Cardiorespiratory changes during microwave-induced lethal heat stress and beta-adrenergic blockade.

Ketamine-anesthetized Sprague-Dawley rats were exposed to 2,450-MHz microwaves at an average power density of 60 mW/cm2 (whole body specific absorption rate of approximately 14 W/kg) until lethal temperatures were attained. The effects of propranolol (2 or 10 mg/kg body wt), nadolol (10 mg/kg), and labetalol (10 mg/kg) on physiological responses (including changes in body temperature, heart rate, blood pressure, and respiratory rate) were examined. Lethal temperatures in the labetalol and both propranolol groups were significantly lower than in saline controls. Survival time was significantly less only in the high-dose propranolol 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 pressure changes were similar to those that occur during environmental heat stress. Heart rate and blood pressure changes among groups were similar. Respiratory rate, however, was significantly elevated during most of the exposure period in the high-dose propranolol animals. This change in respiration, coupled with the significantly lower survival time in these animals, suggests a vital role of respiration in susceptibility to microwave-induced heating.