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We try to better understand the mechanisms underlying defibrillation. A major unresolved question is how defibrillation shocks manage to eliminate reentrant activity from the deeper layers of the myocardium in which the shock effects should be small according to common theory. Figure 1 shows how a shock can indirectly eliminate reentrant activity even from the center of a preparation. A shock causes an initially straight filament to detach from the surfaces of the preparation. Afterwards, the filament assumes a new shape that is in certain situations unstable, so that the scroll wave eventually disappears.

In experiments, we found that defibrillation may be effective in the deeper layers of the myocardium for an even simpler reason: That the polarization caused by a shock deep inside the heart is significantly larger than predicted by current theories. When applying shocks that are barely strong enough to initiate excitation, we found that these shocks do not excite the cathodal surface of the preparation as we had expected, but instead excited tissue deep inside the cardiac wall. Figure 2 shows the activation following such a barely exciting shock: only after a significant delay the surfaces are excited.

Scroll wave
Figure 1. Scroll wave after the application of a defibrillation shock. Activated tissue is shown in yellow, resting tissue is transparent. The red line is the organizing center or filament of the scroll wave. Its tips have been detached from the surfaces to which the shock was applied.
Activation maps
Figure 2. Activation maps of anodal and cathodal surface following a shock of near-threshold amplitude. The significant activation delays on both surfaces and the fact that both delays are of similar order suggest that the initial excitation occurred inside the cardiac wall.