Why do we need supercomputers to understand the electrocardiographic T wave?

OBJECTIVE Propagation of depolarisation and repolarisation in myocardium results from an interplay of membrane potential, transmembrane current, and intercellular current. This process can be represented mathematically with a reaction-diffusion (RD) equation. Solving RD equations for a whole heart requires a supercomputer. Therefore, earlier models used predefined action potential (AP) shapes and fixed propagation velocities. We discuss why RD models are important when T waves are studied. METHODS We simulated propagating AP with an RD model of the human heart, which included heterogeneity of membrane properties. Computed activation times served as input to a model that used predefined AP, and to a "hybrid model" that computed AP only during repolarisation. The hybrid model was tested with different spatial resolutions. Electrocardiograms (ECGs) were computed with all three models. RESULTS Computed QRS complexes were practically identical in all models. T waves in the fixed-AP model had 20 to 40% larger amplitudes in leads V1-V3. The hybrid model produced the same T waves as the RD model at 0.25-mm resolution, but underestimated T-wave amplitude at lower resolutions. CONCLUSION Fixed AP waveforms in a forward ECG model lead to exaggerated T waves. Hybrid models require the same high spatial resolution as RD models.