The "skipping" P waves

ECG case

Vignette: 70yo with h/o HTN, s/p valve replacement c/o one-sided facial numbness and dizziness. Work-up for stroke was negative. CBC - N, chemistry and troponin - N, CXR negative for acute disease.

What is the rhythm?

Image 1 - Long lead II

Long lead II can be interpreted as atrial fibrillation (AF). However, it is very odd for AF to be regular unless there is a complete heart block, AF with a pacemaker and AF with entrance block and junctional rhythm with an exit block. The machine read this rhythm as AF.

Image 2 - Long lead II and V1

Adding long lead V1 revealed a different story. Organized atrial activity can be seen in V1. The PP rate is about 88 bpm. Some of the P waves are distorting the initial and terminal portion of the QRS. 

Image 3 - Ladder diagram 

Image 3 marked in red arrows some hidden P waves. The complexes with red arrows highlight the typical morphology of a QRS with no P wave distortion.

The laddergram also illustrates an interesting pattern. The P waves depolarizing the the QRS is "skipping" the nearest QRS. The initial QRS complexes are conducted with 1:1 pattern and the latter part is conducted with a 2:1 pattern (rate ~ 40 bpm). There is a very long PRI of about 0.56 sec and a left bundle branch block.

Interpretation: Sinus rhythm, first degree AV block, second degree AV block type I (Wenckebach/Mobitz I), LBBB.

This patient eventually got a pacemaker.



  1. The terminal part of the ECG is conducted through 2:1 pattern according to the ladder strip.. There is no progressive PRI prolongation that preceeds a non conducted wave and RR intervals are fixed (no progressive shortening), the rhythm went like one conducted P wave with 1st degree AVB, followed by a non coducted P.. So i think its second degree AVB Mobitz II not Moibtz I.. So what did I miss here?

    1. By following the ladder diagram, the PRI of the last conducted beat is longer compare to the first conducted beat. This is a telltale marker of a Wenckebach. As also seen there is a very long PRI (or long first degree). The Wenckebach cycle is presenting like the "atypical" Wenckebach pattern where the PRI remained the same but prolonged until you see the dropped P wave. The clue that gives the diagnosis is comparing the last conducted PRI and the first conducted PRI.