Predicting Hypotension Related to Spinal Anesthesia for Caesarean Section With Ultrasonography

Establish a diagnosis power of the ΔITVAo ( aortic velocity peak )measured with cardiac Trans thoraciq ultrasonography after passiv leg rising to predict hypotension after spinal anesthesia for elective caesarean. Every patients participate for 30 minutes (time to perform the echography). Outside echographic evaluation, medical taking of participating patients will be exactly the same than what is done for all elective caesarean section.