Simulating T-wave parameters of local extracellular electrograms with a whole-heart bidomain reaction-diffusion model: Size matters!

As a measure of local repolarization time (T_R), the instant of maximum slope (T_up) of the T wave in the local unipolar electrogram is commonly used. Measurement of T_up can be difficult, especially in positive T waves. These difficulties have led some researchers to propose the instant of maximum downslope (_Tdown) as a marker of T_R when the T wave is positive. To improve understanding of T-wave parameters, we simulated electrograms with a bidomain model of the human heart. To test T-wave parameters, we compared them to T_R determined from the local membrane potential. We propose a simple model of the electrogram, which we validated by comparison to the bidomain model. With the simple model, it is straightforward to show that the sign of the T wave is almost uniquely determined by T_R. We then used the bidomain model to simulate the effects of a variety of pathologies and technical difficulties, which the simple model could not account for. Generally, T_up was a much better estimate for T_R than T_down. Regional fibrosis could attenuate local electrogram components and reduce accuracy of T_up as a marker for T_R. In fibrotic tissue, T_down was not related to T_R at all. This investigation of electrogram slopes required the simulation of extracellular potentials with about 100 times more precision than needed for simulation of visually acceptable waveforms alone. This requirement is more difficult to meet in larger models, but it was actually possible for a human-heart model with 60 million nodes. By sacrificing some spatial resolution, we kept the computational requirements within acceptable limits for multiple simulations.