Before induction of anesthesia, an arterial line is installed. The agents of induction of anesthesia and doses are left to the discretion of the attending anesthesiologist. The trachea is intubated, anesthesia circuit and sidestream CO2 sensor were connected to the endotracheal tube and positive pressure ventilation begins with standardized parameters: control assisted mode with no inspiratory effort, tidal volume of 6-8 mL/kg, respiratory rate of 8-10/minute and PEEP of 5 mmHg. Modification of the ventilation parameters is not permitted before and during the PLR maneuvers. Isoflurane and propofol are used for the maintenance of anesthesia. A PA catheter (Paceport, Edwards Lifesciences, Irvine, California, USA) is inserted in the right internal jugular vein. If a femoral line is installed, the FloTrac/EV1000 is connected to the femoral line, if not, the radial line is used for mini-invasive continuous cardiac output monitoring. The a and v wave aspect and ratio of CVP waveform is noted. The RV waveform is inspected for signs of abnormalities such as a non-horizontal slope restrictive pattern (Figure xx)18. The transducers are all placed at the midthoracic level and a "flush test" is performed prior to CO measurements with the FloTrac/EV1000 to ensure absence of resonance or damping of the system.