Skipped P Waves in Wenckebach Conduction

Report:

Sinus tachycardia 120-128/min

First degree AV block

Second degree AV block, Möbitz 1

Periods of 3:2 conduction, with bigeminy

Right bundle branch block

Pacemaker rhythm 72/min

Fusion and pseudofusion beats

Comment:

The key to correct interpretation of this trace is finding the hidden P waves (‘cherchez le P’ of Marriott’s). In the repetitive 3:2 Wenckebach sequences, only the initial P wave of each sequence is seen clearly; the other two are hiding in the QRS complexes. Still, once suspected, they can be made out: the 9th and the 11th QRS in the bottom strip have the beginning of a P wave precede the Q wave of the QRS. These are the blocked P waves; the invisible ones preceding them are conducted with a PR interval of 0.44” and “skip” the blocked ones to activate the QRS!

Bearing the above in mind, the fusion beat (12th QRS) in the top strip is also preceded by a skipped (and dissociated) P wave; its conducted predecessor peaks the preceding T wave. The P wave after the 12th complex is also easily seen, and can be used to obtain the atrial rate directly.

In Fig 73a below, the fact of fusion in the bottom strip can only be proven by comparison with “pure” paced beats of the original strips.