Dissociation of the pressor and cerebral protective effects of phenylephrine

The relationship between the pressor and cerebral protective effects of the alpha-adrenergic agonist phenylephrine was assessed. Sprague-Dawley rats were anesthetized and underwent right carotid artery ligation and placement of arterial and central venous pressure catheters and EEG electrodes. Following a two-hour recovery, the rats were exposed to hypoxia (4.5% chamber O2). Three groups of rats were infused with phenylephrine hydrochloride at 17, 35, and 88 micrograms/kg/min, respectively. A fourth group received normal saline, and a fifth, no infusion. All rats receiving phenylephrine experienced an initial pressor response to 130, 162, and 180 mm Hg, respectively, compared with control animals (119 mm Hg). Cerebral electrical activity was prolonged with the two lower doses of phenylephrine (24 +/- 3 minutes and 21 +/- 4 minutes, respectively), compared with the control response (13 +/- 1 minutes). High-dose phenylephrine decreased survival time (8 +/- 1 minutes), despite higher BP at the time of brain death.