A 32-year-old man is brought to the Emergency Department after being struck by lightning during a winter storm while working outdoors. Witnesses describe a brief collapse followed by confusion. He has no known cardiac history.

On arrival, he is conscious but disorientated. Examination reveals transient erythematous branching skin markings over the upper torso. There is no evidence of major blunt trauma. Initial ECG shows sinus rhythm with QTc prolongation. Shortly after commencing cardiac monitoring, he deteriorates and suffers a sudden cardiac arrest.

Which of the following is the most commonly reported initial arrhythmia following lightning strike?