Experiments with the rabbit heart show that periodic driving of the heart with voltage pulses (5-20 Hz, ~30 V) applied in the bathing solution results in steady periodic patterns of depolarization on the heart surface – standing waves (Gray et al. 2001). Such waves stop all propagating activity in the cardiac tissue, including re-entry and fibrillation. Extension of the bidomain model for the intra- and extracellular spaces of the tissue to include the bathing solution – originally called the ‘tridomain’ model – allows simulation of the standing waves in two dimensions (Aslanidi et al. 2002). Similar to the experiments, standing waves induced by periodic driving of the ‘tridomain’ virtual ventricular tissue have eliminated re-entry in form of a single stably rotating spiral wave, thus providing a defibrillatory effect. Our aim is to show the importance of the bath in such defibrillation. We use the Luo-Rudy family of virtual ventricular tissues (Aslanidi et al. 2001) extended to include external solution bathing the tissue. Spiral waves are first initiated in 3 X 3 cm square bidomain virtual tissue by the phase distribution method, then after 200 ms of simulation external forcing is applied locally along two opposite edges of the tissue. For the bidomain model without a bath, rotation of a spiral wave is stable both before and after the stimuli application. In the bidomain with the bath, there is a threshold driving strength (~12 V), below which the spiral wave rotation is also stable with respect to the stimuli. However, increasing the driving strength above the threshold leads to formation of standing waves in the tissue, which eliminate the spiral wave in agreement with the experimental results by Gray et al. (2001). Measurements of the current flowing from the external solution into the tissue during driving show that it is non-localized, whereas in the absence of the bath only edges of the tissue adjacent to the electrodes are effected by external voltage. The bathing solution redistributes the current flowing from the electrodes, thus mediating the effects of the external driving to the tissue and allowing ‘defibrillation’.

This research was supported by the MRC.