Mechanism of ventricular defibrillation
A summary is presented of findings about the mechanisms by which a shock succeeds or fails in halting ventricular fibrillation. It is shown that earliest activations following a subthreshold defibrillation shock occur in regions of low potential gradient generated by the shock. Activation fronts after subthreshold shocks are not continuations of fronts present just before the shock. An upper limit exists to the strength of shocks that induce fibrillation during the vulnerable period of regular rhythm and correlates with the defibrillation threshold. The response to shocks during regular rhythm just below the upper limit of vulnerability is similar to the response to subthreshold defibrillation shocks. To defibrillate, a shock must not only halt the activation fronts of fibrillation, but it must also not give rise to new activation fronts that reinduce fibrillation.
