Relation Between Mean Arterial Pressure and Renal Resistive Index in the Early Phase of Septic Shock

Learned societes (survival sepsis campaign and ESICM) are currently recommending a mean arterial pressure (MAP) target at 65 mmHg in septic shoc with a potential increase to 85 mmHg in patients with medical history of arterial hypertension. A high renal resistive index on the first day of septic shock was associated with acute renal failure more frequently on the 5th day. A decrease in the renal resistance index was also objectified during the increase in MAP. The main objective of this trial is to study the relationship between the improvement of the resistance index during a test of increase in average blood pressure during septic shock and the improvement of renal function In this interventional monocenter trial, we will measure the renal resistive index after stabilization of the MAP at 65 mmHg for two hours, then after 2 hours of stabilization at 85 mmHg. This part allows us to define the patients "responding" to the renal resistive index (improvement of the resitive renal index when the MAP increases). Then, patients will be randomized into two groups : * first group with a MAP target at 65 mmHg * second group with a MAP target at 85 mmHg. There will be a stratification on the responder character to the renal resistance index. Finally, we will assess renal function on the 7th day (with the assessment of serum creatinine and the change of stage of the KDIGO classification)