Cardiac Responsiveness Assessment by CO2

The increase of cardiac output resulting from increased blood volume (systolic ejection volume) during intravenous fluid administration defines a state of fluid responsiveness. Fluid responsiveness in intensive care patients with circulatory failure (shock) needs to be carefully evaluated because only half of the patients are fluid responsive and excessive fluid administration is harmful. To reliably assess this state, it is currently necessary to perform either invasive cardiac output monitoring or ultrasound evaluation before and after a fluid administration (called fluid challenge). It is either an invasive procedure or a time-consuming technique (that might depend on operator experience and patient echogenicity). The investigators foresee a potential benefit for an objective quick-answering screening tool that does not require additional monitoring. Instantaneous CO2 production rate (VCO2) calculated automatically by the most recent ventilators (Hamilton C5-C6) via the analysis of exhaled gases. There is an established physiological link between cardiac output, arterial oxygen transport to cells and CO2 production by cell metabolism. The variation in End-Tidal CO2 (another parameter derived from exhaled CO2) is conventionally monitored in the operating room; it can show sudden changes in cardiac output. In intensive care, the EtCO2 variation is probably less sensitive than the variation in VCO2 to detect changes in cardiac output. The aim of this study is to show that the variation in VCO2 as shown on ventilators during a fluid challenge test has satisfactory diagnostic performance in the detection of fluid responsiveness in patients with circulatory failure in intensive care.