ECG Blog #155 (PACs – Wandering – MAT )

The ECG in Figure-1 was obtained from a woman in her 50s, who complained of intermittent chest discomfort in recent weeks. She was hemodynamically stable at the time this tracing was obtained.

• How would you interpret her ECG?
• HINT: The rhythm is not sinus ... 

Figure-1: ECG obtained from a woman in her 50s, with intermittent chest discomfort. How would you interpret this tracing?

Interpretation: The interesting part of this tracing is the rhythm. We have numbered the long lead II rhythm strip. What do you see?

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ANSWER: The rhythm is irregular. The QRS complex is narrow. P waves are present — albeit as you look from 1 beat to the next in the long lead II rhythm strip, it should be clear that P wave morphology changes ...

• We identify 3 different shapes of P waves on this tracing (Figure-2). That is, P waves are tall and pointed in front of beats #1,5,6,7 and 8 (RED arrows) — P waves are round in front of beats #2, 9 and 10 (BLUE arrows) — and, P waves are pointed, but not quite as tall in front of beats #3,4,11,12 (GREEN arrows).
• Each of the P waves in this long lead II is conducting. We know this because for each of these 3 different P wave shapes — all beats of that shape manifest the same PR interval.

Figure-2: Colored arrows highlight atrial activity in the long lead II rhythm strip. What is the rhythm? (See text).

The principal differential diagnosis for an irregular rhythm with different-shape P waves that are conducting is: i) Sinus rhythm with multiple PACs; ii) MAT (Multifocal Atrial Tachycardia); and, iii) Wandering pacemaker. Regarding this differential:

• This is not sinus rhythm with PACs — because there is no predominant underlying sinus rhythm. And, although R-R intervals are shorter in some places — there are no beats that are especially early (ie, there are no PACs).
• This is not MAT — because P wave morphology and the PR interval do not change from one-beat-to-the-next. Instead, there is gradual change in the site of the supraventricular pacemaker over the course of several beats. This strongly suggests the presence of a wandering atrial pacemaker.

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What is a Wandering Pacemaker?

Occasionally — the site of the atrial pacemaker may shift (wander) away from its usual site of origin in the SA (Sino-Atrial) Node. In most cases — wandering atrial pacemaker is a benign normal variant that occurs in patients without underlying heart disease. It may result from variationsin vagal tone (that slow SA nodal discharge and allow other atrial sites to temporarily emerge) — or there may be no obvious cause.

• ECG recognition of wandering pacer requires a long enough rhythm strip to appreciate gradual change over a period of beats from one P wave morphology to another. Technically, there should be at least 3 different atrial sites — in order to distinguish a wandering atrial pacemaker from a simple atrial escape rhythm. The clinical reality, is that most of the time — the period of monitoring available for our scrutiny will simply not be long enough to appreciate gradual shift in the site of the atrial pacemaker to at least 3 different sites. As a result, true wandering pacemaker is not a common diagnosis.
• KEY: All too often, even experienced interpreters fail to adequately assess the long lead rhythm strip. On seeing a few upright P waves — the “eye” tends to assume that the rest of the rhythm is also sinus. All it takes to avoid missing subtle deviations from strict sinus rhythm is a few careful seconds in which you ensure that you scrutinize the P wave preceding each QRS complex in the long lead rhythm strip. You’ll be amazed at how disciplined addition of these few extra seconds will easily pick up those cases in which the rhythm is not strictly sinus.
• P.S.: Other than the rhythm — there are minimal nonspecific ST-T wave abnormalities in several leads that do not appear to be acute. These include shallow T wave inversion in lead III, and relative ST-T wave flattening in leads II, aVF, V5, V6. Thus, the wandering pacemaker is likely to be unrelated to the cause of this patient's intermittent chest discomfort.

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COMMENT: The example of wandering atrial pacemaker in Figure-2 is unique in our experience for allowing definitive diagnosis of this rhythm in no more than a short long lead II rhythm strip. Most of the time, a much longer period of monitoring is needed.

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Acknowledgment: My thanks to Joshua Wyeth from Petoskey, MI, for his permission allowing me to use this tracing and clinical case.

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Related ECG Blog Posts to Today's Case:

• ECG Blog #65 — for more on distinction between MAT, Wandering Pacer & Sinus Rhythm with PACs.
• ECG Blog #200 — for more on distinction between these rhythms (plus the 4-minute Audio Pearl in this blog post).