The risk associated with arterial hypotension during general anesthesia for surgery has been demonstrated , but the threshold at which consequences for perfusion of one or more organs appear varies according to the mechanism of hypotension, associated abnormalities (HR, cardiac output, and oxygen transport), and the patient's terrain. Currently, a mean arterial pressure greater than 60 mm Hg and a reduction of less than 30-50% from the value measured before general anesthesia are commonly used treatment thresholds to ensure good perfusion of all organs. Normally, cerebral blood flow is auto-regulated, which allows cerebral blood flow to adapt to oxygen requirements and to different levels of blood pressure, both high and low. However, this protective mechanism may fail for a degree of hypotension that depends on several factors such as the age or vascular status of the patient. The aim of the study is to measure non-invasively, easily and reliably the variations of cerebral perfusion in patients with and without cardiovascular risk factors during controlled variations performed during routine care to set the blood pressure level within the recommended safety standards during general anesthesia. What is the target level of blood pressure tolerable for a patient under general anesthesia? Is there a simple and non-invasive way to measure the level of cerebral blood flow autoregulation and especially its adequacy to the brain's oxygen needs?