Longer action potential duration in the Purkinje network than in the ventricular myocardium delays retrograde activation of the human His‐Purkinje system

His-Purkinje system (HPS) activation during programmed electrical stimulation is delayed in patients susceptible to ventricular arrhythmias. Its mechanism and clinical relevance are unknown. We hypothesized that longer action potential duration (APD) in the HPS than in the ventricular myocardium delays retrograde HPS activation, and is arrhythmogenic for large APD differences. The right ventricular apex of a computational human ventricles model, with short or long HPS APD, was paced with an S1 cycle length of 600 ms for 10 cycles followed by an S2 stimulus. S1S2 coupling intervals (CIs) started at 400 ms and were reduced by 10 ms steps until the loss of stimulus capture. S1 and S2 activation times (ATs) were computed to determine the S₂H₂ time, which, when prolonged, is analogous to delayed retrograde HPS activation. Simulations were compared retrospectively to patients who underwent the same protocol (N = 5). In the model, longer APD in the HPS than in the ventricular myocardium delayed HPS activation as S1S2 CI shortened (60-152 ms), which was similar to patients examined for ventricular arrhythmias (40-160 ms). Delayed HPS activation was due to increased AT (from 11-14 to 31-196 ms) and distance (from 0.3-0.4 to 0.6-5.3 cm) between the myocardial pacing site and the excitable HPS. For the largest difference between myocardial and HPS APD (106 ms), reentry was initiated for S1S2 CI between 270-320 ms. In conclusion, longer APD in the HPS than in the myocardium delayed HPS activation during programmed electrical stimulation. The S1S2 CI-dependent delay was due to slower and longer pathways from the stimulus site to the HPS, which promoted reentrant tachyarrhythmias for the slowest and longest pathways. KEY POINTS: This is the first study to identify the location and mechanism of delayed retrograde activation that leads to increased S₂H₂ and constant S₁H₂ times at short S1S2 CIs during programmed electrical stimulation, a phenomenon discovered in 1978. At short S1S2 CI during programmed electrical stimulation, longer APD in the HPS than in the myocardium can lead to entry points into the HPS further away from the myocardial S2 stimulus site and promote reentrant arrhythmias. This is new knowledge regarding the retrograde activation of the human conduction system. The mechanism of delayed retrograde activation was determined using a state-of-the-art computer model of the human ventricles coupled to a robust model of the HPS. As a result, this study developed a new and extensive set of HPS parameters that future computational studies can use and build upon to simulate retrograde activation of the human ventricles.