ADRENERGIC MECHANISMS AFFECTING CIRCULATORY DYNAMICS IN HYPOTHERMIA.

Abstract : Plasma norepinephrine (NE) concentrations were found to increase in dogs maintained at moderate hypothermic temperatures (25C plus or minus 0.5C). By contrast, tissue NE stores in various parts of the heart and in order organs were not altered significantly when examined after animals were maintained in hypothermia and compared to anesthetized normothermic controls maintained for the same time. Similarly, isolated preparations of rabbit atria maintained at normothermic and hypothermic temperatures showed no significant differences in their NE stores over the first 2 hours of observation. These results indicate that although there may be an increase in NE turnover in hypothermia, as suggested by the increase in plasma NE, the normal balance between synthesis and catabolism of NE is maintained during hypothermia. Infusions of NE reduced the incidence of ventricular fibrillation and the mean lethal temperatures. This was not found with epinephrine infusions. The cardiovascular responses to NE were altered by hypothermia. Both pressor and maximal inotropic responses of injected NE were found to be reduced at hypothermic temperatures. An even greater reduction was found in the maximal chronotropic and inotropic responses following cardiac sympathetic nerve stimulation. These results indicate that effector responsiveness and probably also the release of the adrenergic neurohumor are reduced by hypothermia. This could account for the progressive cardiovascular deterioration observed in prolonged hypothermia. (Author)