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.

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