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Cardiology MCQ

 Question:

A 60-year-old man presents with syncope and dizziness. On ECG, there is progressive PR interval prolongation followed by a dropped QRS complex. What is the most likely type of heart block?

Options:

A. First-degree AV block

B. Second-degree AV block Type II

C. Second-degree AV block Type I (Mobitz I / Wenckebach)

D. Third-degree AV block

✅ Answer: C. Second-degree AV block Type I (Mobitz I / Wenckebach)



Explanation:

Mobitz I (Wenckebach) is characterized by progressive prolongation of the PR interval until a QRS complex is dropped.

Usually occurs at the AV node level and is often asymptomatic but can cause dizziness or mild syncope in some patients.

Mobitz II (Type II): PR interval remains constant before sudden dropped QRS; usually below AV node (His-Purkinje system) and more dangerous, often requiring pacemaker.

First-degree AV block: PR interval is prolonged (>200 ms) but no dropped QRS.

Third-degree (complete) AV block: No conduction from atria to ventricles; atria and ventricles beat independently, often causing syncope.

Clinical relevance: Mobitz I is usually benign, especially in asymptomatic patients. Symptomatic patients may need temporary pacing.



References: 

Harrison’s Principles of Internal Medicine, 21st Edition, Ch. 223; Braunwald’s Heart Disease, 12th Edition, Ch. 48; 

UpToDate: Evaluation and management of AV block.

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