Cardiac Medicine Certification (CMC) Practice Exam

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In 2nd degree AV block Type I (Wenckebach), how is the PR interval affected?

It remains constant throughout the rhythm.
It gradually lengthens before a dropped QRS complex.
It is shorter than 0.12 seconds.
It is consistently longer than 0.20 seconds.

The correct answer is: It gradually lengthens before a dropped QRS complex.

In 2nd degree AV block Type I, also known as Wenckebach, the defining characteristic is that the PR interval progressively lengthens with each successive heartbeat until a QRS complex is finally dropped. This phenomenon is referred to as "grouped beating" or "progressive block." The gradual elongation of the PR interval is a reflection of the increasing delay in conduction through the AV node. As the cycle continues, this progressive lengthening continues until the electrical impulse fails to transmit, leading to a missed or dropped QRS complex. Following this, the cycle resets, and the pattern repeats. This pattern of progressively increasing intervals followed by a dropped beat is what differentiates Wenckebach from other types of AV block, making it crucial to recognize the specific behavior of the PR interval in this scenario.