Hoffman et al. Anesth Analg 2001;93:166-70

Reviewer:  John F. Heath, M.D.

Methods:

• Eight anesthetized dogs
• Brain tissue oxygen pressure (PtO2), middle cerebral artery (MCA) blood flow, and femoral artery/ sagittal sinus blood gases were measured using a cerebral cortical tissue probe, ultrasonic flow probe, femoral artery catheter, and sagittal sinus catheter respectively.
• Measurements were taken at baseline (1.5% isoflurane), then hypotension was induced with either 3% isoflurane (ISO) or sodium nitroprusside (SNP) (20-100 mcg/min) to a target MAP of 60-65 mmHg.
• After 5 min., measurements were repeated.
• A third measurement was taken after IV phenylephrine was infused (0.5-2 mcg/min) to return blood pressure to baseline levels (SNP or 3% ISO maintained at constant levels).
• After one hour of equilibration, each dog received the other hypotensive drug (SNP or 3% ISO), and measurements were again taken with and without phenylephrine.

Results:

• SNP infusion
• unchanged MCA blood flow, venous PO2, and sagittal sinus pH
• however, brain A-V shunting increased 50%, Pt02 decreased 50%, and tissue pH decreased.
• Pt02 returned to baseline levels with blood pressure support using phenylephrine.
• 3% ISO
• unchanged MCA blood flow, venous PO2, and sagittal sinus pH
• also no change from baseline of brain A-V shunting, Pt02, or tissue pH.

• SNP-induced hypotension under 1.5% ISO may produce a greater degree of tissue hypoxia and acidosis compared with 3% ISO.