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NEUROSURGICAL ANESTHESIA

Sodium Nitroprusside Compared with Isoflurane-Induced Hypotension: The Effects on Brain Oxygenation and Arteriovenous Shunting

Department of Anesthesiology, University of Illinois at Chicago, Chicago, Illinois

Address correspondence and reprint requests to William E. Hoffman, PhD, Department of Anesthesiology, M/C 515, University of Illinois at Chicago, 1740 W. Taylor St., Chicago, IL 60612. Address e-mail to whoffman{at}uic.edu

We compared sodium nitroprusside (SNP)-induced hypotension with 3% isoflurane-induced hypotension with regard to brain tissue oxygen pressure (PtO₂), middle cerebral artery (MCA) blood flow, and cerebral arteriovenous shunting. Eight dogs were anesthetized with 1.5% isoflurane. After a craniotomy, a probe was inserted into the left frontoparietal brain cortex to mea-sure tissue gases and pH. Blood flow was measured in a secondary branch of the MCA by a flowprobe. Mea-surements were made during baseline 1.5% isoflurane, during 1.5% isoflurane and SNP-induced hypotension or 3% isoflurane-induced hypotension to a mean pressure of 60–65 mm Hg, and during continued treatment with SNP or 3% isoflurane with blood pressure support to baseline levels with phenylephrine. Shunting was calculated from arterial, sagittal sinus, and tissue (indicating capillary) oxygen content. During hypotension with SNP, PtO₂ decreased 50%, and shunting increased 50%. During hypotension with 3% isoflurane, PtO₂ and shunting did not change. Blood pressure support increased PtO₂ and MCA flow during both SNP and 3% isoflurane treatment. These results show that SNP is a cerebrovasodilator but that hypotension will decrease PtO₂, probably because of an increase in arteriovenous shunting and a decrease in capillary perfusion.

Implications: We measured brain arteriovenous shunting and tissue oxygen pressure(PtO₂)during a 40% decrease in blood pressure induced by sodium nitroprusside (SNP)or 3% isoflurane. Large-dose isoflurane maintainedPtO₂ withno change in shunting. SNP infusion decreasedPtO₂ 50%and increased shunting 50%. This suggests that SNP-induced hypotensiondecreasesPtO₂because of a decrease in capillaryperfusion.