Cardiac Rhythm Series: Second Degree AV Block, Mobitz Type I

Click here to review First Degree AV block before reading this post.

In a Mobitz I (Wenckebach) block, the PR interval becomes progressively longer with each cardiac cycle until a P-wave appears with no QRS to match1, 2. This occurs when the cardiac conduction system delays or blocks an atrial impulse traveling through the AV node and/or bundle of His3. If you look closely at an ECG, you will see the atrial rhythm (P-P interval) is regular but the ventricular rhythm (R-R interval) is irregular2. The P-wave is not moving, but the QRS complex is.

Mobitz I
Characteristics: PR interval progressively widens until a QRS is ‘dropped’; QRS less than 0.12 seconds; regular atrial rhythm but the ventricular rhythm is irregular.

Wenckebach has many possible causes:

  • Cardiac surgery: AV node conduction is affected in approximately 25% of CABG patients1.
  • Digitalis toxicity: Digoxin increase the refractory time of the AV node which slows conduction.
  • Myocardial ischemia/infarction: The AV node is perfused by the right coronary artery in 90% of individuals and the left circumflex coronary artery in the remaining 10%2.

“Longer, longer, longer, DROP– now you have a Wenckebach”

Treatment is not usually required unless the ventricular rate is slow1. Atropine or chronotrope infusion (dopamine or epinephrine) may be administered if cardiac output is inadequate2. The patient may experience symptoms related to decreased cardiac output– lightheadedness, dizziness, or syncope2. If symptomatic, AV pacing (DVI mode) at a slightly faster rate is recommended. If the atrial rate is too fast to overdrive, it can be treated by pacing in DDD mode1.

  1. Bojar, R. M. (2016). Manual of perioperative care in adult cardiac surgery (5th ed.). West Sussex, UK: Wiley-Blackwell
  2. University of Maryland Medical Center Office of Clinical Practice and Professional Development. (2014). Introduction to cardiac rhythm interpretation (6th ed.).
  3. https://emedicine.medscape.com/article/161919-overview
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About Ryan Barnes, BSN, RN, PCCN

Cardiac Surgery RN from Baltimore, DNP student, and Army nurse.

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