Prone Position

\<Protocol\> 1. All patients were ventilated by an anaesthesia ventilator (Zeus, Dräger, Lübeck, Germany) under general anesthesia. 2. Ventilation strategy: * Pressure controlled ventilation volume guaranteed (PCV-VG) mode or volume control ventilation (VCV) mode * tidal volume 6-8 mL/kg (predicted body weight) * Inspiratory/expiratory (I/E) ratio 1:1.5 * Inspired oxygen concentration (FIO2) 0.5 with air * 3.0 L/min of inspiratory fresh gas flow * Positive end-expiratory pressure (PEEP) of 5 cm H2O * Respiratory rate (RR) of 10 to 14 min-1 3. After prone positioning and hemodynamic condition was stabilized, patients were ventilated with VCV, PCV-VG mode, VCV with pause following a randomized order, previously determined for each patient. Before each ventilator setting was started, lung recruitment maneuvers were performed to set the lungs to a defined baseline status. * ventilation order : one of the following examples is taken by randomization. VCV 15 min -- VCV pause 15min -- PCV-VG 15min VCV 15 min -- PCV-VG 15min -- VCV pause 15min VCV pause 15min -- VCV 15 min -- PCV-VG 15min VCV pause 15min -- PCV-VG 15min -- VCV 15 min PCV-VG 15min -- VCV 15 min -- VCV pause 15min PCV-VG 15min -- VCV pause 15min -- VCV 15 min