Phenytoin improves hemodynamic tolerance and survival after severe hypoxia

Three groups of 12 rabbits each, anesthetized with ketamine (30 mg/kg, intramuscularly) and spontaneously breathing N2O-O2, were given intravenous injections of either placebo (group 1), thiopental (TP) (40 mg/kg, intravenously; group 2), or phenytoin (PNT) (15 mg/kg, intravenously; group 3). Four minutes later, succinylcholine was given and, while ventilation was controlled with N2O alone, changes in systolic blood pressure (SBP) and heart rate (HR) were measured. Tachycardia (greater than 210 beats/min) occurred in groups 1, 2, and 3-111, 107, and 151 sec later, respectively. SBP decreased below 100 torr at 189, 192, and 362 sec, respectively, and below 30 torr 245, 216, and 433 sec after the onset of hypoxia (P less than 0.05), respectively. Bradycardia (less than 30 beats/min) appeared 248, 219, and 439 sec (P less than 0.05), respectively. The onset of severe bradycardia and hypotension was significantly (P less than 0.05) delayed by PNT but not by TP. All rabbits in the placebo group died, while 3 and 8 of the animals given TP and PNT, respectively, survived; survival rate was significantly (P less than 0.05) increased by PNT but not by TP. Phenytoin appears to sustain cardiovascular function during hypoxia better than thiopental does, but phenytoin may not be more effective than thiopental in increasing survival.