Intraoperative hypotension - update on pathophysiology and clinical implications

The incidence of intraoperative hypotension is high. An evidence-based definition of intraoperative hypotension, however, is still a matter of debate. Major risk factors include patient age, an ASA physical status 3 or higher, a given combination of regional and general anaesthesia, the duration of surgery, and the emergency status. The haemodynamic significance of intraoperative hypotension is related to the fact that the cerebral, renal and the myocardial blood flow and its autoregulation depend on perfusion pressure. The mean arterial pressure (MAP) seems more appropriate for intraoperative haemodynamic control than the systolic arterial pressure (SAP), since pulse pressure and SAP strongly depend on stroke volume and arterial elastance. Based on physiological considerations and several observational studies, a lower limit of 60 mm Hg is generally accepted in patients without risk factors. However, in patients with arterial hypertension a relative decrease in MAP by >30% should be assumed as the lower threshold limit for haemodynamic intervention. Similarly, impaired autoregulation, a significant arterial stenosis or specific problems associated with intraoperative beach-chair position increase the lower limit of MAP which is necessary to ensure an adequate organ blood flow. Several retrospective studies comprising large patient cohorts demonstrated that intraoperative hypotension is associated with an increased 1-year mortality. A causal relationship, however, has not yet been verified. Treatment of intraoperative hypotension should not only rely on vasoactive agents to control the decreased systemic vascular resistance, but should also focus on other causes, which may include hypovolaemia, a redistribution of the blood volume, or impaired myocardial performance.