Changes in heart rate associated with high-pressure convulsions in rodents.

The effect of compression in heliox atmospheres on heart rate in mice and in rats has been explored. In the absence of high-pressure neurological syndrome (NPNS) convulsions, the general relations between heart rate and pressure in mice from 14 days old to adulthood, as well as in 6- to 8-day-old rats, resemble each other, and also the results reported by others for liquid-breathing mice. Rats, 29 days old and adult, are different in that little bradycardia is observed. Type I (clonic) HPNS seizures are not associated with any additional changes in heart rate. Type II (tonic) seizures are invariably associated with profound transient bradycardia, recovery from which begins about the time the tonic seizure phase ends. The seizure-associated bradycardia can be abolished by atropine pretreatment. Data concerning relation of seizure-associated mortality, drug effects, and age and species differences are presented. The bearing of the results on the questions of seizure types in different species, neuroanatomical bases for HPNS seizures, and high-pressure death are discussed.