Phenylephrine increases cerebral blood flow during low-flow hypothermic cardiopulmonary bypass in baboons

Background: Although low-flow cardiopulmonary bypass (CPB) has become a preferred technique for the surgical repair of complex cardiac lesions in children, the relative hypotension and decrease in cerebral blood flow (CBF) associated with low flow may contribute to the occurrence of postoperative neurologic injury. Therefore, it was determined whether phenylephrine administered to increase arterial blood pressure during low-flow CPB increases CBF. Methods: Cardiopulmonary bypass was initiated in seven baboons during fentanyl, midazolam, and isoflurane anesthesia. Animals were cooled at a pump flow rate of 2.5 1 · min^-1 · m^-2 until esophageal temperature decreased to 20°C. Cardiopulmonary bypass flow was then reduced to 0.5 1 · min^-1 · m^-2 (low flow). During low-flow CPB, arterial partial pressure of carbon dioxide (P(CO₂)) and blood pressure were varied in random sequence to three conditions: (1) P(CO₂) 3039 mmHg (uncorrected for temperature), control blood pressure; (2) P(CO₂) 50-60 mmHg, control blood pressure; and (3) P(CO₂) 30-39 mmHg, blood pressure raised to twice control by phenylephrine infusion. Thereafter, CPB flow was increased to 2.5 l · min^-1 m^-2, and baboons were rewarmed to normal temperature. Cerebral blood flow was measured by washout of intraarterial ¹³³Xe before and during CPB. Results: Phenylephrine administered to increase mean blood pressure from 23 ± 3 to 46 ± 3 mmHg during low-flow CPB increased CBF from 14 ± 3 to 31 ± 9 ml · min^-1 · 100 g^-1, P < 0.05. Changes in arterial P(CO₂) alone during low flow bypass produced no changes in CBF. Conclusion: Although low-flow CPB resulted in a marked decrease in CBF compared with prebypass and full-flow bypass, phenylephrine administered to double arterial pressure during low-flow bypass produced a proportional increase in CBF.