A 68-year-old woman presents to her primary care provider for dizziness. She often feels lightheaded and like she may “pass out” but never actually loses consciousness. She denies dyspnea on exertion or orthopnea. The patient had a myocardial infarction 6 months ago, and she now has multiple drug-eluting stents. Her ECG upon discharge from that hospitalization was normal. The patient has been taking all of her medications as instructed. Her medical history is otherwise significant for rheumatoid arthritis and migraines. Her home medications include methotrexate, atorvastatin, clopidogrel, and aspirin. Her temperature is 98.6°F (37°C), blood pressure is 118/75 mmHg, pulse is 92/min, and respirations are 12/min. On physical exam, she has an irregular heartbeat with normal cardiac sounds. Her abdomen is non-distended and she has mild lower extremity edema. Her current ECG can be seen in Figure A. Which component of the cardiac conduction system is most likely affected in this patient?
Bundle branches
Bundle of His
Atrioventricular node
Sinoatrial node
Atrioventricular node
This patient who presents with lightheadedness and is found to have progressive prolongation of the PR interval on ECG most likely has Mobitz type 1 (Wenckebach) 2nd-degree heart block. Mobitz type 1 heart block is usually caused by atrioventricular node dysfunction.

Second degree heart block can be divided into Mobitz type 1 and Mobitz type 2 patterns. The Mobitz type 1 pattern is usually a result of dysfunction at the atrioventricular (AV) node, whereas the Mobitz type 2 pattern is usually caused by dysfunction below the AV node in the His-Purkinje system. In Mobitz type 1 2nd-degree heart block, the PR interval becomes progressively longer with each beat before the dropped QRS complex. In Mobitz type 2 2nd-degree heart block, the PR intervals are consistent in length and the dropped QRS complexes occur at regular intervals. The easiest way to determine whether the PR interval is getting longer is to compare the PR interval just before the dropped beat to the PR interval immediately following the dropped beat. Patients with Mobitz type 1 blocks generally do not require treatment unless symptomatic whereas those with Mobitz type 2 blocks generally require pacemaker placement.

Clark and Prystowsky review the pathophysiology of heart block. They discuss how a type 1 2nd-degree heart block is caused by dysfunction of the atrioventricular node. They recommend symptomatic treatment of this condition.

Figure/Illustration A is an ECG that demonstrates the progressive prolongation of the PR interval (blue lines) until a beat is dropped. This progressive prolongation is characteristically seen in Mobitz type 1 (Wenckebach) heart block.

Incorrect Answers:
Answer B: A bundle branch block would produce a classic right or left bundle branch block pattern on ECG (RSR') described colloquially as "bunny ears." Bundle branch blocks are defined in part by a QRS duration > 120 ms and can be seen in lead V1. Treatment is with control of underlying risk factors such as hypertension.

Answer C: The bundle of His conducts the electrical impulse from the atrioventricular node to the ventricles. Dysfunction at the level of the bundle of His would more likely cause Mobitz type 2 2nd-degree heart block or 3rd-degree heart block rather than the Mobitz type 1 heart block seen in this patient. Treatment is with pacemaker placement.

Answer D: Purkinje fibers rapidly conduct along the ventricular walls to create synchronized contraction of the ventricles. Aberrant conduction of the Purkinje fibers leads to premature ventricular complexes. Treatment is with beta-blockers if patients are symptomatic.

Answer E: The sinoatrial node is the heart’s primary pacemaker. Dysfunction at this level can lead to sick sinus syndrome. Sick sinus syndrome can manifest as many types of arrhythmias, including bradycardia-tachycardia syndrome, in which there is alternating bradycardia with paroxysmal tachycardia. Sinoatrial node dysfunction can be treated with pacemaker placement.

Bullet Summary:
Mobitz type 1 (Wenckebach) 2nd-degree heart block is characterized by progressive prolongation of the PR interval before a dropped QRS complex and is usually caused by dysfunction of the atrioventricular node.