The effects of antihypertensive medication on the control of the cardiovascular system during halothane anaesthesia in rats.

The effects of hydralazine, clonidine, propranolol and methyldopa medication on the control of the circulatory system during halothane anaesthesia were studied in spontaneously hypertensive (SH) rats. Special attention was directed to the problem of circulatory emergencies. Under 1 and 3% halothane anaesthesia, the mean arterial pressure was lowest in methyldopa-treated rats. During 3% anaesthesia, plasma renin activity was markedly increased in the methyldopa group and decreased in the propranolol group. Hydralazine medication suppressed the pressor responses to dopamine and metaraminol, whereas clonidine, propranolol and methylopa increased the response to dopamine. These sympathetic agents induced more cardiac arrhythmias in SH controls than in normotensive ones. These arrhythmias were antagonized by hydralazine. The SH controls also tolerated haemorrhagic shock more poorly than did normotensive control rats. Among the pretreated animals, tolerance to this shock was highest in hydralazine-and clonidine-treated animals and lowest in the methyldopa group. The results suggest that during halothane anaesthesia SH rats are more prone to disturbances in the control of circulation than are normotensive controls. Hydralazine and, to a lesser extent, clonidine have a protective action against these disturbances, but the effect of methyldopa seems to be disadvantageous.