Just a paced rhythm… Or is it? Conclusion

Ok, so I posted a 12 Lead ECG on Facebook this past Monday, June 2nd, 2014, which  generated some interesting comments and thoughts…

EMS 12-Lead on Facebook: 67yom with Dyspnea and Diaphoresis

 This ECG was obtained from a 67 year old male, complaining of difficulty breathing for the past 2 hours, and presented diaphoretic, with no other signs of hypoperfusion.

Past medical history:

  • Hypertension
  • Congestive Heart Failure (CHF)
  • Hypothyroidism

Vital signs:

  • Blood Pressure: 110/88 mmHg
  • Heart Rate: Between 80 -94 beats/min and regular
  • Respiratory Rate: 18 breaths/min
  • SpO2: 97%

Now to the fun part…

As most noticed, it is indeed a paced rhythm, with unipolar leads which cause a bigger and more noticeable “pacer spike”, preceding every QRS. The QRS  are wide in appearance, with an arrow on the bottom of the ECG pointing to the pacing origin.

Some interpreted this as a RBBB due to the dominant R wave in V1, however, this finding in the presence of a pacemaker, suggest LV or septal pacing, with a superior axis, moving away from the inferior leads, as well as the lateral leads, due to impulses originating and moving away from the apex, which causes the axis to shift towards the right upper quadrant of the Hexaxial Reference System, at approximately -118 degrees.

But there is more to it than just a paced rhythm.

Some suggested that since the rhythm is paced,  no other diagnosis could be made, but this is not the case here, but, that’s ok, this is why we discuss this type of cases, to make us better clinicians. Remember, during ischemic events, cardiac tissue may not transmit this potentials properly, whether paced or natural conduction, affecting both depolarization and repolarization phases, which is exactly what we are looking at when evaluating an ECG. A pacemaker can still show ventricular beats and even fail to produce capture.

This is a paced rhythm with Infero-postero-lateral Myocardial Infarction, with 100% RCA occlusion found after Cath Lab activation.

Primary ST-T changes are caused by ischemia and/or infarction, while Secondary ST-T changes are caused by repolarization abnormalities. The later is what we typically see during a normal LBBB or paced rhythm.

Sgarbossa’s Criteria was developed to identify STEMIs in the presence of a LBBB or paced rhythm.

  • Concordant ST Segment Elevation > 1 mm in at least one lead = 5 points
  • Concordant ST Segment Depression > 1 mm in V1-3= 3 points
  • Excessive discordant ST Segment Elevation > 5mm


Discordance: ST-T vector moving away from the terminal portion (the last wave) of the QRS

Concordance: ST-T vector moving the same direction as the terminal portion (the last wave) of the QRS

3 points or more has a 90% specificity  for MI. And then there’s Dr. Smith’s Sgarbossa Rule which replaces the 3rd criterion, increasing both sensitivity and specificity.This replacement looks for an ST/S ratio > -.25 , dividing the ST segment elevation by the depth of that preceding S wave.  This can also applied to discordant depression.

 inf-con-elev2 (2)



As you can see, we have > 5 points with 2 different criterion found

How does it apply to both LBBB and paced rhythms?

  • A LBBB produces a tall wide R wave in leads I, aVL and V6 since impulses traveling towards these leads, which look at the Left Ventricle (LV),  are creating a delayed depolarization
  • In the meantime, the Right Ventricle (RV) depolarizes normally because the Right Bundle Branch is conducting normally. This produces an rS pattern (small  r wave and deep S wave) in V1-3 because impulses  are moving away from these leads, causing a negative deflection (S wave), but impulses travel from myocyte to myocyte, causing a delayed conduction making the S wave wide.
  • A typical paced rhythm, will also have fast RV conduction, as the pacer wires are often implanted on said wall, while the LV is being depolarized slowly


Because of this, a paced rhythm will often present with a LBBB morphology, therefore, this criteria can apply to both LBBBs and paced rhythms, however, it has been shown to be a predominant factor in LBBBs over paced rhythms. Keep in mind that you may not always see changes while the patient may still be experiencing an acute ischemic event, but with a proper assessment and good ECG evaluation, you may just make a difference in someone’s life.

