The study is about comparing blood samples obtained from the superior vena cava and the pulmonary artery. The investigators will compare the values of PCO2 and PO2 from these two sites, in order to see if exists a PCO2 gap and a PO2 gap, different from zero. Moreover the investigators want to find out if these gaps are related with changes in the cardiac output, the hematocrit or if they can predict post operative dysfunctions.

Blood samples will be taken at six different times as scheduled by the protocol. Along with the two venous blood samples, an arterial blood sample will be taken too, from a radial of femoral artery catheter.

Inclusion Criteria:

• age > 18 years
• patient undergoing cardiac surgery with extracorporeal circulation
• hemodynamic monitoring with a central venous catheter and a Swan Ganz catheter placed in the internal left giugulare vein

Exclusion Criteria:

• severe tricuspid insufficiency
• EF < 50%
• SOFA score >1 in at least one of all the criteria

Will be enrolled adult patients only , eligible for cardiac surgery (CABG, valvular replacement, aortic surgery) with extra corporeal circulation.