EMS 12-Lead: The 12 Rhythms of Christmas – 2:1 AV-Block

This article is the seventh in our latest series, The 12 Rhythms of Christmas, where each day we examine a new rhythm disorder. It’s a continuation of the theme behind last year’s 12 Leads of Christmas.

Our last two posts have examined type I and type II AV-block, so it’s only fitting that we continue our theme with a topic that combines the two:

While most everyone has heard of first degree, type I, type II, and complete AV-block, comparatively few people recognize 2:1 AV-block as a valid rhythm diagnosis in its own right. That’s a shame, because 2:1 AV-block is a rather interesting finding. To understand why, consider the following dilemma:

  • Type I AV-block presents with progressively increasing PR-intervals until a P-wave is blocked

Figure 1. Increasing PR-intervals in the setting of type I AV-block. Measurements are in milliseconds.

  • Type II AV-block presents with fixed PR-intervals until a P-wave is blocked

Figure 2. Fixed PR-intervals with type II AV-block. Measurements are in milliseconds.

It should be clear from the two examples above (and the others in this series) that both forms of AV-block can present with various—and actively varying—conduction ratios: 5:4, 4:3, 3:2, etc… That doesn’t affect our ability to diagnose the rhythms, and in fact, it can be helpful to see how the PR-intervals behave with different ratios of P-waves to QRS-complexes.

  • What do you do, however, when every-other P-wave is blocked?

Figure 3. 2:1 AV-block of uncertain mechanism.

For the rest of this discussion please follow this link or click on the rhythm strips above.

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