Thursday, 4 April 2013

The Wenckebach Counter-Intuition

We are going to depart a little from our usual format today and address an ECG related question we asked yesterday.

This all stem's from this week's ECG which showed a Type I 2nd Degree AV block, a.k.a Wenckebach conduction. 



Features of Wenckebach

So we know the features of Wenckebach conduction, which are:

  • Progressive PR lengthening resulting in non-conducted P wave
  • Progressive R-R interval shortening
  • R-R interval length of dropped beat less than twice shortest R-R cycle
  • Grouped beating
But let's stop for a second, the PR gets longer whilst the R-R gets shorter, that doesn't seem to make sense. 

Why given the progressive PR prolongation do you get R-R interval shortening ?

Let's look at some of features of Wenckebach conduction again:
  • Progressive PR lengthening
    • BUT the magnitude of the lengthening progressively decreases
    • The greatest increase in PR is from the 1st to 2nd beat following a dropped complex
  • Regular P waves
    • P-P interval is relatively fixed
We'll that has certainly cleared everything up !

This is quite a migraine inducing concept to think through and it made more sense when I went through a Wenckebach example.


Wenckebach Example

So let's assume:
  • P wave occurs every second
    • We have regular P wave activity with a relatively fixed P-P interval
  • PR intervals increases by 0.1s then 0.08s then 0.06s
    • We have progressive lengthening but with a decreasing magnitude
  • Let's assume the first PR interval is 0.2s
  • So the PR intervals are 0.2s -> 0.3s -> 0.38s -> 0.44s
  • Now let's start our conduction at time 0 seconds


Time (s)      0         0.2         1        1.3         2         2.38        3       3.44        4

Event          P        QRS       P        QRS      P        QRS        P      QRS       P

  • This means the R-R intervals are 1.1s -> 1.08s -> 1.06s
    • Progressive shortening
  • This is also the reason why the largest R-R interval is less than twice the shortest R-R interval, I'll let you do the math's on this one.
I found drawing a 'ladder diagram' helped me organise my thoughts so I scanned my scribbling and stuck it here.


Click to enlarge
You can see how ladder diagrams can be used at Nelson's EKG site here.


Christopher's response to our Wenckebach question

Christopher , a fellow ECG enthusiast and regular contributor to the comments section, posted an excellent explanation for our PR/RR puzzler. His comments can be found on the blog here, and I've pasted them below.

The amount the PR interval prolongs becomes less with each beat, leading to the shortening R-R interval (presuming a fixed P-P).

I found it to be an abstract concept to wrap my head around at first!

If you think of the R-R in terms of the PRi, it is a bit easier. With a fixed PRi you have a fixed R-R (we'll assume a fixed P-P). If you simply have a longer PRi the R-R does not change, and the same goes for a shorter PRi.

The PRi dictates when the R's occur relative to the P's.

If you vary the PRi from beat-to-beat, then you'll change when the R's occur from beat-to-beat. If you change when the R's occur, you'll change the R-R.

Going back, constant PRi's do not affect the R-R. Thus the amount which varies from beat-to-beat is the change in the R-R.

In AV Wenckebach the amount of beat-to-beat prolongation which occurs lessens (or stays the same), with the largest delta-PRi occurring in the first beat (e.g. 200ms, 260ms, 290ms, 310ms, drop; deltas of 60, 30, and 20). So, if the amount we vary decreases with each beat, the R-R will decrease with each beat.

Put in math form, assuming k is our baseline PRi and we have a repeating series of PRi's, where P is PP interval:






 

I hope that helps others understand it, I had to draw it a lot when I first noticed the decreasing R-R's.