TFI method uses a tetra-polar electrode system, which are located in the neck and thorax of the patient, and through which alternative current of 4 mA at 100 KHz goes. This allows a signal of ECG and the first derivative of the impedance, which is afterwards processed to obtain heart rate, cardiac output and stroke volume. Through the pulse wave method, which requires inserting a catheter into the femoral artery, cardiac output can also be obtained, in addition to intra-arterial pressures. That method requires a pre-calibration through by getting the cardiac output through transpulmonary thermodilution. To get the transcutaneous oxygen saturation, a conventional pulsioximeter is used.
Oxygen consumption is studied through indirect calorimetry.
The same device informs of CO2 production. El O2 is carriage is deducted once cardiac output, hemoglobin and O2 saturation are known. The goal of the system is to be able to read the data from different monitoring systems and to store all the data in a centralized way, an thus be able to work together with all the information. The input to the computer is done though an RS-232 multiplex, which allows all the data to be able to enter to the computer using a single communications port. Graphically, the global configuration of the system can be seen in fig.