Wednesday, 3 April 2013

ECG of the Week - 1st April 2013 - Interpretation

We didn't have any clinical information on this week's ECG, but that shouldn't stop us being able to review the ECG.


Click to enlarge

  • Ventricular rate 48 bpm
  • Atrial rate 66 bpm
  • Sinus
  • Regular atrial activity
    • P-P interval relatively fixed
    • R-R interval progressively shortens
  • Progressive PR prolongation culminating in a non-conducted p wave
    • 4:3 & 3:2 relationship (atrial:ventricular activity)
  • Normal (-50 deg)
  • PR - Progressive prolongation (~210 - 360 ms)
  • QRS - Normal (100ms)
  • QT - 480ms (QTc Bazette ~ 480 ms)
  • Slight concave / flat ST elevation V2-4
  • No ST depression
  • P Wave Inversion V1,V2, aVL
    • ? V2 inversion secondary to lead placement as other P wave morphology appears normal

  • AV Block
    • 2nd Degree
    • Mobitz Type I
    • Wenckebach
Clinical Implication
  • I don't have any clinical information on this case, so I don't know the likely cause or outcome.
  • Type I second-degree AV block
    • Can occur during sleep in healthy people
    • It is NOT normal during waking hours
    • Can result in significant exercise limitation if occurs during waking hours
  • Symptomatic patient may require atropine +/- chemical +/- electrical pacing.
  • Cardiology referral should be made for patients found to have a Wenckebach conduction for specialist opinion on management, further investigation, and PPM consideration

Multiple causes, as Clare had pointed out in her comments, which include:
  • Ischaemia / Infarction
  • Drugs - anti-arrhythmic, lithium, alcohols
  • Inflammatory - myocarditis, endocarditis, Lyme's disease
  • Metabolic
  • Infiltrative diseases - amyloid, sarcoid
  • Obstructive Sleep Apnoea
  • Athletic Heart

Features of Wenckebach

Thanks to Jason for highlighting the cardinal features of Wenckebach 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
Something to think about ...
Click on the question link to read our follow-up post on 'The Wenckebach Counter-Intuition'
References / Further Reading 

Life in the Fast Lane
  • Wenckebach Phenomenon here
  • Wenckebach Squared ECG Case here
  • Second-Degree Atrioventricular Block here
  • Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.

1 comment:

  1. 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:

    PRi := { k, k60, k90, k110, k, k60, ... }

    The n'th R-wave (we'll ignore drops, and P is the PP interval):
    R_n := nP + PRi_n

    With an R-R:

    RR_n := R_n+1 - R_n

    := ((n+1)P + PRi_n+1) - (nP + PRi_n)

    := (n+1)P - nP + PRi_n+1 - PRi_n

    := P + (PRi_n+1 - PRi_n)

    If you plug in the PRi's in order you'll find you get a decremental series:

    RR := { P + 59k, P + 30k, P + 20k, ... }

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

    (apologies if the math formats oddly)