COMPARATIVE EFFECTS OF ANESTHETIC AGENTS ON CARDIAC IRRITABILITY DURING HYPOTHERMIA

Factors affecting the incidence of ventricular fibrillation and asystole in hypothermia were studied in dogs. Four types of anesthesia were used and the responses of the heart to mechanical stimulation, hypercapnia, and hypoxia were observed. Temperature was found to be the most important factor, for fibrillation did not occur in 40 experiments at 27 C but was frequent in 50 experiments at 20 C. At the lower temperatures fibrillation was most often seen during cyclopropane-induced anesthesia and least often during anesthesia induced by thiopental-nitrous oxide supplemented with lidocaine. When the lidocaine was goven simultaneously with pentothal, subsequent induction of hypothermia to 20 C in 10 dogs resulted in spontaneous fibrillation in 4 and asystole in 1; in another series of 10 dogs, when lidocaine was given after hypothermia to 20 C had been induced, spontaneous fibrillation occurred in only 1, while asystole occurred in 4. No criteria were found that would distinguish hearts liable to fibrillation from hearts liable to asystole. Although the incidence of fibrillation was reduced with the use of lidocaine, there was an increase in heart block and asystole.