The circus movement theory explains hypothermia-induced ventricular fibrillation, highlighting the critical role of decreased conduction velocity and heart size.
The mechanism of ventricular fibrillation in hypothermia is best explained by the circus movement theory. The main factor responsible for the initiation of a circus movement type of fibrillation is the marked reduction in conduction velocity which is not counterbalanced by a proportional prolongation of the refractory period, in other words, an increase in the conduction time/refractory period ratio occurs. Maintenance of a fibrillary state in hypothermia is dependent also on the size of the heart. Small hearts either fail to fibrillate or show a spontaneously reversible type of fibrillation. Moreover, shortening the conduction pathway by cutting a fibrillating heart will abolish the arrhythmia. Sympathomimetic amines which increase conduction velocity and so reduce the conduction time/refractory period ratio are also capable of decreasing the incidence of fibrillation. The sequence of events which may lead to development of ventricular fibrillation in hypothermia is discussed.
Covino et al. (1962) studied this question.