CARDIORESPIRATORY CHANGES DURING MICROWAVE-INDUCED LETHAL HEAT-STRESSAND BETA-ADRENERGIC-BLOCKADE

Ketamine-anesthetized Sprague-Dawley rats were exposed to 2,450-MHz mi crowaves at an average power density of 60 mW/cm(2) (whole body specif ic absorption rate of similar to 14 W/kg) until lethal temperatures we re attained. The effects of propranolol (2 or 10 mg/kg body wt), nadol ol (10 mg/kg), and labetalol (10 mg/kg) on physiological responses (in cluding changes in body temperature, heart rate, blood pressure, and r espiratory rate) were examined. Lethal temperatures in the labetalol a nd both propranolol groups were significantly lower than in saline con trols. Survival time was significantly less only in the high-dose prop ranolol group. In all groups, heart rate increased continuously during exposure; blood pressure increased until colonic temperature reached 41-41.5 degrees C and then decreased. These heart rate and blood press ure changes were similar to those that occur during environmental heat stress. Heart rate and blood pressure changes among groups were simil ar. Respiratory rate, however, was significantly elevated during most of the exposure period in the high-dose propranolol animals. This chan ge in respiration, coupled with the significantly lower survival time in these animals, suggests a vital role of respiration in susceptibili ty to microwave-induced heating.