Lower limb orthopedic surgery is commonly realized under spinal anesthesia. This loco-regional anesthesia induces a peripheral vascular resistance decrease by vasodilatation resulting in hypotension. A severe and prolonged hypotension can compromise regional perfusion and worse outcome especially in very elderly patients. Moreover the venous vasodilatation observed after spinal anesthesia decreases cardiac preload resulting in a cardiac output decrease. Several authors had identified the prevention of hypotension as a key role during spinal anesthesia although none prophylactic treatment has been identified. Spinal anesthesia single injection (SA) with low dose of local anesthetic or continuous spinal anesthesia with very low dose bolus injections cause fewer episodes of hypotension. Despite these techniques, hypotension can occur. Phenylephrine is an alpha adrenergic agonist and cause vasoconstriction preventing hypotension. Prophylactic phenylephrine infusion for preventing hypotension has been demonstrated during spinal anesthesia for cesarean delivery. The investigators want to assess for the first time the prophylactic phenylephrine infusion for preventing hypotension in elderly patients undergoing orthopedic lower limb surgery under spinal anesthesia single injection.