Agreement of CNAP pressure with invasive pressure is evaluated dependent on hemodynamic variables achieved by a PICCO monitor.~Hypothesis: Low cardiac output and volume status but not high vascular resistance affect the accuracy of CNAP. Body mass index and age may have impact on the accuracy of PPV measured non-invasively on the basis of volume-clamp technique.
Agreement of CNAP pressure with invasive pressure is evaluated dependent on hemodynamic variables achieved by a PICCO monitor.~Hypothesis: Low cardiac output and volume status but not high vascular resistance affect the accuracy of CNAP.