The aim of the present study is to investigate whether changes in systolic peak velocity (V peak-CA) and in flow time (FT) using carotid artery Doppler during an End-Expiratory Occlusion Test (EEOT) predict fluid responsiveness in patients with septic shock and lung protective mechanical ventilation in ICU.

All patients will be in supine position (trunk elevated 30), sedated, paralyzed and mechanically ventilated in the volume control mode. Tidal volume will be set at 6-8 ml/kg of predicted body weight.

They will be all monitored by an EV1000TM/Volume View (Edwards Lifesciences Corporation, Irvine, CA 92614) for measurement of cardiac index through transpulmonary thermodilution (TPTD) and pulse contour analysis. Cardiac index and the other hemodynamic parameters derived from pulse contour analysis will be continuously recorded over a 20-sec period.

Phase 1 (baseline): a first set of TPTD will be performed to assess the cardiac index (CI), the stroke volume index (SVI), the stroke volume variation (SVV), the systemic vascular resistance index (SVRI). The mean arterial pressure (MAP), heart rate (HR), central venous pressure (CVP) were also recorded. A carotid doppler was performed to measure the systolic peak velocity (CDPV) and the flow time (FT) (see below).

Phase 2 (EEOT): A 20-second EEO will be than applied through a touch of ventilator for measuring the total end-expiratory pressure. MAP, HR, SVI, CVP, SVRI, pulse contour-derived CI were averaged during the 5 last seconds of the EEO because the maximal hemodynamic effects of the occlusion were observed at this time and because the EV1000TM monitor updates the data every 20 seconds. During this pause a carotid Doppler will be performed and the last 5 seconds will be recorded.

The effects of EEOT on cardiac index will be measured by pulse contour analysis and not by TPTD because these effects must be assessed by a real-time monitoring technique. In practice, the investigators will observe the continuously changing values of pulse contour analysis-derived cardiac index while performing the Doppler measurements.

Phase 3 (fluid challenge): The patients then will receive a 10-minute infusion of 500 mL saline or lactate ringer (7 ml/kg). A last set of hemodynamic measurements, including CI, MAP, HR, SVI, CVP, SVRI, and carotid Doppler, will be recorded after fluid infusion.

As soon as the cardiac index value started to increase, the investigators will consider that it had reached its maximum. At this precise time, they will freeze the image of the echograph and performed the Doppler measurements on the values displayed during the previous seconds. If pulse contour analysis-derived cardiac index will increase 5% during the EEOT, compared to the baseline value, the patient will be consider as responder to the test.

Catecholamine's infusion, mechanical ventilation settings and bed position will be kept constant during the study period.