Experimental procedure Before start of anaesthesia, an unused carbon dioxide absorber will be applied (Drägersorb, Dräger Medical, Lübeck, Germany) to the anaesthesia ventilator (Dräger Primus™, Dräger Medical, Lübeck, Germany). All patients will be preoxygenated with 100% oxygen for 3-4 minutes with a fresh gas flow of 5 liters/minute. Anaesthesia will be induced with 2 µg/kg fentanyl and 1.5-3.0 mg/kg propofol. Rocuronium 0.6 mg/kg will be administered for muscle paralysis. Ventilation will be assisted manually with 100% oxygen via a semiopen circle system (4.5 L volume) until tracheal intubation and then by means of a ventilator with an FiO2 at 0.35 in nitrogen. The respiratory rate will be 15/min throughout the study and VT will be adjusted as to achieve a PETCO2 at 4.5 kPa. No positive end-expiratory pressure (PEEP) will be applied. Propofol 8 mg/kg/h will be infused until an arterial cannula have been inserted in the radial artery. Stroke volume (SV) and cardiac output (CO) will be assessed in 20 patients (10 in each group) with LIDCO Rapid® (LiDCO Ltd, Cambridge, UK). Transoesophageal echocardiography (TEE) will be used to assess cardiac output in 6 patients (3 in each group) with Philips CX-50®, TEE ultrasound X7-2t (Philips Ultrasound, Bothell, WA, USA). After a control (time zero) sample of arterial blood (3 ml) have been obtained, the ventilatory mode will be altered as follows: In the group with increased tidal volume (IVT), initial plateau pressure (Pplateau) will be monitored and then VT will be increased until Pplateau will be 0.04 cm H2O/kg over the initial Pplateau. The PETCO2 will be then adjusted to 4.5 kPa with a flexible corrugated hose (disposable plastic tube, Medcore, AB Uppsala, Sweden) placed between the Y-piece of the anaesthesia circle system and the heat and moisture filter (HME) attached to the endotracheal tube. This flexible corrugated hose increased the dead-space volume and provided adjustable rebreathing of carbon dioxide. In the group with normal tidal volume (NVT), a PEEP to10 cmH2O will be applied. When required, VT will be then adjusted to maintain PETCO2at 4.5 kPa. In both groups, inspiratory:expiratory ratio was 1:2 including an inspiratory plateau of 10%. When PETCO2 values will be stable at 4.5 kPa, sevoflurane administration will be started with a vaporiser (sevoflurane Dräger Vapor 2000: Medical, Lübeck, Germany) set to 3%. After 5 minutes the fresh gas flow will be adjusted to 1.0 L/min with an unchanged vaporiser setting throughout the anaesthesia period.