Simulation of protective zones during quatrefoil reentry in cardiac tissue

An S3 stimulus can exert a protective effect by terminating reentry induced by an S2 stimulus. Our goal was to examine the mechanism by which an S3 pulse terminates reentry and the role that virtual electrodes and break excitation play in this process.

In our simulation, the bidomain model represents the electrical properties of the tissue and the Beeler-Reuter model represents the membrane kinetics. Quatrefoil reentry is initiated by S1-S2 stimulation, and then a third stimulus S3 is applied at different intervals after S2. All stimuli are applied through the same unipolar electrode. For some S2-S3 intervals, the S2 and S3 wavefronts interact destructively, terminating reentry (protective zones). For other S2-S3 intervals, S2 and S3 wavefronts interact constructively, and reentry continues. Protective zones appear recurrently, with approximately the period of the S2 reentrant circuit. The protective zones are wider for anodal stimulation than for cathodal stimulation.

Virtual electrodes, break excitation, and S2-S3 timing all play important roles in determining the electrical response of the tissue.