Although no specific Sgarbossa’s criterion study has been published on a particular pacemaker setting, once the particular electrophysiologic  function is understood, abnormalities such as the ones mentioned above can be identified, even if no particular criteria has been specifically developed for that particular scenario.




  • Peter Hammarlund says:

    Not sure if I would say that there are more than 5 Sgarbossa points. The concordant ST depression is in aVL, and you only get 3 points for concordant ST depression in leads V1-V3. The majority of the QRS complexes in the inferior leads are negative and thus there is no concordant ST elevation, but rather unappropriate discordance (using the Smith rule). However, regardless of how the Sgarbossa criteria are defined the findings you point out are clearly abnormal in a paced rhythm and clearly suggestive of an inferior STEMI and the unappropriate discordance in V2 is suggestive of posterior involvment. The best thing to do would be to compare the ECG with an old one (pacemaker patients luckily always have old ECGs) and I think that the changes would be so apparent that this RCA occlusion wouldn’t be missed. Last, I would like to point out that the Sgarbossa criteria were developed to identify STEMI equivalents in the precence of LBBB, but not in paced rhythms. However, as this case shows, they can definitely be useful.

    • Ivan Rios says:

      Thank you very much for taking time to reply. For the most part, most vectors are negative, however, there is concordance in the inferior leads. Although, these leads are predominantly negative, the last wave of the QRS in lead III and aVF are R waves followed by elevated ST deviation which is abnormal indicating primary ST changes (MI), considering secondary ST changes in a pacemaker and ventricular conduction delays are discordant, away from the last wave of the QRS whether positive or negative predominance. Also V2 has concordant ST depression, again, the last portion of the QRS, whether positive or negative, is an S wave, with the ST deviation going the same direction, not a normal secondary ST change, as well as aVL. Sgarbossa in Gusto trial, developed the criteria for both LBBB and RV paced rhythms, not just LBBB. A lot of articles focus more on the LBBB, however, it was intended for both, since they both have a LBBB morphology for the most part. When it comes to Dr. Smith’s modified rule, it does not meet the ST-S ratio, which is approximately .18, not > -.25, but even without a criteria or comparing an old ecg, these findings are obvious and clear for MI.

      • Peter Hammarlund says:

        First off, I would like to say that I am, by no means, an expert and you’ve probably read more articles than I have on the matter. It’s an interesting subject to discuss and you’re more than welcome to prove me wrong 🙂
        Yes, the Sgarbossa criteria were evaluated by Sgarbossa in patients from the GUSTO-1 trial with paced rhythms, but with a much less impressive result (and a rather small number of patients and LV pacing was excluded). Thus, in her article in NEJM 1996 they only discuss the Sgarbossa criteria in the presence of LBBB. I don’t think we actually disagree in the matter. Rather it’s a question of semantics.
        What we probably disagree on is the definition of discordance which in my opinion is when the ST segment is in the opposite direction of the majority of the QRS complex and not just the end, at least when using the Sgarbossa criteria. However, you may be right that the ST segments should be assessed in relation to the end of the QRS complex, but in that case I don’t think you should refer to the Sgarbossa score. Whether the Sgarbossa criteria are fulfilled or not, I believe there is no disagreement on that this is a clearly abnormal ECG that should prompt you to consider a STEMI equivalent.
        You don’t have an old ECG for comparison? That would be highly interesting.

        • Ivan Rios says:

          I wish I had a prior ECG to compare it to. I definitely enjoy the discussion and any opportunity to share opinions, studies and cases. Keep it up and once again, thank you for taking time to participate. You’re opinions are very much appreciated.

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