Noninvasive pulse pressure variation for the guidance of perioperative fluid therapy is compared with the gold standard of invasive measurements. Cardiac output is derived by the PICCO monitor.

Hypothesis: Noninvasive CNAP PPV predicts fluid responsiveness. A grey zone for the prediction of fluid responsiveness will be defined.

Accuracy of PPV measured by two devices,PICCO and CNAP, is compared with the manually calculated PPV from the arterial line waveform. Established tests like passive leg raising and end-expiratory occlusion are used for prediction of fluid responsiveness. Intraoperative events like volume challenge are monitored.

Inclusion Criteria:

• major surgery
• need for invasive arterial measurements

Exclusion Criteria:

• age under 18
• cardiac dysrhythmia
• vascular pathology at punction site

Patients undergoing major surgery with the need for invasive arterial pressure monitoring