Cardiac arrest and a bifid T wave

A 35-year-old woman without a history of cardiac disease was seen in the emergency department after an abrupt syncopal episode. She denied the use of any medications, and her family history was unremarkable. Her cardiac echocardiogram and serum electrolytes were normal. What are the important features of the electrocardiographic (ECG) tracings (Figure 1), and what is the most likely diagnosis?