Managing hypotension

Hypotension (low blood pressure) during anaesthesia is usually defined as a mean arterial pressure of less than 60 mmHg, although some anaesthetists prefer to maintain a higher mean pressure in anaesthetised patients [1]. Tissue blood flow is determined by the pressure gradient from artery to vein, so as arterial pressure rises flow will increase; however, autoregulation (vasoconstriction of resistance vessels in the face of rising perfusion pressure) ensures constant flow over a range of pressures between approximately 55 and 180 mmHg [2]. When arterial pressure falls below this, autoregulation fails and tissue blood flow reduces. This can have fatal consequences especially in horses, where reduced muscle blood flow may cause post-anaesthetic myopathy ...