A 67-year-old man is brought to the emergency department by his son after a syncopal episode. The son was helping his father clean out the garage when the father said he felt dizzy. As the son was helping him into a chair, the patient lost consciousness. He woke up spontaneously about a minute later without any disorientation or confusion. ECG demonstrates bradycardia with a regular rhythm and narrow QRS complexes. There is complete desynchronization between the P waves and QRS complexes. Which of the following locations is most likely pacing this patient's ventricles?

Question

A 67-year-old man is brought to the emergency department by his son after a syncopal episode.  The son was helping his father clean out the garage when the father said he felt dizzy.  As the son was helping him into a chair, the patient lost consciousness.  He woke up spontaneously about a minute later without any disorientation or confusion.  ECG demonstrates bradycardia with a regular rhythm and narrow QRS complexes.  There is complete desynchronization between the P waves and QRS complexes.  Which of the following locations is most likely pacing this patient's ventricles?

Quizplus · Accepted Answer

11ec160b_3c94_5cbb_9249_374091aaf388_MD0001_00 The electrical impulses in the myocardial conduction system are normally initiated by the sinoatrial (SA) node at a rate of 60-100/min. These impulses are then transmitted through the atria to the atrioventricular (AV) node, then on to the His bundle, bundle branches, Purkinje fibers, and ventricular myocardium. Most of the conduction system has its own intrinsic pacemaker, which is normally suppressed by the more rapid SA node pacemaker but triggers when a signal from further up the conduction system is not received. This patient's ECG shows complete dissociation of P waves and QRS complexes consistent with third-degree (complete) AV block. In third-degree AV block, electrical impulses coming from the SA node are blocked before being transmitted to the ventricles; the point of blockade usually is within the AV node or His bundle. The cells located immediately distal to the blockade (eg, in the His bundle) never receive the impulse from the SA node and therefore begin generating their own pacemaker impulse that is transmitted to the ventricles. On ECG, the SA node impulses continue to march out as P waves, and the His bundle impulses generate QRS complexes (an escape rhythm) at the intrinsic rate of the His bundle pacemaker (eg, 40-60/min). Because the atria and the ventricles are not communicating, the P waves and QRS complexes have no relation to one another. (Choices B and D) When electrical impulses are initiated below the AV node and His bundle, the heart rate typically slows to 25-40/min (based on the intrinsic pacemaker rate of the bundle branches and Purkinje system). On ECG, the QRS complexes are typically wide compared to the narrow QRS complexes initiated by the AV node and His bundle. (Choice C) The ventricular myocardium does not have an intrinsic pacemaker and typically must receive electrical impulses from the Purkinje system. (Choice E) When the SA node is initiating impulses through a normal conduction system, the heart rate is 60-100/min. ECG shows P waves quickly followed by a narrow QRS complex, representing normal atrial depolarization followed by normal ventricular depolarization (normal sinus rhythm). Educational objective: The sinoatrial node is the main pacemaker of the cardiac conduction system, typically initiating electrical impulses at a rate of 60-100/min. Other parts of the conduction system (eg, atrioventricular node, His bundle) have their own intrinsic pacemakers, and they initiate impulses at a slower rate when impulses from the sinoatrial node are blocked.

A 67-Year-Old Man Is Brought to the Emergency Department by His