Which rhythm has a dropped QRS complex without prior PR prolongation?

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Multiple Choice

Which rhythm has a dropped QRS complex without prior PR prolongation?

Recognizing how AV blocks appear on ECG is the key concept here. When a P wave appears and a QRS complex is dropped without any preceding change in the PR interval, this pattern points to a second-degree AV block of the Mobitz II type. The important detail is that the PR interval on the conducted beats remains constant; there is no gradual lengthening before the dropped beat. In contrast, Mobitz I shows progressive PR prolongation before a QRS is dropped, so the absence of PR prolongation before the missing QRS rules out that pattern. A junctional rhythm can have different P/QRS configurations and isn’t defined by a dropped beat occurring without PR change, and normal sinus rhythm would have every P wave followed by a QRS with a stable PR interval. Thus, the rhythm described is Mobitz II second-degree AV block.

Recognizing how AV blocks appear on ECG is the key concept here. When a P wave appears and a QRS complex is dropped without any preceding change in the PR interval, this pattern points to a second-degree AV block of the Mobitz II type. The important detail is that the PR interval on the conducted beats remains constant; there is no gradual lengthening before the dropped beat. In contrast, Mobitz I shows progressive PR prolongation before a QRS is dropped, so the absence of PR prolongation before the missing QRS rules out that pattern. A junctional rhythm can have different P/QRS configurations and isn’t defined by a dropped beat occurring without PR change, and normal sinus rhythm would have every P wave followed by a QRS with a stable PR interval. Thus, the rhythm described is Mobitz II second-degree AV block.

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