A 68-year-woman presents to the emergency department complaining of chest pain for the past 30 minutes. The pain is retrosternal in location and it radiates to her neck. She has no history of cardiac conditions or similar episodes of chest discomfort, and her past medical history includes hypertension and dyslipidemia.On physical examination, she is diaphoretic and in moderate distress. The blood pressure is 150/90 mm Hg, the heart sounds are normal, and the lungs clear on auscultation. The ECG is shown in Figure below. Which of the following mechanisms is the most likely cause of her condition?