Which rhythm has a fixed PR interval with occasional non-conducted P waves and dropped QRS complexes without PR progression?

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Multiple Choice

Which rhythm has a fixed PR interval with occasional non-conducted P waves and dropped QRS complexes without PR progression?

The pattern tests a second-degree AV block where the conduction to the ventricles fails intermittently but the beats that do get through keep a constant timing. Here, the P waves march through regularly, the PR interval on conducted beats stays fixed, and occasionally a P wave is not followed by a QRS, producing a dropped beat. That combination—regular P-P, constant PR on conducted beats, and non-conducted P waves with dropped QRS—defines Mobitz II. This differs from Mobitz I, where the PR interval lengthens progressively before a dropped QRS, and from a junctional rhythm, which usually shows a different relationship between P waves and QRS and often no pattern of intermittent non-conducted P waves with preserved PR. In a third-degree block, there’s complete AV dissociation with no consistent relation between P waves and QRS, and the blocks aren’t just intermittent drops with a fixed PR. Mobitz II is the pattern described.

The pattern tests a second-degree AV block where the conduction to the ventricles fails intermittently but the beats that do get through keep a constant timing. Here, the P waves march through regularly, the PR interval on conducted beats stays fixed, and occasionally a P wave is not followed by a QRS, producing a dropped beat. That combination—regular P-P, constant PR on conducted beats, and non-conducted P waves with dropped QRS—defines Mobitz II.

This differs from Mobitz I, where the PR interval lengthens progressively before a dropped QRS, and from a junctional rhythm, which usually shows a different relationship between P waves and QRS and often no pattern of intermittent non-conducted P waves with preserved PR. In a third-degree block, there’s complete AV dissociation with no consistent relation between P waves and QRS, and the blocks aren’t just intermittent drops with a fixed PR. Mobitz II is the pattern described.

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