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Second-degree AV Block Type I

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Dawn's picture

Supraventricular Tachycardia With Wenckebach Conduction

Wed, 03/16/2016 - 22:55 -- Dawn

This ECG was obtained from a patient in a walk-in health clinic.  We do not have any other information on the patient.  We thank Joe Kelly for donating this interesting ECG to the GURU. 

IRREGULAR RHYTHM    If you march out the P waves, you will see that they are regular, at a rate of approximately 130 bpm.  But the QRS complexes are not regular, and there are fewer QRS complexes than P waves.  

WENCKEBACH CONDUCTION   Looking closely at the PR intervals, you will notice that they progressively prolong.  This “pushes” the QRS complexes progressively toward the right.  Eventually, the T wave – and the refractory period – will land on the next P wave.  That P wave will be unable to conduct to the wave, and no T wave of course, so the next P wave will conduct with a shorter PR interval.

We are including a short rhythm strip from this patient, with conduction marked with a laddergram. 

Dawn's picture

Second-degree AV Block,Type I, With 2:1 Conduction

Fri, 04/04/2014 - 23:17 -- Dawn

This ECG is a follow-up to last week's ECG of the WEEK, which presented an AVB that was mostly conducted 2:1, and proved to be a "Type II" block when it conducted 3:2 with consistent PR intervals at the end.  We often just use the term, "2:1 AVB", rather than try to discern the Mobitz type, realizing that the most important feature of a 2:1 block may be that it automatically cuts the heartrate in half.  That is, there are half as many QRS complexes as P waves, which can result in a very slow pulse.  

However, some AV blocks occur at the AV node level, and some occur below the node.  The latter type tend to be more ominous, and less likely to self-correct.  Type II (or sub-Hisian) blocks nearly ALWAYS need to be paced.  So, it is beneficial to make the correct diagnosis when possible.  One helpful sign is that Type II blocks frequently show signs of bundle branch pathology, and have wide QRS complexes.  It is possible for Type I blocks to have wide complexes, or bundle branch block, but it is also more likely that they will have narrow QRS complexes, indicating normal conduction through the ventricles.

The BEST way to determine the type of block from the  ECG is to get lucky and find a moment of 3:2 or better conduction.  Then, the PR intervals can be assessed for progressive prolongation (Type I) or no change (Type II).   This ECG, while not of the best print quality, clearly shows the progressive prolongation of the PR intervals in the last two QRS complexes, unmasking this rhythm as Type I, Wenckebach. 

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