# 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:

## 2:1 AV-Block

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 PR-intervals that progressively lengthen 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 interrupted by the occasionally blocked P-wave.

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 changing—conduction ratios: 5:4, 4:3, 3:2, etc… That doesn’t affect our ability to diagnose the rhythms; 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.

It’s not a problem you really think about until you first run into it.

At first glance it seems clear that there are constant PR-intervals in Fig. 3, but it is important to note that in order to diagnose type I or type II AV-block, we must be able to see two consecutively conducted P-waves. A constant PR-interval is only meaningful if we see it in consecutively conducted P-waves. While the PR-intervals of the conducted P-waves in Fig. 3 are all the same, we cannot know if they are truly fixed unless we see two P-waves in a row conduct to the ventricles.

I keep repeating that statement because it is so important. If you’re looking at a 2:1 AV-block with a constant conduction ratio (i.e. every-other P-wave is conducting across the entire strip), it’s impossible to differentiate a type I from a type II mechanism.

Think about it: when every second P-wave is blocked you can’t tell if the PR-interval is progressively lengthening (since the AV-node “resets” with each blocked P). As a result, it’s also impossible to determine whether the AV-block we are observing is due to a type I or type II mechanism.

I think I’ve said the same thing about five times but the distinction is important because it can have implications for the patient’s prognosis and management.

### What’s a helpless young electrocardiographer to do??

Unless we have strong proof of the underlying mechanism, it is proper etiquette to make no assumptions and simply call the rhythm in Fig. 3 “2:1 AV-block.” Your peers may try to pressure you to pick sides and call it either type I or type II, but the right move is to remain uncommitted and call it what it is: 2:1 AV-block.

For those with a background in physics. it’s almost like the setup for the Schrödinger’s cat thought experiment[1]: If we do not open the box to see if the cat is alive or dead, it may be both at the same time. Likewise, we know it must be of of the two types of AV-blocks governing the 2:1 phenomenon, but since we can’t tell which, we just assume it is both simultaneously. Thankfully, in our case, if we do manage to get a peek at the actual mechanism, there’s not a 50% chance of killing a helpless feline.

### Why not guess?

I suppose if you were so inclined you could just assume all 2:1 AV-blocks were due to a type I mechanism and you’d be right more often than not (since type I AV-blocks are more common than type II), but that seems a bit messy for my taste. Plus, if you look at the middle portion of Fig. 2, you’ll note that there is a brief spell of 2:1 AV-block in that proven example of type II AV-block.

### Does a bundle branch block help the diagnosis?

Recall that type II AV-blocks are a manifestation of extensive infra-AV-nodal conduction system disease and usually present with a bundle branch block (BBB). So, if you see a 2:1 AV-block with a BBB, does that mean it’s more like to be due to a type II block?

Nope.

There’s no rule saying type I AV-blocks can’t have a fixed BBB as well. In fact, seeing the two together is quite common—and again, since type I AV-blocks are more common than type II’s, the odds are probably even that a given 2:1 AV-block with a BBB is either type I or type II in origin.

In fact, let’s look at the 12-lead from that rhythm strip in Fig. 3.

Figure 4. 12-lead showing 2:1 AV-block with a RBBB.

While there is no bifascicular block, there is a clear RBBB with a prolonged PR-interval on the conducted P-waves. If I was a gambling man I’d be tempted to wager that there was a type II mechanism underlying the 2:1 AV-block.

Good thing I’m not, because I’d be wrong. Here are some more rhythms strip from that same patient showing a clear type I mechanism as the cause of his AV-block (they may seem familiar from our last post on type I AV-block).

Figure 5.

Figure 6.

Figure 7.

Figure 8.

### So there’s no shortcut?

The only way to determine if there’s a type I or type II mechanism is to search for two consecutive P-waves?

Yep.

Here’s a subtle example that gives away the type I AV-block underlying the 2:1 AV-block. Though the strip appears pretty similar to Fig. 3, there’s one small difference. See if you can spot it.

Figure 9.

Did you see the two consecutive P-waves that manage to conduct with a lengthening PR-interval?

Figure 10.

### Speaking of subtle…

2:1 AV-block can present with astounding subtlety. Here are some examples; would you spot them all?

Figure 11. 2:1 AV-block of uncertain mechanism.

Figure 12. 2:1 AV-block later proven to be due to type II AV-block.

Figure 13. Can you spot the hidden 2:1 AV-block?

Figure 14. This is actually the same patient as Fig. 13, but now the 2:1 AV-block is more apparent.

Figure 15. I actually had this categorized as “sinus brady with first degree AV-block” in my collection, but when I pondering its use for some of the other articles in this series something caught my eye… There is actually extremely subtle 2:1 AV-block!

Thanks for following the series, and be sure to catch up below on any of the other rhythms you may have missed!

#### Check out the rest of The 12 Rhythms of Christmas (updated as new posts come out)!

1. I’m sorry to anyone who actually knows some physics out there; I realize it’s not a great analogy and a disservice to the great thought experiment. If I recall correct, SchrÃ¶dinger was also proposing the experiment to demonstrate a perceived fallacy underlying quantum mechanics, not to support it.

### 1 Comment

• Rui says:

Hi! Brilliant article!
What are the causes for 2:1 AV block?