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Intrathecal and to a lesser extent epidural local anaesthetics block the sympathetic nervous system, resulting in vasodilatation and hypotension. Usually there is a compensatory tachycardia associated with a fall in diastolic pressure. Subsequently, systolic blood pressure falls earlier than would occur during actual blood loss.

This sympathetic blockade exacerbates other simultaneous causes of hypotension such as haemorrhage and results in earlier decompensation.

A ‘high’ spinal will also affect the sympathetic nerves controlling the heart rate, causing a bradycardia and profound hypotension.