Prof. Monty MYTHEN
Professor of Anaesthesia and Critical Care, Department of Perioperative Medicine, University College of London, United Kingdom
Professor Michael (Monty) G Mythen
MB BS MD FRCA FFICM FCAI (Hon)
Smiths Medical Professor of Anaesthesia and Critical Care at University College London.
Chair, Evidence Based Perioperative Medicine (EBPOM). Founding Editor-in-Chief of Perioperative Medicine; Editorial Board British Journal of Anaesthesia.
Founding Editor-in-Chief TopMedTalk.
Founding Board Member Perioperative Quality Initiative (POQI)
Co-President International Board of Perioperative Medicine
Intraoperative hypotension is a common occurrence in patients undergoing general anaesthesia for major non-cardiac surgery. Whilst the incidence of hypotension is a function of the definition, significant amounts of hypotensive load are seen at levels that are associated with an increased risk of patient harm. In subjects undergoing predominately major abdominal surgery, management of intraoperative blood pressure using a noradrenaline infusion to maintain systolic blood pressure within 10% of baseline was associated with less post-operative organ dysfunction. However, all subjects also underwent optimal fluid loading prior to receiving vasopressor infusions. Approximately two thirds of blood volume reside in the venous circulation, and this can be divided into two functional volumes, the unstressed and stressed volumes, the latter which determines mean systemic filling pressure returning blood to the heart. The unstressed volume acts as a reservoir that can be recruited to maintain stressed volume and hence maintain central circulation volume in hypovolaemic states. Vasopressors can increase venous tone changing the relative ratio of these compartments, decreasing unstressed volume and increasing stressed volume to maintain or increase stroke volume and cardiac output. The splanchnic circulation is one of the largest blood reservoirs and hence this redistribution of volume may compromise intestinal blood flow, especially in covert hypovolaemic states, where administration of a vasopressor may mask changes in commonly measured haemodynamic parameters. Intestinal mucosal hypoperfusion has clinical sequelae and has been associated with increased post-operative complications, costs and subsequent mucosal cell damage leading to serotonin release, resulting in nausea and vomiting.
Reduced perfusion of the gut is an early manifestation of hypovolaemia. This is associated with post-operative complications and poor patient outcomes through ischaemic damage to the mucosa.