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
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
This is quite a migraine inducing concept to think through and it made more sense when I went through a 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.
|Click to enlarge|
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.