A 78-year-old man is brought to the emergency department due to lightheadedness that began earlier in the day. This morning, the patient felt unsteady and almost passed out when he tried to get out of bed. He has not had similar symptoms before and he has had no chest pain or shortness of breath. He has a history of coronary artery disease requiring percutaneous coronary intervention. The rhythm strip of the ECG obtained in the emergency department is shown in the exhibit.
Which of the following is the most likely cause of this patient's current symptoms?

Question

A 78-year-old man is brought to the emergency department due to lightheadedness that began earlier in the day.  This morning, the patient felt unsteady and almost passed out when he tried to get out of bed.  He has not had similar symptoms before and he has had no chest pain or shortness of breath.  He has a history of coronary artery disease requiring percutaneous coronary intervention.  The rhythm strip of the ECG obtained in the emergency department is shown in the exhibit. 
 Which of the following is the most likely cause of this patient's current symptoms?

Quizplus · Accepted Answer

This patient's ECG demonstrates third-degree atrioventricular (AV) block. Third-degree (complete) AV block involves a total lack of communication between the atria and ventricles due to AV node dysfunction. It can result from ischemia, infiltrative disease (eg, sarcoidosis), infection (eg, Lyme disease), or age-related fibrosis with cellular degeneration. Because conduction signals from the atria cannot communicate with the ventricles, the intrinsic pacemaker of the His bundle or ventricles is triggered, resulting in a junctional or ventricular escape rhythm. On ECG, there is dissociation of P waves and QRS complexes, with P waves marching out at the intrinsic rate of the sinoatrial node (~75/min) and QRS complexes at the intrinsic rate of the His bundle or ventricles (~45/min). The QRS-complex escape rhythm in this patient is particularly slow (ie, <30/min). The slow ventricular rate in third-degree AV block leads to reduced cardiac output, potentially causing dyspnea, fatigue, lightheadedness, and syncope. Definitive management requires placement of a permanent pacemaker. (Choice B) Degenerative changes involving the sinus node can lead to sick sinus syndrome, characterized by bradycardia with sinus pauses (delayed P waves) and sinus arrest (dropped P waves). However, there would be no dissociation of P waves and QRS complexes (ie, P waves are always promptly followed by a QRS complex). (Choice C) Ectopic electrical foci in left ventricular (LV) myocardium are a common cause of monomorphic ventricular tachycardia, typically in the setting of myocardial ischemia or LV systolic dysfunction. ECG demonstrates wide QRS complexes that are rapid and regular, and P waves are not visible. (Choice D) A reentrant circuit involving the cavotricuspid isthmus occurs in atrial flutter. ECG shows saw-toothed flutter waves; QRS complexes may demonstrate a regular or irregular rhythm depending on how consistently flutter waves are conducted through the AV node. (Choice E) Sporadic depolarization of atrial cardiomyocytes occurs in atrial fibrillation. On ECG, QRS intervals are irregular and P waves are absent. Educational objective: Third-degree (complete) atrioventricular block involves dysfunction of the atrioventricular node, resulting in total lack of communication between the atria and ventricles. On ECG, there is dissociation of P waves and QRS complexes, with P waves marching out at the intrinsic rate of the sinoatrial node and QRS complexes at the intrinsic rate of the His bundle or ventricles (escape rhythm).

A 78-Year-Old Man Is Brought to the Emergency Department Due