Arterial Hypotension

Immediately following the induction of general anesthesia, arterial hypotension may occur with an incidence of about 40% in patients classified as ASA 3 or 4. Perioperative arterial hypotension is associated with increased perioperative morbidity and mortality. The increased mortality associated with perioperative arterial hypotension has been reported up to 30 and 90 days after surgery. This association appears to be both dose-dependent and time-dependent (severity and duration of arterial hypotension). EchOptimal study aims to determine whether non invasive preoperative stroke volume optimization using transthoracic echocardiography (TTE) can reduce the incidence and severity arterial hypotension following induction of general anesthesia. This is prospective, randomized, controlled, double blinded study approved by ethics comittee. The primary objective of this study is to compare the incidence of arterial hypotension (mean arterial pressure \< 65 mmHg) within the first 15 minutes following the induction of general anesthesia, between 2 groups : * standard current care (stroke volume optimized after induction of general anesthesia using oesophageal doppler) and * preopertaive stroke volume optimization using transthoracic echocardiography measument of subaortic velocity time integral.