THE EFFECTS OF PHENOXYBENZAMINE ON IMMEDIATE-EARLY GENE EXPRESSION AFTER HYPOTHERMIC CIRCULATORY ARREST

Abstract

Introduction. Cardiopulmonary bypass (CPB) and hypothermic circulatory arrest (HCA) can cause significant neurologic complications in infants, including seizures, developmental delays, choreoathetosis, and diminished intelligence. Brain injury is multifactorial, but the pathophysiology is due to diminished substrate supply to neurons and the excessive release of excitatory amino acids within the brain. [1,2] Our institution routinely uses the long-acting alpha-blocker, phenoxybenzamine (PB), in all neonates and infants receiving blood primes. This study evaluated the effects of PB on postoperative metabolic acidosis and the expression of the ischemia/stress-associated immediate-early gene, c-fos, in neurons of neonatal lambs subjected to CPB and HCA. Methods. Neonatal lambs (4-6 kg) anesthetized with isoflurane were cannulated for CPB via the right atrium and ascending aorta. Group A (n=12) received saline vehicle and Group B (n=6) received PB, 1 mg/kg iv, prior to CPB. The animals were cooled to 16[degree sign]C and subjected to 90 or 120 mins of HCA. CPB was resumed, the animals were rewarmed to 38[degree sign]C, and weaned from CPB. Arterial blood gas samples were obtained before and after CPB. One hour after terminating CPB, the brains were perfusion-fixed, removed, and analyzed for Fos-related proteins using immunohistochemistry techniques. Neurons positive for intranuclear Fos were identified and quantitated within the subregions of the hippocampal formation, the area of the brain most vulnerable to ischemic injury after CPB and HCA. Results. All animals survived the CPB and HCA. Cooling times and mean blood pressure on bypass were the same in both groups. Pump flows on CPB were significantly higher (p < 0.01) at 20[degree sign]C in PB-treated animals averaging 200 ml/kg compared to 75 ml/kg in vehicle-treated animals. PB treated animals had significantly less (p < 0.05) metabolic acidosis than Group A animals 15 min after weaning from CPB: base excess -2.5 +/- 2 vs. -7.2 +/- 3, respectively. Fos related proteins were expressed in neurons throughout the hippocampal formation after 90 and 120 min of HCA in Group A. PB significantly reduced Fos protein expression in CA1 and CA3 neurons after 90 and 120 min HCA. Discussion. PB facilitated higher flows at the same mean blood pressure on CPB at all temperatures. PB-treated animals had significantly less metabolic acidosis following CPB and HCA and decreased expression of immediate-early genes associated with cellular stress. We speculate that PB increases blood flow in the cerebral microcirculation maintaining substrate supply more effectively during hypothermic CPB and enhances the restoration of cerebral blood flow after HCA.