Mobitz type I second degree AV block (Wenchebach)

Mobitz type II second degree AV block

Complete (third degree) heart block

DESCRIPTION

We’re going to focus on when a P wave is supposed to be conducted into the ventricle but isn’t, which means second or third degree AV block. When a P wave conducts to the ventricle more slowly than it’s supposed to (longer than 0.2 seconds, right?) that’s first degree AV block (see above), and we’re not very impressed. I suppose to the purist it isn’t even an arrhythmia. Similarly unimpressive to us are P waves which come so early they can’t be conducted, the ever-present-but-so-hard-to-find blocked premature atrial complex (PAC) (see below). This isn’t considered heart block at all because those P waves aren’t supposed to be conducted because they’re too early. But I just called them blocked, didn’t I? Go ask your attending.

Third degree heart block is the simplest: it’s where the P waves and the QRS complexes are completely independent of each other AND P waves which should be conducted are not. That second criterion is there so you wouldn’t be tempted to call things like junctional rhythm with AV dissociation (see page 12) heart block. That, in turn, is to be distinguished from the third degree about heart block, which is when your attending grills you about heart block you were supposed to look up yesterday.

Second degree AV block occurs when an occasional P wave isn’t conducted, and there are two types: Mobitz I and Mobitz II. Mobitz I (or Wenkebach, J. S. Bach’s 32nd child) is characterized by the little rhyme I heard last week for the very first time:

Longer, longer, longer, dropped,

That’s the sign of Wenckebach!

The poem is actually a couplet, but has nothing to do with paired extrasystoles; it merely is a bit of doggerel intended to remind you that if the PR interval prolongs in beats preceding the non-conducted P wave, the rhythm is classified as Mobitz I second degree AV block, with the block probably occurring within the AV node, frequently drug-related (those ubiquitous slower-downers again!), and is often not that dangerous. This poem will be on the test! On the other hand, if the blocked P wave is not preceded by a prolonging PR interval, this is a Mobitz II type, possibly infranodal, and possibly portending a worse prognosis.

Figure shows 2:1 AV block, of unknown Mobitz

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