The ECG Blog

Second Degree AV Block Mobitz 1; Wenckebach with 4:3 conduction + Ventricular and Junctional Escape Beats

September 6, 2009

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Patient/anamnesis: n/a

ECG interpretation/discussion: This ECG is a printout from a telemetry station, derived from a 5 lead patient monitoring.  So, what have we here? The underlying rhythm is sinoatrial, but both PR and RR interval varies. Remember, Wenckebach is not the name of a certain type of block, but rather a type of conduction. This is often misunderstood, as Second Degree AV Block Type 1 or Mobitz 1 is also often labelled Wenckebach block. A more precise term would be Wenckebach periodity, phenomenon, conduction. Wenckebach conduction is usually considered benign and can be recognized by the following criteria:

1) PR interval is progressively prolonged until a P wave is blocked

2) The shortest PR interval is the one immediately following the dropped beat. The longest PR interval is the one immediately before the dropped beat. The incremental change in PR interval is in the beginning of the Wenckebach cycle, thus between the first and second PR interval in a sequence.

3) The RR intervals progressively shorten until a QRS is ‘dropped’ due to the non-conducted atrial/sinoatrial impulse.

Now, looking at this ECG, the two first beats are at the end of a Wenckebach cycle. After the second QRS a non-conducted P wave occurs. The following beat is wide and bizarre and is a ventricular escape beat that occurs due to the long preceding pause. After the escape beat, a new Wenckebach cycle starts. The PR interval lengthens until a P wave is blocked. After this pause, a narrow QRS is preceded by a P wave with a very, very short PR interval. This is a junctional escape beat. Then, the Wenckebach cycle restarts.

When counting P waves and QRS complexes in the cycles, we’ll see that for every three QRS complexes there are four P waves, since one of the QRS complexes gets dropped repeatedly. This gives a 4:3 atrioventricular (AV) ratio, which is also called 4:3 conduction.

What a beauty! Thanks to my colleague and fellow ECG-dork for bringing me this rare gem!

Filed under: 1° AV Block, 2° AV Block, 2° AV Block Type 1 (Mobitz I / Wenckebach), Arrhythmias, Escape beats, Junctional Escape Beat, Ventricular Escape Beat

Tags: 2AVB, heart block, junctional escape, second degree av block, ventricular escape, wenckebach, Wenckebach phenomenon