During the VF episodes, changes in sinus node CL measured from the stored atrial electrogram (EGM) from the implanted device will be recorded. Specifically, investigators will be measuring the average CL over the last 5 seconds before defibrillation (VF-SNCL) and will be comparing it with the average sinus node CL during the 5 seconds prior to VF induction (Baseline-SNCL). In addition, continuous BP monitoring from an existing arterial line during and following defibrillation will be stored onto a data recording system. BP recovery will be defined as the difference in mean BP during the 5 seconds before defibrillation compared with the first 30 seconds after successful defibrillation. Post-shock pacing will always be set at 40bpm in a VVI mode.