Brain Retractor Edema during Induced Hypotension: The Effect of the Rate of Return of Blood Pressure

Abstract

This study evaluated the hypothesis that the postoperative formation of cerebral edema may be influenced by the rate of blood pressure return after induced hypotension in a graded brain retractor injury. Nineteen cats underwent unilateral craniotomy, isoflurane-induced hypotension to a mean of 50 mm Hg, and application of a brain retractor at 20 mm Hg of pressure for 1 hour. Blood pressure was returned to normal either within 3 minutes or over 20 minutes. The degree of cerebral edema formation was determined by Evans blue dye and coronal magnetic resonance imaging. All animals showed extravasation of Evans blue dye in the retracted hemisphere that was most marked at the periphery of the retractor. T1 relaxation times were significantly prolonged in the retracted hemispheres of both the fast return and slow return groups (18.8% and 17.8%, respectively) and more so at the Evans blue sites (42.8% and 40.8%), although not so strikingly beneath the retractor itself (6.3% and 7.8%). T2 relaxation times were similarly prolonged but to approximately half the degree of the T1 times. In the nonretracted hemisphere, drug-induced hypotension alone did not result in significant acute cerebral edema or blood-brain barrier alteration. There was no significant difference between the fast and slow groups in Evans blue extravasation or magnetic resonance changes. Thus, in a retractor-induced brain injury, restoration of arterial pressure to normal either gradually or rapidly did not influence the degree or extent of edema formation.