 Anesthesia induction will be performed with usual anesthetics without any opioids and followed by the placement of a laryngeal mask airway. Optimal anesthesia level will be maintained with inhalation anesthesia so as to obtain a bispectral index of 30 to 50. Subjects will be ventilated with controlled mechanical ventilation CMV by the anesthesia ventilator until the presence of spontaneous breathing can be affirmed. A carbon dioxideflow sensor Adult Combined CO2Flow Sensor; Novametrix medical systems INC., Wallingford, CT will be placed between the LMA and the breathing circuit. The sensor will be connected to a noninvasive cardiac output monitor NICO; Noninvasive Cardiopulmonary management System, model 7300; Respironics Corp., Murrysville, PA. NICO data end-tidal CO2 ETCO2, tidal volume and flow waveforms will be recorded and stored on a personnel computer. At the start of surgery, the anesthesia care provider will administer opioids to control nociceptive stimuli from surgery. Figure 2 shows the procedural course of the study. Once spontaneous breathing is observed, PSV will be applied with pressure support PS of 3, 8, 13 and 18 cmH2O in this order, with slope of 1.0 second and no PEEP. The fraction of inspired oxygen will be titrated to achieve SpO2 over 98. Five minutes will be allowed to achieve equilibrium at each PS level after which an additional recording of one minute of data will be obtained. Once the PSV is completed at 18 cmH2O, ventilation will be carried out in reverse order of PS; 13, 8 and 3 cmH2O of PS. If at any point in the course of the study the subjects ETCO2 is higher than 60 mmHg, obvious body motion is observed or abnormal vital signs NIBP; 16090 mmHg or 8050 mmHg, HR; 100 mmHg or 40 mmHg, SpO2; 94 at a given fraction of inspired O2 occur, the study will be terminated and routine care will be provided. 