Adrenoceptor mechanism involved in thiopental-induced potentiation of halothane-epinephrine arrhythmias in dogs.

Although thiopental is known to potentiate halothane-epinephrine arrhythmias, the precise mechanism of this potentiation is obscure. The authors investigated the comparative role of alpha 1 and beta adrenergic actions in the thiopental-induced potentiation of halothane-epinephrine arrhythmias in dogs. Adult mongrel dogs were anesthetized with halothane alone (1.3%) or thiopental (20 mg kg-1) plus halothane and monitored continuously for systemic arterial pressures and for premature ventricular contractions. The arrhythmogenic doses of phenylephrine and isoproterenol were determined during the two anesthetic methods and the effect of thiopental on the arrhythmogenic action of alpha 1 and beta agonists was examined. Thiopental failed to exert a significant potentiation of arrhythmogenic effect of phenylephrine or isoproterenol, when these agents were administered separately. On the other hand, the potentiation of arrhythmogenicity by thiopental was remarkable in the case of combined administration of both the agonists, that is, thiopental enhanced the synergistic interaction between phenylephrine and isoproterenol for inducing arrhythmias during halothane anesthesia. In addition, the potentiation was more prominent when a low dose of isoproterenol and a high dose of phenylephrine was combined than that when a high dose of isoproterenol and a low dose of phenylephrine was given in combination. The results indicate that thiopental significantly potentiates the arrhythmogenic interaction of alpha 1 and beta adrenergic agonists administered concurrently, although individual potentiation of these agonists is not significant.