The 12 Rhythms of Christmas: Type II AV-Block

This article is the sixth 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.

Type II AV-Block

Except for first degree,  type II is probably the simplest of the AV-blocks to identify. There’s really only two major criteria:

  1. A P-wave suddenly and unexpectedly fails to conduct to the ventricles.
  2. The PR-intervals of the P-waves that do conduct are fixed and equal.

Let’s examine the ECG below, which exhibits both of those findings. The computer, however, disagrees and suggests and alternate diagnosis. Which of is right?

Mobitz II AV-block

Figure 1. This ECG has P-waves that don’t conduct; is it type II AV-block?

The first step in our rhythm analysis is to examine the atrial rate for regularity. Why is that important? Because:

The most common cause of a pause is a non-conducted PAC.

Non-conducted PAC’s

Let’s digress for a moment and examine an ECG that the computer interprets as actually showing type II AV-block. It was obtained on a 33 year old male with a chief complaint of pleuritic chest pain x 3 weeks.

Non-conducted PAC's

Figure 2. Sinus rhythm with frequent non-conducted PAC’s interpreted as type II AV-block by the computer.

The first thing you’ll notice on the above ECG is that there are several pauses in the regular sinus rhythm. Close inspection, however, reveals the culprit…

Non-conducted PAC's

Figure 3. Non-conducted PAC’s.

There are P-waves buried in the T-waves preceding each pause! Not only are they P-waves, but they arrive markedly early, confirming their identity as PAC’s. But why aren’t they followed by QRS complexes?

Recall that the existence of the absolute refractory period in the heart. The myocardium works in such a way that it cannot depolarize a second time immediately after it has already depolarizated. The AV-node exhibits that same trait, but to a greater degree in that it takes a longer time to “recharge” than the myocardium. Not only does that allow ample time for the ventricular cells to fully repolarize before the next impulse can discharge them, it’s also a protective mechanism that discourages excessive heart rates by limiting the rate at which atrial impulses can traverse the AV-node and cause the ventricles can contract.

If the AV-node didn’t protect the ventricles, everyone in atrial fibrillation (with atrial rates of 400–600 bpm) would also be in ventricular fibrillation because every atrial depolarization would be transmitted to the ventricles.

The PAC’s above simply arrive so early following the preceding P-waves that the AV-node fails to conduct them. If the AV-node didn’t prevent their conduction the patient would be at risk of an R-on-T phenomenon and V-fib or V-tach. The AV-node saves the day!

Here’s some more examples from the same patient of the AV-node doing it’s thing.

Non-conductec PAC's

Figure 4. Sinus rhythm with non-conducted PAC’s. This is the same patient as Fig. 2.

Non-conducted PAC's

Figure 5. Non-conducted PAC’s from Fig. 4 are highlighted.

Non-conducted PAC

Figure 6. A single non-conducted PAC. This is the same patient as Fig. 2 and Fig. 4.

Blocked or Non-conducted?

Note that in the discussion above I refer to the PAC’s as “non-conducted,” but you might have seen them referred to as “blocked” PAC’s” in other sources. That is a conscious choice I make to avoid confusion regarding whether the failure to conduct the P-waves is pathological or not.

In Fig. 2 thru Fig. 6 the AV-node is behaving normally and doing it’s job of filtering out overly premature atrial discharges. Contrast that with the cases in our last article on type I AV-block, where the AV-node was behaving abnormally and progressively delaying conduction until a P-wave was eventually blocked.

In my mind (and that of Dr. Henry Marriott, whose texts first taught me about this distinction), a block is something pathological, where the AV-node is failing to transmit atrial discharges that should be conducted.

Non-conducted P-waves, on the other hand, are something decidedly more benign. The AV-node is behaving normally; the atrial complexes just arrive so early that we don’t want them conducting anyway! That is normal physiology, and I don’t want it confused with something pathological. Once you start noticing non-conducted PAC’s (they’re uncommon but certainly not rare), you’ll also notice how folks that don’t understand that distinction over-react to their presence and confusing them with true AV-block.

Though the PAC’s might be caused by an underlying issue (we’re not sure if the patient in Fig. 2 thru Fig. 6 was experiencing recurrent pericarditis), their failure to conduct is not the problem.

Back to our first case

So the first thing we want to examine is the PP-intervals to make sure that we’re not missing any PAC’s.

Normal sinus rhythm with tpye II AV-block

Figure 7. The PP-intervals from Fig. 1.

Though there are some minor variations in the PP-intervals, they are acceptable for what we expect with normal sinus rhythm. Additionally, though the PPi’s preceding the second two dropped P-waves appear shorter than the other PPi’s (705 and 710 ms), the last dropped P-wave is preceded by the longest PP-interval on the strip: 750 ms. It’s unlikely that the other P-waves arriving “early” played a role in their failure to conduct.

The next thing we’re worried about is whether there is PR-elongation. Type I AV-block is more common than type II AV-block and can present subtly. The solution is to just compared the PR-intervals.


Fixed PR-intervals in type II AV-block

Figure 8. Fixed PR-intervals from Fig. 1.

All of the conducted P-waves share the same exact PR-interval.

With our two major criteria met, we can say with reasonable confidence that we are truly looking at type II AV-block in Fig. 1.

Icing on the cake

There are some other less important features that also contribute to the diagnosis of type II AV-block in Fig. 1.

First, let’s look at the patient’s age. He’s 84 years old, while the patient in Fig. 2–Fig. 6 was only 33. An 84 year old is much more likely to be experiencing a malignant form of AV-block.

Second, type II AV-block is usually caused by disease of the conduction system below the level of the AV-node, while type I AV-block is typically due to abnormal conduction within the AV-node and non-conducted PAC’s are due to normal AV-nodal behavior. What else is below the AV-node? The bundle branches:

Figure 9. Just the 12-lead from Fig. 1.

Figure 9. Just the 12-lead from Fig. 1.

The 84 year old in our initial case also demonstrates a right bundle branch block. He also shows significant left axis deviation that could be due to a left anterior fascicular block (LAFB), though that is less certain. Since type II AV-block is usually due to significant conduction disturbances in the bundle branches and major Purkinje fibers, it is almost always accompanied by at least some manner of bundle branch block, if not true bifascicular block. There is also a slightly prolonged PR-interval that could be to impaired conduction through the AV-node or in that last conducting posterior fascicle (see our discussion on first degree AV-block)

So, in summary, we have an ECG from an elderly man showing:

  • Normal sinus rhythm
  • No signs of ectopy
  • Fixed PR-intervals
  • Slightly prolonged PR-intervals
  • Right bundle branch block
  • Possible left anterior fascicular block
  • Frequent dropped P-waves

Given the overall picture, we can say with near certainty that what we are looking at is type II AV-block.


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

The 12 Rhythms of Christmas: Sinus Tachycardia
The 12 Rhythms of Christmas: Sinus Bradycardia
The 12 Rhythms of Christmas: Atrial Flutter
The 12 Rhythms of Christmas: First Degree AV-Block
The 12 Rhythms of Christmas: Type I AV-Block

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