Monday, March 1, 2021

Blog #199 (ECG-MP-16) – AFib – MAT – Wandering Pacemaker - PACs

The ECG in Figure-1 was obtained in the ED from an older man with a long smoking history. He presented with acute shortness of breath. 

  • What is the rhythm?
  • Is this an unexpected finding in this type of patient?
  • Could this be a wandering atrial pacemaker?
  • WHY is it important to recognize the rhythm in Figure-1?


Figure-1: ECG obtained from an older man with a long smoking history — who presented with acute shortness of breath (See text).



NOTE #1: Some readers may prefer at this point to watch the 3:20-minute ECG Video before reading My Thoughts regarding the rhythm in Figure-1. Feel free at any time to review to My Thoughts (that appear below ECG MP-16).



Today’s ECG Media PEARL #16 (3:20 minutes Video) — What is MAT (Multifocal Atrial Tachycardia)? ECG Recognition — What to do about it — How to distinguish from a Wandering Pacemaker ...






MY Approach to Assessing the ECG in Figure-1:

We are only given a single lead rhythm strip to assess. That said — this should be sufficient for accurate interpretation:

  • The rhythm is rapid — and irregularly irregular.
  • The QRS complex is narrow.
  • P waves are present. Looking closely — P wave morphology (and the PR interval) appear to be changing constantly!

IMPRESSION of the Rhythm: By far — the most common cause of an irregularly irregular supraventricular (narrow QRS) rhythm is AFib. The fact that P waves are present rules out AFib as the diagnosis.

  • In order to clarify our observation that P waves are present but constantly changing in shape and in their PR interval — we have labeled the different shapes of P waves in different colors in Figure-2:


Figure-2: I have labeled the different P wave shapes from Figure-1 with different colors (See text).


My Assessment of Figure-2: Realizing that we only have a single lead to assess — and realizing that there are some artifactual undulations in the baseline — it should be apparent that there are multiple different P wave morphologies in Figure-2:

  • Virtually all P wave shapes in Figure-2 are positive — albeit some P waves are almost equally biphasic (RED arrows). The PINK arrow highlights a P wave which is biphasic, but with a tiny and slender negative component.
  • YELLOW P waves highlight pointed P waves — GREY P waves are peaked but a little less pointed — and ORANGE P waves are peaked but much smaller.
  • BLUE P waves are tiny, if not isoelectric.
  • The GREEN and PURPLE P waves look rounder — and are partially hidden at different points within the ST segment.

BOTTOM LINE: The rhythm in Figure-2 is MAT (Multiple Atrial Tachycardia). I'd emphasize the following:

  • MAT is not a common diagnosis. As a result, in order to differentiate MAT from the much more commonly encountered irregularly irregular rhythm (which is AFib) — we need to be certain we are seeing multiple different P wave morphologies that are constantly changing. The colored arrows in Figure-2 make it evident that we are! 
  • MAT almost always occurs in one of 2 common predisposing settings. These 2 settings are: i) In patients with severe, often longstanding pulmonary disease; and/orii) In acutely ill patients with multi-system disease (ie, sepsis, shock, electrolyte and/or acid-base disorders). As a result — I’m hesitant to diagnose MAT in the absence of one of these 2 settings. The fact that the patient in today’s case is a long-term smoker who presents with acute dyspnea is therefore in support of the diagnosis of MAT.
  • PEARL: It is EASY to overlook the diagnosis of MAT because: i) The diagnosis is not common; andii) P waves with changing morphology may not always be evident in all 12 leads — so IF the 1 lead you are monitoring happens to be one in which P waves are not well seen — then you might assume the irregular rhythm in front of you was AFib. 12 leads are better than one! It is especially important to always obtain a 12-lead ECG whenever you see an irregularly irregular rhythm in association with one of the 2 predisposing conditions that MAT is pront to occur in.
  • MAT is not a Wandering Pacemaker. MAT is a pathologic rhythm in which P wave morphology changes from one-beat-to-the-next. In contrast — a wandering pacemaker is often a benign rhythm (if not a normal variant) — in which there is gradual shift in P wave morphology over a period of several beats, most often occurring in an otherwise healthy and asymptomatic patient.
  • The importance of recognizing MAT — is that treatment is different than the treatment of AFib. Most of the time, the heart rate with MAT will not be excessive — so all that is usually needed is to identify and correct the underlying predisposing cause (ie, optimize oxygenation; treat sepsis, electrolyte or acid-base disorders). This is distinctly different than treatment objectives for AFib, which in addition to correcting the cause — consist of rate slowing and medical or electrical cardioversion interventions.


Beyond-the-Core: There are 2 additional advanced ECG findings in the lead II rhythm strip from today’s case:

  • A number of P waves that occur at an early point within the ST segment are non-conducted. In Figure-2 — these include P waves highlighted by the 1st BLUE arrow — the 2 ORANGE arrows — and the PURPLE arrow. (NOTEI acknowledge one could argue that the 2nd ORANGE arrow is an r’ rather than a P wave).
  • The QRS complexes of beats #3, 7, 12 and 16 are all somewhat smaller in amplitude than the other QRS complexes. I suspect the reason for this, is aberrant conduction that is facilitated by the Ashman phenomenon — since each of these beats manifest a relatively short coupling interval — and — each of these beats follow a relatively longer preceding R-R interval (For detailed explanation of the Ashman Phenomenon — See ECG Blog #70).


  • NOTE: For a Summary of KEY points related to MATSee Figure-3 below.


Additional Relevant ECG Blog Posts to Today’s Case: 

  • ECG Blog #65 — for an example of MAT in a patient with chronic pulmonary disease (plus more on the differential diagnosis of MAT).
  • ECG Blog #200 — for an example of Wandering Atrial Pacemaker.

I link to 2 additional illustrative Cases taken from Dr. Smith’s ECG Blog. For each of these posts — Please scroll down to the bottom of the page to see My Comment. These cases provide insight to assessment for MAT:

  • The January 5, 2020 post in Dr. Smith’s ECG Blog — for an example of MAT.
  • The September 30, 2019 post in Dr. Smith’s ECG Blog — for an example of “MAT”, but without the tachycardia ...


Figure-3: Summary of KEY points related to MAT.


  1. Excellent. Thank you. I thought that a WAP tends to be regular unlike a MAT.

    1. @ Polly — Wandering Pacemaker will tend to be more regular than the obviously irregularly irregular rhythm of MAT — but there CAN be variation in the rhythm. Please see ECG Blog #200 ( — which suggests a probable wandering pacemaker that is NOT regular — :)