Sudden cardiac death due to ventricular fibrillation impacts 50 000 athletes per year. The most common finding at autopsy amongst young adults is sudden cardiac death with unexplained origins due to the absence of structural cardiac disease. A case study to investigate the electrophysiological properties in a male of 19 years old after suffering a fatal sudden cardiac arrest during a marathon event. A coronary-perfused left ventricular preparation was imaged using high resolution optical mapping of both epicardial and endocardial surfaces using a voltage-sensitive dye, Di-4-ANEPPS. Activation time, repolarization time and action potential duration were assessed during paced activity through the His bundle and endocardial and epicardial surfaces. Conduction system activation origins were identified as distant endocardial origins during His bundle pacing. Short-coupled S1S2S3S4 pacing was used to determine effective refractory period (ERP). Pacing protocols were repeated in the presence of isoprenaline (ISO) at 100nM, 500nM and 1μM to mimic stress conditions. No structural abnormalities were identified at the macroscopic level. At pacing cycle lengths of 667ms, a total myocardial activation time of 63ms was observed. Negligible changes were found in the presence of isoprenaline at all tested concentrations (up to 64ms). However, the latency of activation of conduction system activation origins following His pacing was shortened by ISO from 61ms to 42ms, 44ms and 16ms for 100nM, 500nM and 1μM. At short cycle length (250ms) His pacing, considerable AT latency reduction from 146ms to 43ms was observed under ISO at 100nM, but insensitive to further increases of ISO concentration. Local APD and RT gradients at His pacing origins were greatly enhanced by ISO (from 3ms/mm & 3ms/mm to 40ms/mm & 41 ms/mm) at short cycle lengths. The ERP of the His was up to 55ms shorter than endocardial pacing. Origins of ectopic activity coincided with His paced responses and frequency increased with ISO. The cardiac conduction system showed a greater dynamic range of excitability under short cycle length and isoprenaline stresses compared to the myocardium and likely underlie local repolarization gradients at Purkinje termini.