Discordant ST-Segment Elevation in LBBB or Paced Rhythm

If you’ve been following the Prehospital 12-Lead ECG blog for a while, you know that I’m advocate of using Sgarbossa’s criteria to help identify acute STEMI in the presence of left bundle branch block (LBBB) or paced rhythm.

According the Sgarbossa’s original criteria, 5 mm of discordant ST-segment elevation is required to identify AMI in the presence of LBBB.

Why 5 mm when normally we require only 1 or 2 mm of ST-elevation?

Because in the setting of left bundle branch block or paced rhythm, it’s normal for the ST-segment and T-wave to be defected opposite the main deflection of the QRS complex!

That’s why it’s necessary to consider the depth of the QRS complex when examining the amount of discordant ST-segment elevation. The deeper the S-wave, the greater the secondary ST-T wave abnormality in the opposite direction!

In the original article I wrote on the topic, I showed this example 12-lead ECG to show why the 5 mm criterion is problematic.

As you can see, this 12-lead ECG shows sinus rhythm with left bundle branch block and > 5 mm of discordant (opposite the QRS complex) ST-elevation in leads V1, V2, and V3 (the right precordial leads). The T-wave are huge!

The problem is, this patient was not experiencing acute myocardial infarction. The ST-segments are elevated > 5 mm because the S-waves are extremely deep (off the bottom of the ECG paper for leads V2 and V3).

Had we used the modified criterion of discordant ST-elevation that is = or > to 0.2 the QRS complex (ST/QRS ratio) we would have seen that in lead V1 the S-wave is 50 mm deep. Thus, we would require at least 12.5 mm of ST-segment elevation to consider this finding positive for acute STEMI.

(Credit to Dr. Smith’s ECG Blog)

There’s another way the modified criterion can help you!

Consider this 12-lead ECG that shows a ventricular paced rhythm. It’s been in my collection for many years, and I regret that I no longer recall where it came from.

This ECG does not meet Sgarbossa’s criteria for diagnosing AMI in the presence of LBBB. With the exception of lead V6, the paced QRS complexes show appropriate T-wave discordance, and none of the ST-segments are elevated to 5 mm or more.

But wait! The ST-segments are elevated far greater than 0.25 the depth of the QRS complex in leads II, III, and aVF! This patient is experiencing acute inferior STEMI!

The intrinsic QRS complex in the right precordial leads also shows an > R/S ratio in lead V1 and V2 and ST-segment depression suggesting posterior extension, which clinches the diagnosis.

So remember, when using Sgarbossa’s criteria, huge QRS complexes can cause false positive and tiny QRS complexes can cause false negatives, unless you use the modified rule that considers ST-segment elevation as a percentage of the QRS complex!

See also:

Identifying AMI in the presence of LBBB – Sgarbossa’s Criteria Part I

Identifying AMI in the presence of LBBB – Sgarbossa’s Criteria Part II

“New” LBBB – What’s the big deal?

Sgarbossa’s Criteria – New Graphic

Found on the Lifenet Receiving Station (LBBB with concordant ST-depression in leads V3 and V4)

62 year old male CC: Chest pain (LBBB with ST-elevation > 0.2 the QRS complex)

58 year old female CC: Chest pain

58 year old female CC: Chest pain – Conclusion (meets all 3 of Sgarbossa’s criteria)

STEMI best seen in PVC (Dr. Smith’s ECG Blog)


  • Christopher says:

    Do any of the algorithms used by ECGs consider this criteria?

  • Tom B says:

    C.Watford – I suspect the GE-Marquette 12SL interpretive algorithm considers the depth of the S-wave for two reasons.First, you'll notice the 12-lead ECG with the huge S-waves in the right precordial leads is not giving the ***ACUTE MI SUSPECTED*** message.Second, LBBB is not causing a lot of false positives in Southern California (where they use interpretive algorithms for their STEMI system).Tom

  • Steve Smith says:

    Great post!!I don't think any of the algorithms attempt to diagnose MI in the presence of LBBB. Do they? I am quite sure they do not program the ratio (ST/S ratio of 0.25) into the algorithms.I have added new cases to my study and re-analyzed, and found that 0.20 is a better cutoff (by a small margin). The normal ratio is from 0.09 to 0.15. You should start to get suspicious with a ratio higher than 0.15.Steve Smith(Dr. Smith's ECG blog)

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