Loss of Ventricular Pacing

The patient is a 62-year-old woman with a history of paroxysmal atrial fibrillation (AF), sinus node dysfunction (SND), and ventricular tachycardia status/post dual-chamber implantable cardioverter-defibrillator. Her medication list includes metoprolol, lisinopril, potassium chloride, magnesium, and dofetilide. She is seen at the antiarrhythmic clinic for her routine scheduled visit. She has been doing fairly well, with rare, skipped beats. She denies near-syncope, syncope, lightheadedness, dizziness, and shortness of breath.

Her laboratory data show serum potassium level 4.4 mmol/L, serum magnesium level 2.2 mg/dL, creatinine level 1.01 mg/dL, and glomerular filtration rate 63 mL/min/1.73 m2.

An electrocardiogram (ECG) is performed (Figure 1).

Figure 1

Figure 1

Which one of the following is the most likely explanation for this ECG?

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