What defines First Degree AV Block on an ECG?

• A. Prolonged PR interval with irregular P waves
• B. Normal PR interval with occasional QRS dropped
• C. Short PR interval with delayed QRS
• D. Prolonged PR interval (>0.20 s) with P waves before every QRS

Answer: (D)

The key idea is that the time it takes for the impulse to travel from the atria to the ventricles is lengthened, but every atrial beat still gets through. In first-degree AV block, conduction through the AV node is slowed, so the PR interval is longer than normal, yet there is a 1:1 relationship between P waves and QRS complexes. The PR interval is consistently prolonged beyond 0.20 seconds, and every P wave is followed by a QRS complex. The QRS complexes are typically normal in shape and width.

This distinguishes it from other patterns: if QRS sometimes drops, that points to a higher-degree AV block; a short PR interval suggests conditions like pre-excitation or other conduction abnormalities; irregular P waves with a prolonged PR interval would not fit the classic first-degree pattern where P waves are regular and each one conducts to the ventricle.