Scenario: Below is a 12-lead electrocardiogram (ECG) of an 80-year-old unresponsive man obtained by paramedics en route from the patient’s home to the emergency department. It was unclear how long the patient had been unconscious, but distal pulses were intact. He required respiratory support with a bag valve mask. Per a family member, his history included prior myocardial infarction (MI) with coronary artery bypass graft surgery, heart failure, and an implantable cardioverter defibrillator (ICD) placed a “few years ago.” He remained unresponsive except for localized pain upon arrival at the emergency department and was intubated. What is the underlying mechanism of the QRS complexes?

Ventricular paced rhythm at a rate of 40 bpm.

In this scenario, the myocardial conduction cells fail to generate automatic, or intrinsic, electrical impulses; thus, the ICD initiated ventricular pacing to support cardiac output, but at a slow ventricular rate. Unfortunately, the ICD did not...

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