This article is the fifth 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.
Before we delve into today’s particular rhythm I’d like to discuss a bit of terminology. My first ECG textbook was the 8th edition of Marriott’s “Practical Electrocardiography,” which I chased with a couple more of Dr. Marriott’s books. As a result, I tend to describe arrhythmias using the same language he used in his texts.
We should all be aware that there are four main types of atrio-ventricular block:
- “First degree”
- “Second degree type I,” also known as “Mobitz type I,” “Mobitz I,” or (commonly) just “Wenckebach”
- “Second degree type II,” also known as “Mobitz type II,” “Mobitz II,” or (rarely) just “Hay”
- “Third degree,” also known as “complete AV-block” or “complete heart block”
First and third degree AV-block are pretty straightforward, though I tend to prefer the term “complete AV-block” for the latter in order to avoid confusion over the different “degrees.” The two second degree blocks, however, are as mess. As a result, I find it preferential to refer to them as simply “type I” and “type II,” dropping the bulky eponyms entirely. Additionally, we don’t even need to preface those terms as “second degree” since there are not multiples types of either first degree or complete AV-block, leaving us with:
- First degree AV-block
- Type I AV-block
- Type II AV-block
- Complete AV-block
As a final point, note that I always try to use the term “AV-block” and not simply “block” when talking about conduction abnormalities of the AV-node. That is because there are other locations where the above blocks can develop (sometimes concurrent with AV-block), such as the sino-atrial node (SA exit block).
Type I AV-Block
Type I AV-block is a pretty interesting phenomenon, first described in 1899 by Dutch anatomist Karel Wenckebach. It is characterized by progressive lengthening of the PR-intervals that culminates in a dropped (fully blocked) P-wave and a pause in the ventricular rhythm, reseting the AV-node so that the cycle can repeat.
Let’s look at an example:
Even those with limited experience in basic dysrhythmias should be able to identify the increasing PR-intervals…
That, however, is where most folks’ knowledge ends. But I expect most of the folks reading this blog, if they don’t already, would like to understand why this pattern occurs.
It turns out that the healthy AV-node exhibits a normal behavior called “decremental conduction;” where the faster you stimulate the node with impulses, the slower it conducts. In fact, the “refractoriness” of the AV-node can be tested in the EP lab by purposely pacing the atria at increasing rates until the AV-node starts exhibiting slower and slower conduction (with longer and longer PR-intervals). Eventually the conduction slows to such an extent that it fails entirely for a beat, resulting in a blocked P-wave and a brief pause in the ventricular rate. This act “resets” the AV-node and allows it to conduct more rapidly on the next P-wave. The cycle, however, continues as the relentless barrage of atrial impulses slows the AV-node to a greater and greater extent until another P-wave is blocked and the AV-node resets again.
That is an important protective mechanism because it helps control the ventricular rate in the setting of atrial fibrillation, where the atrial rate can exceed 500 /min. In most patients with type I AV-block, however, this decremental conduction has become pathological to the point that atrial impulses at even normal rates—which should conduct regularly with no signs of decremental conduction—start to exhibit the phenomenon.
To further examine this phenomenon in type I AV-block, let’s start off with a normal sinus rhythm. There is always some minor variation in the sinus rate, but these P-waves march out with fairly regular PP-intervals.
Now let’s talk about one of the hallmark finding of Wenckebach phenomenon and our sign of decremental conduction on the surface ECG: RP/PR reciprocity.
While everyone should be familiar with the PR-interval, it’s partner the RP-interval is not nearly as well known. Since the PR-interval is the time from a P-wave to the next QRS-complex, the RP-interval is the time from a QRS-complex to the next P-wave.
One of the most hallmark features of type I AV-block is a reciprocal relationship the PR and RP-intervals: The shorter an RP-interval, the longer the next PR-interval.
Starting at the left side of Fig. 4, we see the first RP-interval (in red) is 520 ms; the PR-interval that follows it is 400 ms. Next, we encounter a shorter RP-interval of 450 ms, followed by a longer PR-interval at 460 ms. The next P-wave arrives with an even shorter RP-interval at about 390 ms (not shown). This RP-interval turns out to be too short to conduct and the P-wave is dropped.
