Caring for critically ill patients with a subarachnoid haemorrhage and preventing its most prevalent and devastating complication, vasospasm, requires an in-depth understanding of the mechanisms which underpin the physiology of SAH. This is essential to provide appropriate nursing practice derived from theory. All too often practitioners are asked to follow unsubstantiated regimes without question of the origins of practices. This paper approaches the physiological theory underpinning the mechanisms surrounding subarachnoid haemorrhage and the altered cerebral and extracerebral dysfunction which can occur. Physiological theory is analysed to generate nursing interventions which may be individually tailored to provide comprehensive nursing care with a sound underpinning to its practice. The foundations of effective management of SAH lies within prevention, early diagnosis, and correction of complications [Neurosurg. Clin. North Am. 9 (3) (1998) 595]. In order for such identification to take place, it is essential to have an understanding of the physiological theory that underpins the basis of care interventions. These interventions should compliment all other theoretical input that influences patient care and nursing practice, contributing to a holistic, dynamically formulated plan of care.