Welcome back to Master USMLE! I’m Dr. Amin, and today we’re breaking down heart blocks—a high-yield ECG topic you need to master for Step 2 CK.
Let’s get straight to it.
First-degree AV block is simple. The PR interval is prolonged—greater than 200 milliseconds, but every P wave is followed by a QRS. No dropped beats. This is usually benign and doesn’t need treatment. Think of it as a slow-moving train—delayed but always arrives.
Mobitz Type I, also called Wenckebach, follows a pattern. The PR interval gradually lengthens until a QRS drops, then the cycle repeats. This happens at the AV node, and it’s usually not dangerous. It actually improves with exercise and worsens with vagal maneuvers. Imagine a student delaying assignments longer and longer until they finally miss a deadline.
Mobitz Type II is different—it’s serious. The PR interval is constant, but QRS complexes drop unpredictably. This means the block is lower, in the His-Purkinje system, and it can progress to complete heart block. These patients need a pacemaker. Think of it as a faulty power circuit—it works fine until, suddenly, the lights go out.
Complete heart block, or third-degree AV block, is a total electrical disconnect. P waves and QRS complexes are completely independent—there’s no conduction between them. The atrial rate is faster than the ventricular rate, and without intervention, the heart will fail. The only treatment? A pacemaker. Picture a couple in a relationship that’s completely fallen apart—they’re not talking, not listening, and doing their own thing.
Quick review.
First-degree AV block – Long PR, no drops. Benign.
Mobitz Type I – PR gets longer, then QRS drops. Usually benign.
Mobitz Type II – Constant PR, random QRS drops. Needs a pacemaker.
Complete Heart Block – P waves and QRS are independent. Emergency pacemaker needed.
And that’s it!
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