Fluid administration is a daily intervention on the intensive care unit to improve cardiac output (CO) and stabilize circulation in critically ill patients. Simultaneously, the volume status of the patient is very difficult to assess. Too little volume leads to inadequate organ perfusion followed by ischemia and organ failure. Too much volume may worsen heart failure and cause pulmonary and peripheral edema and contribute to further tissue injury and organ dysfunction. Although dynamic indices have been shown to be more accurate predictors of fluid responsiveness, this relevant and complex task is usually guided by static clinical variables and the specialist's interpretation due to the fact that the interpretation of dynamic parameters is not fully developed and that they are not universally available. This lack of understanding is partially because of the complex interaction with mechanical ventilation. The investigators hypothesize that knowing the distribution of ventilatory pressures will make it possible to index dynamic parameters to tidal volume and improve their predictive value concerning the volume status of the patient. In addition, it would be of interest to be able to predict fluid responsiveness in a non-invasive way, especially in critically ill patients. Up to now, continuous non-invasive cardiac output monitoring using Nexfin in critically ill patients has not been validated and also not tested for its ability to predict fluid responsiveness. The present research proposal evaluates the possibility and accuracy of the model flow analysis obtained by non-invasive finger arterial pressure measurements to determine fluid responsiveness using passive leg raising. It will also be compared to a more invasive method (that is currently used in the clinic) to assess its ability to measure absolute CO levels accurately. It may make it possible to assess fluid responsiveness in a non-invasive and patient friendly way.