Excito-oscillatory dynamics as a mechanism of ventricular fibrillation

Background

The instabilities associated with reentrant spiral waves are of paramount importance to the initiation and maintenance of tachyarrhythmias, especially ventricular fibrillation (VF). In addition to tissue heterogeneities, there are only a few basic purported mechanisms of spiral wave breakup, most notably restitution.

Objective

We test the hypothesis that oscillatory membrane properties act to destabilize spiral waves.

Methods

We recorded transmembrane potential (V_m) from isolated rabbit myocytes using a constant current stimulation protocol. We developed a mathematical model that included both the stable excitable equilibrium point at resting V_m (−80 mV) and the unstable oscillatory equilibrium point at elevated V_m (−10 mV). Spiral wave dynamics were studied in 2-dimensional grids using variants of the model.

Results

All models showed restitution and reproduced the experimental values of transmembrane resistance at rest and during the action potential plateau. Stable spiral waves were observed when the model showed only 1 equilibrium point. However, spatio-temporal complexity was observed if the model showed both excitable and oscillatory equilibrium points (i.e., excito-oscillatory models). The initial wave breaks resulted from oscillatory waves expanding in all directions; after a few beats, the patterns were characterized by a combination of unstable spiral waves and target patterns consistent with the patterns observed on the heart surface during VF. In our model, this VF-like activity only occurred when the single cell period of V_m oscillations was within a specific range.

Conclusion

The VF-like patterns observed in our excito-oscillatory models could not be explained by the existing proposed instability mechanisms. Our results introduce the important suggestion that membrane dynamics responsible for V_m oscillations at elevated V_m levels can destabilize spiral waves and thus may be a novel therapeutic target for preventing VF.

Keywords: Fibrillation, Myocyte, Restitution, Reentry, Computer Model