Mechanism of ventricular fibrillation in hypothermia.

• Two general theories of cardiac fibrillation currently prevail: the circus movement theory of Garrey and Lewis, and the ectopic focus theory of Sherf and Prinzmetal. Surprisingly, little systematic work has been done to establish whether either of these theories applies to the cardiac fibrillation that occurs spontaneously when mammals are cooled. The circus movement theory postulates repetitive re-excitation of cardiac tissue by a cardiac impulse re-entering an original area of excitation. As stated by Lewis, three factors are important to the development of circus movement fibrillation: (1) the length of the conduction pathway, (2) the conduction velocity, and (3) the duration of the refractory period. A relatively long conduction pathway, a slow conduction velocity, and a short refractory period would favor fibrillation. Any one or a combination of these events would allow re-entry of the cardiac impulse into the original area of excitation after it is no longer refractory. Myocardial threshold need not be altered. By contrast, the ectopic focus theory postulates repetitive excitation arising spontaneously in hypei'irritable foci. Thus, either a general or local decrease in myocardial threshold should be present prior to the onset of this type of fibrillation. Length of conduction path, conduction velocity, and duration of refractoriness would not affect ectopic focus fibrillation in a predictable way. Previous studies have indicated that during progressive hypothermia there is either no change or