Simple hypotension may be defined as a systolic BP below 85 mmHg for healthy patients who are otherwise undergoing an uncomplicated anaesthetic. If there are associated symptoms, such as rash or bronchospasm, then anaphylaxis should be suspected. For patients with significant co-morbidity, especially cardiovascular disease, significant hypotension may be present at a higher systolic BP.

Hypotension is a common side-effect of both IV and volatile agents. It is most common during induction of anaesthesia and is proportional to the amount of anaesthetic agent used; 2 MAC of isoflurane produces more pronounced hypotension than 1 MAC.

Regional anaesthesia, either spinal or epidural, may also cause hypotension.