The objective of fluid challenge is to increase oxygen delivery (DO2) in order to improve/restore tissue oxygen consumption (VO2). However the fluid challenge volume to administer stills in debate, some studies recommend to administer 250 mL. Previous studies suggested that 500 mL of fluid challenge administration may improve cardiac output and oxygen delivery. The relation between the amount of fluid expansion and oxygen delivery and oxygen consumption was not yet been studied in a randomized study in critical care patients. This is the purpose of this study.

Inclusion Criteria:

• Patient for whom the physician in charge decided to infuse fluid challenge because of signs of acute circulatory failure (systolic arterial blood pressure < 90mmHg, and/or mean arterial blood pressure< 65 mmHg, and/or the need of vasopressive amine infusion, and/or skin mottling, and/or diuresis <0.5 mL/kg/h for more than 2 hours, and/or arterial lactate level>2mmol/L.
• Echogenic patient
• Patient with a Stroke Volume (SV) variation with passive leg raising (PLR) more than 10%.
• Patient with regular sinus rhythm.
• Patient who received protocol information.

Exclusion Criteria:

• Age Under 18-year-old.
• Pregnancy.