This prospective trial was designed to assess the performance of a non invasive method to estimate cardiac output in mechanically ventilated cardiac surgery patients and its outcome is compared to the values obtained from the invasive measurement system based on thermodilution. The pulmonary blood flow (PBF), which is the volume of blood that actively participates in the gas exchange per unit of time, is the directly estimated variable that this technique employ to calculate cardiac output. The method investigators studied is theoretically based on the application of an algorithm which derives from a modified version of the Fick equation in two different phases; the former involves measurements during the steady state, whilst the second starts when a sudden perturbation into the carbon dioxide (CO2) elimination process is introduced. The algorithms investigated in this work requires the analysis of the expired gas content during both normal breathing and prolonged expiration, providing a non-invasive estimation of the artero-venous content of CO2, and consequently allows the calculation of PBF according to the Fick method. At this purpose, a metabolic monitor was used; it sampled gas from the "Y" piece of the mechanical ventilator's breathing circuit through a suction pump. In order to obtain the prolonged expiration, a pneumatic system, with an ad hoc designed orifice resistance (5 cmH2O•L-1•min),has been made and experimentally characterized to adapt the breathing circuit to this application. Experimental data of CO2 and O2 concentrations were recorded and processed after the measurement session thanks to an ad hoc developed LabView application. This application performs the following tasks: it converts the gas fractions into partial pressures, segments the trends of partial pressures, executes the data-reduction and, after obtaining the values of venous carbon dioxide tension (PvCO2) and arterial carbon dioxide tension (PaCO2) obtained, it calculates the CO value using the above mentioned algorithms.