NBRC Sleep Disorders Specialty (SDS) Practice Exam

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What characterizes a 2nd degree AV block Mobitz I (Wenckebach)?

Constant PR interval with a dropped QRS

Increasingly longer PR interval with a dropped QRS

A 2nd degree AV block Mobitz I, also known as Wenckebach, is characterized by a specific pattern in the PR intervals on an electrocardiogram (ECG). In this type of block, the PR interval progressively lengthens with each heartbeat until a QRS complex is dropped. This means that during the series of heartbeats, the time between the P wave and the QRS complex gradually increases until a QRS is missed entirely. After the dropped QRS, the cycle usually resets, and the PR interval starts again from a shorter duration.

This unique behavior distinguishes Mobitz I from other types of AV block. It reveals underlying issues in the conduction system, typically occurring at the level of the AV node, but with less clinical significance compared to other blocks. The increasing length of the PR interval is a key feature that aids in both identifying the condition and understanding its mechanism.

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Sudden drop of a QRS without elongation of PR

Normal PR interval with dropped beats

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