Thiopental is well known ultra-short acting anesthetic agent and the pharmacological actions of the drug is extensively studied. However, still there are some effects of the thiobarbiturate particulary on cardiovascular and on circulatory systems uncertain, even though deep interests are focuced by many workers on the actions of the drug to the systems up to date.
.Development of cardiac arrhythmias by the intravenous administration of thiopental was reported by a few laboratories, however, there are only vague knowledge of the thiopental as a possible inducer of the cardiac arrhythmias.
Present study was. carried out to pursue the characteristic and the nature of the cardiac arrhythmia induced by thiopental, on 22 mongrel dogs. Thiopental, 8-10 mg/kg, wasadministered via femoral vein at the rate of 2-2.5 mg/kg/sec. Electrocardiogram femoral arteriogram and/or left ventricular pressure curves were monitored through-out the experiments.
The induced cardiac arrhythmias were analyzed and possible mechanism of development of the cardiac arrhythmia were discussed as follows:
1. The incidence of the arrhythmia development was 72.8% on 22 dogs and mean latent time, time of appearence of the arrhythmia after the administration of the thiopental, and duration of the arrhythmia were 59.5 seconds and 11.2 minutes respectively
2. Almost all arrhythmia induced by thiopental revealed regular sinus rhythm with alternating ventricular extrasystols forming bigeminy. The bigeminy on E C G appeared as a pattern of pulsus alternans on the femoral arteriogram.
3. Intravenous administration of propranolol, 10-20 mg/dog, abolished the thiopental induced arrhythmias in 92.3% on 13 dogs. Furthermore, it was unable to induce arrhythmia by repeated doses of thiopental while the propranolol remains effective in the dogs.
4. As a possible mechanism of the arrhythmias induced by thiopental, the prolongation of relative refractory period of the myocardium and develop ment of hyper-excitable ectopic focus by the administration of thiopental are suggested. Possibly, recving re-entered impulse of spread normal sinus ?? the ectopic focus might generate premature ventricular beat regularly after the sinus rhythm. It was assumed that abolition of the thiopental induced arrhythmia by the propranolol administration is due to depression of the excitability and prolongation of the contractile duration of the myocardium by effects of the drug.
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