 In these patients n10 the pleurae will not be opened. During aortic cross-clamp period the ventilator will be adjusted to provide dead space ventilation only i.e. 5cm PEEP, ventilator frequency 5min and the minute ventilation 1,5 liter. Fio2 50. The operating field will be insufflated with CO2 at a flow rate of 10 L minute starting 2 minutes before cardiac cannulation and continued until 10 minutes after termination of the CPB. 