Wide QRS Complex Rhythm in an Unresponsive Community-Based Patient

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 include atrial pacing; hence, atrial "kick" has been lost. The QRS complexes in all ECG leads are wide (ie, prolonged in duration), which is expected with ventricular paced rhythms because the pacing impulse travels across the ventricles more slowly outside the normal His-Purkinje conduction network. The wide QRS and dominant S wave in V1 are due to the initial depolarization in the right ventricle (location of pacer wire) that then spreads to the left ventricle. This morphology should not be confused with left bundle branch block, or a ventricular escape rhythm. Concomitant prior MI and heart failure in this patient may have also affected the myocardium, which may delay ventricular depolarization even more. Finally, although the ventricular paced rhythm may confound the ST segment in V3 and V4, acute MI should be considered given this patient's history and unstable condition.Chart review indicated that this patient had been followed by a cardiologist for his ischemic heart failure with an ejection fraction of 39%. Current practice guidelines recommend an ICD to prevent sudden cardiac death due to life-threatening ventricular tachycardia and ventricular fibrillation in patients with ischemic cardiomyopathy and severe heart failure with reduced ejection fraction. This patient received an ICD without any significant events until this event at home. Generally, ICDs provide cardioversion (synchronized) or defibrillation (unsynchronized) and pacing support. In some cases, severe bradycardia following an ICD shock may occur with subsequent ventricular pacing support to maintain cardiac output.Initially the exact mechanism of this patient's unresponsive episode was unclear; thus, the ICD was interrogated to determine the potential cause. This assessment would also help determine if the ICD was functioning properly. Concurrently, other diagnostic evaluation is indicated to rule out other potential causes (eg, acute myocardial ischemia, electrolyte depletion, toxins). Subsequent interrogation of the patient's ICD showed that he had 3 episodes of ventricular fibrillation ~2 hours before this event, prompting his ICD to appropriately deliver multiple discharges. His first 2 troponin test results were negative for acute MI, with nonsignificant delta troponin. After stabilization, the patient regained consciousness 5 hours after arrival in the emergency department. However, several hours later, he developed significant ST-segment depression in multiple ECG leads, with a positive third troponin test result. He was taken urgently for percutaneous coronary intervention of the ungrafted distal left anterior descending coronary artery. Remarkably, he was discharged home after 8 days.