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Cushing's 'variant' response (acute hypotension) after subarachnoid hemorrhage. Association with moderate intracranial tensions and subacute cardiovascular collapse.

BACKGROUND AND PURPOSE
Hypertension is considered common and appropriate with subarachnoid hemorrhage (SAH), maintaining cerebral perfusion. Hypotension, in contrast, is considered rare and detrimental. This study was designed to assess the frequency of each in both acute and subacute phases of primary SAH.


METHODS
SAH was created by arterial rupture in spontaneously breathing rats under urethane anesthesia without craniotomy (n = 32). Arterial pressure and intracranial pressure (ICP) were monitored invasively.


RESULTS
After extensive extravasation, the mean ICP rose acutely from 8 +/- 1 to 53 +/- 4 mm Hg over 2.4 +/- 0.3 minutes. Acute pressor changes occurred transiently in 71%. The most common acute response was hypotension (63%). Hypertension, in contrast, was rare (6%); the remainder was invariant (29%). Hypertension was associated with significantly lower maximum ICP values (39 +/- 4 versus 69 +/- 4 mm Hg, P < .001) with a negative correlation between hypotension and delta ICP (r = -.7, P < .01). Distinct and independent of acute responses, hypotension also occurred subacutely as a cardiovascular collapse (38%).


CONCLUSIONS
In contrast to popular belief, the most common acute response with SAH is hypotension; hypertension is rare. This, in fact, is in full agreement with Cushing: hypertension was seen only with gradual delta ICPs. In contrast, a "variant" to the classic response (hypotension) occurred with sudden delta ICPs. In the present study, hypotension stanched SAH at lower maximum ICP values, and thus with less cerebral compression. Despite this, cardiovascular collapse developed in a large proportion irrespective of acute change. Such collapse without prior hypertension (94%) implies a nonadrenergic etiology.