Our simulation results (121 group of dual-channel inhibition settings, 16 CLs; 1936 simulations in total) can be categorized into control (16 cases), AP prolongation (864 cases), AP alternans (110 cases), EAD events (242 cases) or no excitation (704 cases). In Fig 2, the three-dimensional (3D) parameter space defined by pacing CLs and concurrent blockade of ICaL and IKr was color-coded according to excitation patterns observed in the PVS. While moderate ICaL blockade (≤60%) can promote occurrences of AP alternans at short CLs, no electric propagation and excitation can be observed with major inhibition of ICaL (≥70%); EAD events preferentially occur with major blockade of IKr (≥50%) at long CLs. In Fig 3A, representative subtypes of simulations were visualized as stacks of 16 two-dimensional (2D) spatiotemporal maps of membrane potential with pacing CL ranging from 250 ms to 1000 ms. In Fig 3B, selected AP morphologies of P, Endo, M, and Epi cells (located in the center of each tissue layer) at CL = 500 ms were plotted. Under control conditions, electric stimuli trigger robust antegrade excitation under all pacing CLs with AP restitution and morphologies, depolarization and repolarization sequences, consistent with experimental findings[12]. With partial concurrent blockade (20%) of IKr and ICaL, few changes in the excitation patterns can be observed, while major concurrent blockade (60%) of IKr and ICaL can induce beats to beat “all or nothing” excitation patterns at pacing CL<500 ms. With the complete inhibition of IKr and partial blockade of ICaL (20%), abundant EAD events with complex spatiotemporal patterns can be observed at all pacing CLs.
Spatially synchronized EAD events
In Fig 4A, under control conditions, electric stimuli can trigger normal antegrade excitations at CL=750 ms with a QT interval of 247.2 ms and a Tpe of 20.4 ms. AP morphologies and underlying ionic currents (ICaL and INaL) of selected cells (indicated as colored lines) were plotted to provide additional insights into spatiotemporal maps of excitation. At the same CLs (750 ms), a major reduction in IKr (80%) induced substantial AP prolongation and spatially synchronized EAD events with inverted and biphasic T-waves (QT=566.4 ms; Tpe=44.4 ms) (Fig 4B). During spatially synchronized EAD events, all types of tissues simultaneously developed EAD upstrokes at -22 mV, with different EAD amplitudes (31.18 mV in Epi cells; 15.11 mV in P cells). These synchronized EAD events were identified when EAD upstrokes can be observed in all cell types with minimal time delay comparing to other EAD events observed in our simulation. When combined with a minor reduction in ICaL (20%), the effects of an 80% reduction in IKr were “weakened” to induce beat-to-beat occurrences of synchronized EADs and T-wave alternans (QT=323.2 ms, 596.2 ms; Tpe=36.0 ms, 32.8 ms) (Fig 4C).
Localized EAD events
Interestingly, as pacing CLs increase (from 750 ms to 850 ms; Fig 5A), localized EAD events may exclusively arise from midmyocardial tissues to significantly prolong QT (443.2 ms; ↑37%) and Tpe (82.4 ms; ↑129%) intervals, with late-appearing (QT prolongation) and pointed (exclusive EADs in the midmyocardium) T-wave morphology[24] and T-wave alternans with reduced beat-to-beat variations in QT intervals (QT=443.2 ms, 596.9 ms; ↓47%) and increased beat-to-beat differences in Tpe (Tpe=82.4 ms, 33.6 ms; ↑1425%). In Fig 5B, under certain conditions (complete inhibition of IKr and 20% reduction of ICaL at CL=550 ms), localized EAD events can synchronously arise from two neighboring tissue types, i.e., M and Epi tissues and led to inverted, asymmetric broad-based T-wave morphology[25] in the first beat (QT=504.8 ms; Tpe=172.8 ms) and T-wave alternans with substantial beat-to-beat variations in Tpe intervals (Tpe=172.8 ms, 43.2 ms).
Localized-EAD-induced unidirectional propagation
In Fig 6, localized EAD events can act as a focal source of arrhythmogenic and trigger spontaneous antegrade or retrograde excitations in the PVS. With a 70% reduction in IKr at CL=500 ms, EAD can spontaneously arise from the midmyocardium and trigger unidirectional electric propagation towards the epicardium with a time delay in the onset of EAD and reactivation of ICaL between the M and Epi cells (Fig 6A), leading to an inverted and notched T-wave (QT=503.6 ms; Tpe=162.9 ms). In Fig 6(B)(C), with 60% reduction of IKr, spontaneous EAD in the midmyocardium (beat #2) can trigger retrograde excitation in the PVS at CL=650 ms with a conduction velocity (CV) of 0.2 m/s, and sequential development of EAD upstrokes from M cells to Endo and P cells (Fig 6B) with a broad-based asymmetric T-wave (QT=488.3 ms; Tpe=172.4 ms); at a slower pacing rate (CL=850 ms), a much slower spontaneous retrograde conduction (CV=0.1 m/s) can be observed (beat #1) with an increase in both QT interval (514.4 ms) and Tpe(192.0 ms) and a broad-based T-wave.
Spatially desynchronized EAD events and oscillatory excitation patterns
In addition to spatially synchronized EAD events, as seen in Fig 4(B)(C), spatially discordant development of EADs can also be induced under certain conditions, typically with a biphasic T-wave morphology. In Fig 7(A), with 60% IKr inhibition at CL=900 ms, partially synchronized EAD development in P, Endo and M cells (beat #1) preceded the beat-to-beat spontaneous onset of EAD in the epicardium by 90 ms, with no EAD-induced electric propagation from M to Epi cells, as seen in Fig 6(A). These spatiotemporal excitation patterns give rise to interesting spatial T-wave alternans with alternating biphasic (QT=594.4 ms; TDR=83.2 ms; with notching) and monophasic (QT=489.6 ms; TDR=168.0 ms; broad-based) T-wave morphologies. With 70% IKr inhibition at CL=750 ms (Fig 7B), focal EAD arising from the midmyocardium triggered bidirectional excitations towards both the epi- and endocardium with a late-appearing spontaneous onset of EADs in the P cells, leading to a biphasic T-wave without notching.
In Fig 7(C), complete inhibition of IKr (CL=600 ms) can induce severe and complex spatiotemporal patterns of EADs in all tissue types. The first electric stimulus successfully triggered antegrade propagation of excitation with spatially synchronized EADs in the Endo-, M- and epicardium; after the application of the second electric stimulus, spatially discordant development of oscillating EAD waveforms with alternating EAD occurrences in the M- and epicardium or in the Purkinje tissue and endocardium was observed, leading to focal reentrant behaviors and a sustained oscillatory ECG pattern typically observed during ventricular tachycardia[26]. In Fig 7(D), with a complete blockade of IKr and a 10% reduction in ICaL (CL=400 ms), localized EAD events in the epicardium during previous repolarization may collide with the current wavefront of depolarization and trigger self-terminated oscillatory EAD patterns. Two electric stimuli (CL=400 ms) were applied during the simulation, and while two complete APs could be observed in the Purkinje tissue (stimulus-to-response ratio of 2:2), there was only one single AP with oscillatory EADs in the epicardium (stimulus-to-response ratio of 2:1).