POTENTIATION BY THIOPENTAL OF CYCLOPROPANE–ADRENALINE CARDIAC ARRHYTHMIAS

Thiopental has been used for induction of anaesthesia in most, experimental and clinical investigations of cyclopropane–adrenaline cardiac arrhythmias. We have compared the effects of adrenaline under cyclopropane (cyclo) anaesthesia in dogs with those observed under thiopental (thio) anaesthesia and thiopental–cyclopropane (thio–cyclo) anaesthesia. Bigeminal rhythm occurring in response to small doses of adrenaline is observed in less than half the animals under cyclo or thio anaesthesia, but is observed in over 90% when the two anaesthetic agents are present. The dose of adrenaline necessary to produce multifocal or ventricular fibrillatory rhythms also appears to be greater under cyclo than under thio–cyclo, but exact quantification is difficult because of the rapid development of tachyphylaxis to larger doses of adrenaline. The duration of the effect of thiopental outlasts by many hours its usual anaesthetic action. Intravenous injections of "subeffective" doses of adrenaline produce bigeminal rhythm if they are preceded by injection of small doses of thiopental into the left circumflex coronary artery. This rarely occurs if thiopental is injected into the left anterior descending coronary artery. Previous evidence has suggested that the bigeminal beat arises in the atrioventricular node or upper bundle of His and that the left circumflex coronary artery supplies this area. It is thus probable that thiopental exerts its action in the upper part of the conduction system. It is concluded that induction of anaesthesia with thiopental plays a considerable role in the "cyclopropane–adrenaline" cardiac arrhythmias described in the literature.