The purpose of this study is to compare changes of minimally invasive arterial pulse contour cardiac output with changes of intermittent and continuous thermodilution cardiac output by pulmonary artery catheter in hemodynamic unstable patients with rapid changing vascular tone (changing dosage of vasoactive drugs or inotropics, or volume challenge). Simultaneously, global parameters of oxygen delivery and consumption will be compared with regional flow parameters and tissue oxymetry (near infrared spectrometry and laser-Doppler). While continuous thermodilution cardiac output is used for patient management, pulse contour cardiac output, intermittent thermodilution cardiac output and tissue oxymetry is only used for monitoring.

Inclusion Criteria:

• Indication for invasive hemodynamic monitoring with pulmonary artery catheter and arterial catheter
• Hemodynamic instability (need for vasoactive drugs, inotropics or volume).

Exclusion Criteria:

• Age < 18.
• Patient with contraindications for the placement of arterial cannula.
• Patient with history or clinical findings of aortic valve regurgitation.
• Atrial fibrillation.
• Patient being treated with an intra-aortic balloon pump.
• Limitation of therapy due to bad prognosis