Sodium nitroprusside compared with isoflurane-induced hypotension: The effects on brain oxygenation and arteriovenous shunting

We compared sodium nitroprusside (SNP)-induced hypotension with 3% isoflurane-induced hypotension with regard to brain tissue oxygen pressure (Pto(2)), 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 measure tissue gases and pH. Blood flow was measured in a secondary branch of the MCA by a flowprobe. Measurements 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, Pto(2) decreased 50%, and shunting increased 50%. During hypotension with 3% isoflurane, Pto(2) and shunting did not change. Blood pressure support increased Pto(2) and MCA flow during both SNP and 3% isoflurane treatment. These results show that SNP is a cerebrovasodilator but that hypotension will decrease Pto(2), probably because of an increase in arteriovenous shunting and a decrease in capillary perfusion.