Non-invasive estimation of cardiac output in mechanically ventilated patients: a prolonged expiration method

A non-invasive method for the estimation ofcardiac output in mechanically ventilated patients isdescribed. The method is based on prolonged expiration, andrelies on measurement of gas concentrations and flow rate. Apneumatic system, with an ad hoc designed orifice resistance,has been made and experimentally characterized to adapt thebreathing circuit to this application. Cardiac output iscalculated using two algorithms and the results are comparedwith the ones obtained by thermodilution. To this purpose,we prospectively enrolled twenty mechanically ventilatedpatients, who had undergone cardiac surgery, and bothalgorithms show good correlation with thermodilution(R>0.8). The application of the first algorithm gave meancardiac output values slightly lower than those obtained bythermodilution (26%), while the application of the secondalgorithm gave higher values (+30%). Difference standarddeviations between paired measurements is 0.72 L min-1 forthe first algorithm and 1.07 L min21 for the second one.Standard deviation obtained by the application of the firstalgorithm is slightly lower than those relative to otherminimally invasive techniques. Through prolonged expiration,and standardization and automation of the procedureon mechanically ventilated patients, the proposed systemallows to obtain a non-invasive estimation of cardiac output.