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.

Inclusion Criteria:

• Patient hospitalized in intensive care.
• Patient sedated and mechanically ventilated in Volume Assisted Controlled mode.
• Patient with hypotension (MAP <65 mmHg OR SAP <90 mmHg) AND / OR under continuous infusion of Norepinephrine / Dobutamine / Epinephrine
• Patient equipped with a cardiac output measurement system by transpulmonary thermodilution (PICCO®) with a femoral arterial module; as well as a central venous route in the superior or inferior vena cava territory.

Exclusion Criteria :

• Patient under 18 years old
• Patient under guardianship without the possible consent of the guardian
• Pregnant woman
• Pathologies and maneuvers strongly affecting CO2 purification: Diagnosed pulmonary embolism, Prone positioning for less than 60 minutes, Exogenous intake of sodium bicarbonate less than 60 minutes ago (with the exception of an renal replacement therapy in place for more than 60 minutes)
• Recent change in respiratory quotient: Change in enteral or parenteral nutrition solution for less than 3 hours
• Pathology affecting CO2 metabolism: Fever> 39 ° c, Intense chills.
• Patients with unstable ventilation over the hour before inclusion.

This study concerns adult subjects in intensive care with a state of circulatory failure (shock), in whom the indication of fluid vascular filling is to be discussed.