With the aim to reduce the risks for the patient, a big research effort has been made to investigate minimally invasive or non-invasive methods. Investigators sought to evaluate the effectiveness of a non invasive method to estimate CO in mechanically ventilated patients.The method is based on prolonged expiration, and relies on measurement of gas concentrations and flow rate. Investigators designed, realized, and characterized a system to induce passive prolonged expirations when connected to the patient circuit.