The primary objective of the study is to compare two strategies of intraoperative blood pressure management in high-risk surgical patients: 1- Continuous infusion of noradrenaline to maintain arterial blood pressure of no more than 10% below its baseline value; 2- Conventional treatment of hypotension (defined as a blood pressure of below 80 mmHg or a decrease of more than 40% from baseline) using intravenous bolus of Ephedrine chorhydrate.