Sinus Arrest Prolonged Pause Sinus Node Dysfunction

A 72-year-old woman with hypertension presents after a syncopal episode while shopping. She takes amlodipine only. On telemetry, the rhythm strip shown captures an abrupt cessation of sinus activity with a 4.8-second pause containing no P waves and no QRS complexes, followed by a junctional escape beat. The pause is NOT an exact multiple of the preceding P-P interval. The structure marked **B** in the diagram represents this finding. Which of the following best describes the pathophysiologic mechanism underlying this rhythm disturbance in this patient's age group?