The next P-wave that arrives enjoys a luxurious 1240 ms RP-interval, allowing the AV-node to conduct relatively rapidly with a PR-interval of only 320 ms. The pattern then repeats, although it does not progress as rapidly towards a dropped P-wave the second time around (see Table 1).
That brings up an exceedingly important feature of type I AV-block…
Because of the patterns that emerge from the Wenckebach phenomenon, you’ll notice that the QRS complexes in the above figures appear to demonstrated two clusters, separated by a relatively long pause after the dropped P-wave.
It may seem like lengthening PR-intervals are the most prominent feature of type I AV-block, but it turns out that grouped beating is even more important. It can be difficult to follow P-waves or pick up subtle PR-interval elongation or see dropped complexes that are buried, but clumps of QRS complexes are easy to spot.
On the topic of grouped complexes, it’s apparent at this point that there is great variability in the patterns that type I AV-block can form. All of the following strips were obtained from the same patient over a short period of time (not continuous):
In this post we’ve seen everything from 8:7 conduction ratios (Fig. 1) all the way down to 2:1 (Fig. 13), and these ratios can be fairly fixed (Fig. 6 and Fig. 7) or quite variable (Fig. 8–14). There are even larger ratios possible, from 14:34 up to 20:19 or more! These longer, more subtle patterns are known as atypical or long-cycle type I AV-block. For more on them check out these posts from our friend Dr. Arnel Carmona over at ECG Rhythms.
There are two more features of Wenckebach phenomenon that come to light after studying its QRS patterns…
With a classic Wenckebach period, due to the timing of the RP/PR reciprocity and the elongation of the PR-intervals, you’ll notice that the RR-intervals decrease progressively across each group of complexes. Let’s look at that first rhythm strip again and measure out the RR-intervals.
Wait! If you’re paying attention, you’ll notice that, while the RR-interval decreases at the beginning of the large group (925 ms, 860 ms, 850 ms), it stabilizes in the middle and actually increases at the end (855 ms, followed by 905 ms).
The key is that I stated progressive RR-shortening occurs in the classic Wenckebach pattern. It turns out that not too many type I AV-blocks fit that ideal (but we love them in spite, or even because of it…).
In fact, it’s pretty common for the last RR-interval in a grouping to be longer than expected; and sometimes it’s even the longest RR in the whole series.
Another classic finding, but one that is more common and we actually see in Fig. 16, is that the longest RR-interval is less than twice the shortest RR-interval.
Note that the first big RR gap measures 1575 ms, while the shortest cycle preceding it is 910 ms. Since 1575 ms is less than 1820 ms (2*910 ms), that fits with type I AV-block. The same goes for the next series, where 1565 ms is less than 1700 ms (2*850 ms).
The “footprints of Wenckebach” were summarized by Dr. Marriott as:
- Grouped QRS complexes, especially pairs, and trios (which are easy to miss)
- Progressive shortening of the RR-intervals across a grouping
- The longest RR-interval is less than twice the shortest
Keep in mind, however, that many type I AV-blocks do not follow the rules! While there is sometimes no apparent trigger for this rule-breaking, here are some common culprits.
- Variations in the sinus rate, which affect everything downstream (RP-intervals, PR-intervals, and the ventricular response)
- Variations in AV-conduction due to fluctuations in the sympathetic and parasympathetic nervous systems
- Ectopic beats interrupting the process (PAC’s, PVC’s, even concealed PJC’s…), as seen in Fig. 10
We’ll go over some of the common mimics of type I AV-block later this week but for now, while it’s possible that you’re blissfully unaware of them, get out there and spot some Wenckebachs!
- Blame it on some interesting history involving Wenckebach, Hay, and Mobitz. In short, Wenckebach first described type I block, Hay first described type II (both without the benefit of electrocardiography!), and Mobitz then named them “type I” and “type II” twenty years later.
Check out the rest of The 12 Rhythms of Christmas (updated as new posts come out)!