Thursday, March 4, 2021

Blog #200 — Wandering Pacemaker (vs MAT)?

There is no clinical information is available for the ECG and 2-lead rhythm strip shown below in Figure-1.

  • HOW would you interpret this tracing?
  • What treatment is likely to be needed? 

Figure-1: How would you interpret this ECG? (See text).



Editorial Comment:

It is always challenging to interpret tracings without the benefit of clinical information. That said — this situation is common in clinical practice. My experience in this area derives from the 30 years during which I was charged with interpreting all ECGs ordered by 35 medical providers at a primary care clinic — as well periodic stints during which I interpreted hospital tracings without the benefit of any history. 

  • The challenge lies with having to decide which tracings in the “pile of ECGs to be interpreted” were those for which I needed to pull the medical chart (or call the provider) because of ECG findings of immediate potential concern.
  • Obvious time constraints made it impossible to pull the chart for each ECG that I was given to read (I’d never get anything else done if I did so).
  • I therefore became well versed in the skill of limiting the charts that I would pull to those patients whose ECGs showed findings I thought were important and potentially indicative of an acute situation that may have been overlooked.



MY Thoughts on the ECG in Figure-1:

As always — systematic interpretation of any ECG should begin with assessing the cardiac rhythm. In general — lead II and lead V1 are the 2 best leads on a 12-lead tracing for assessing atrial activity — and we have the advantage in Figure-1 of a simultaneously-recorded 2-lead rhythm strip of both of these leads. By the PsQsand 3R Approach:

  • The rhythm in Figure-1 is clearly irregular.
  • The QRS complex is narrow (ie, not more than half a large box in duration = ≤0.10 second)
  • The rate varies from 50/minute — to just under 100/minute.
  • More than 1 P wave morphology is present. That said — P waves do appear to be related to neighboring QRS complexes, because the PR interval for the P wave shapes that we see remains constant (See Figure-2).


Figure-2: I’ve labeled the different P wave shapes with different-colored arrows (See text).


MY Thoughts on Figure-2:

There are 2 different P wave shapes in Figure-2.

  • The tracing begins with sinus beats (ie, RED arrows highlight 3 similar-looking upright-in-lead-II P waves — all with the same PR interval).
  • P wave shape then changes for beats #4, 5 and 6 (ie, BLUE arrows highlighting an almost isoelectric, if not negative P wave with fixed PR interval).
  • The atrial focus then shifts back, with return to sinus P waves for beats #7, 8, 9 and 10 (ie, return of RED arrows highlighting similar-looking, upright P waves in lead II — albeit with variability in the R-R interval).
  • The rhythm in Figure-2 concludes with a slowing-down of the ventricular rate, as the 2nd atrial focus returns, in which the P wave is almost isoelectric (ie, BLUE arrows for beats #11 and 12).

BOTTOM LINE regarding Figure-1: The rhythm in Figure-2 is most consistent with a Wandering Atrial Pacemaker. This is because the change from one atrial site to the next occurs gradually over a period of several beats.

    Technically, for a rhythm to be classified as a wandering pacemaker — there should be gradual shift between at least 3 different atrial sites. Since we only see 2 different atrial sites (highlighted by RED and BLUE arrows) in Figure-2 — we would need a longer period of monitoring to prove this rhythm is a wandering pacemaker. That said — wandering pacemaker is the most logical explanation for this rhythm. In support of this conclusion — the rest of the 12-lead ECG looks benign, consistent with the common finding of wandering pacemaker as a type of normal variant occurring most often in asymptomatic, otherwise healthy young adults. No treatment is needed for this rhythm. So, even though we have not been provided with any clinical history on this patient — I would be comfortable concluding that the ECG in Figure-2 looks benign.
  • PEARL: The reason it is uncommon (if not rare) in clinical practice to see a wandering atrial pacemaker — is that most providers do not pay long enough attention to beat-to-beat change in P wave morphology needed to identify gradual shift between at least 3 different atrial sites.


SUMMARY: Review of the KEY features of wandering atrial pacemaker is the theme below for our ECG Media Pearl #17 (a 3:30 minute audio recording).

  • Written review of wandering pacemaker appears below in Figure-3.
  • Review of MAT is covered in our ECG Blog #199.


Today’s ECG Media PEARL #17 (3:30 minutes Audio) — What is a Wandering Atrial Pacemaker (as opposed to MAT)?



Figure-3: Written review of wandering atrial pacemaker.

ADDENDUM (3/4/2021):

I received the following note from David Richley regarding today’s tracing: “I think I would use different terminology to describe this because to me the atrial pacemaker doesn’t so much ‘wander’ as ‘jump’. I would describe this as sinus arrhythmia with junctional escape rhythm at 60-65/minute every time the sinus node discharge rate slows to below that rate. I interpret the escape beats as junctional rather than atrial, because athough the P waves, (which are initially negative in II, aVF and V4-V6 — and positive in aVR) precede the QRS — the PR segment is very short, suggesting an AV nodal origin. However, we describe this phenomenon — I do agree that it’s likely to be completely benign.


MY Thoughts: Dave’s comment is one of the reasons why: i) The diagnosis of wandering pacemaker requires clear demonstration of shift in the atrial pacemaker in at least 3 different sites. We only see 2 different sites here; andii) The diagnosis of wandering atrial pacemaker is not common. 

  • It’s impossible to rule out Dave’s theory from the single tracing we have.
  • That said — the BLUE arrow P wave site may or may not be of AV nodal origin (you can see a similar, near-isoelectric P wave with short PR interval from a low atrial site).
  • I also considered the possibility of the BLUE arrow P waves representing junctional escape — but decided against it because the difference in R-R interval from what we see between beats #9-10 vs what we see between beats #10-11 is more than what I’d expect based on the cadence of rate variation I see from beats #7-10.
  • Bottom Line: We both agree there is a shift in the pacemaker site in a rhythm that is likely to be benign. And, we both agree that additional monitoring would be needed for a definitive response. THANK YOU Dave!

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