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

Circulatory effects of renin-angiotensin system antagonists during halothane anaesthesia in hypertensive rats

The circulatory effects of captopril, an angiotensin I-converting enzyme inhibitor, and saralasin, a competitive angiotensin II antagonist, were studied during halothane anaesthesia in spontaneously hypertensive (SH) rats. Captopril decreased blood pressure significantly in unanaesthetized rats. Pretreatment with indomethacin, a prostaglandin synthesis inhibitor, did not modify the antihypertensive action of captopril. During 1 MAC halothane anaesthesia, the mean arterial pressure (MAP) in unmedicated SH control rats was maintained at a relatively high level (16.2 +/- 0.7 kPa, mean +/- s.e. mean), while in captopril-treated rats MAP decreased to 8.8 +/- 1.1 kPa. Indomethacin somewhat inhibited MAP decrease in the captopril-medicated group. Saralasin infusion in halothane-anaesthetized rats decreased MAP in the same way as captopril alone. The tolerance to haemorrhagic shock was markedly impaired in rats receiving captopril or saralasin, compared to untreated controls. During halothane anaesthesia, the plasma renin activities in the captopril, captopril + indomethacin, and saralasin groups were significantly higher than in untreated animals. Plasma kininogen was unaffected by any of the medications. The results suggest that the renin-angiotensin system is important in maintaining blood pressure in halothane anaesthesia, and that the tolerance to haemorrhagic shock is particularly impaired by drugs inhibiting the renin-angiotensin system.

The combined effects of antihypertensive drugs and anaesthetics (halothane and ketamine) on the isolated heart

The effects of three concentrations of halothane or ketamine were investigated on isolated rabbit hearts, which were perfused with hydralazine, clonidine, propranolol or methyldopa. In hearts not subjected to the influence of an anaesthetic, clonidine was the only drug stimulating myocardial function. In those perfused with halothane or ketamine alone, both anaesthetics exerted a negative chronotropic and inotropic action in a dose-related manner. Ketamine markedly increased the coronary flow. Clonidine distinctly reduced the myocardial depression caused by halothane or ketamine. Hydralazine had no marked effects with either of these anaesthetics, except that it sensitized the hearts to the arrhythmic action of a high concentration of halothane. Propranolol, when combined with halothane, aggravated myocardial depression and decreased coronary flow. With ketamine, propranolol caused no other harmful interactions, apart from inhibiting the increase in coronary flow caused by this anaesthetic. Methyldopa intensified the myocardial depression induced by halothane, but tended to diminish that caused by ketamine. The results suggest that clonidine has a stimulatory cardiac action when combined with either of these anaesthetics. Disadvantageous interactions may exist between methyldopa or propranolol and halothane.

Effects of antihypertensive medication on the cardiovascular response to ketamine in rats

The effects of hydralazine, clonidine, propranolol and methyldopa medications on the cardiovascular response to ketamine anaesthesia were studied in spontaneously hypertensive (SH) rats. Except in propranolol- and methyldopa-treated animals, blood pressure was increased by ketamine in all groups. The heart rate was increased only in the normotensive control rats, whereas it was reduced in both the unmedicated and all of the medicated SH rats. Plasma renin activity under ketamine anaesthesia was markedly higher in normotensive control rats than in any of the SH groups. The pressor responses to dopamine and metaraminol were not altered by propranolol medication, but hydralazine suppressed these responses, while they were markedly intensified by clonidine and methyldopa treatment. Similarly, small additional doses of ketamine caused a pressor response only in clonidine- and methyldopa-treated animals, whereas in the other groups the response was depressive. The tolerance to haemorrhagic shock during ketamine anaesthesia was notably improved by hydralazine and methyldopa medication, but impaired by propranolol treatment. The results suggest that, with special reference to shock tolerance, hydralazine and methyldopa, and to a lesser extent also clonidine, have a favourable effect on the circulation under ketamine anaesthesia, whereas the action of beta-blockade may be harmful.