Concordance, discordance and Sgarbossa's Criteria

New LBBB no longer considered an equivalent of STEMI

I rarely post cardiology clinical tips here on EMS1, primarily because my friend Tom Bouthillet, the EKG Yoda, does it better. But every so often, I do think of something useful that Tom hasn't covered, and those instances are carefully recorded in Kelly's Diary of Notable Achievements.

The last entry was written in crayon, and reads, "Made it all the way through recess without getting beaten up by my sister. Go Team Me!"

I know most of us learned, way back in 12-lead EKG class oh-so-many years ago, about the importance of differentiating a STEMI from its various imposters; ectopic or paced beats, left ventricular hypertrophy, early repolarization, left bundle branch block, and so on.

And one thing stressed in those lessons is that diagnosis of anterior STEMI was impossible in the presence in the presence of left bundle branch block, and that a newly-occurring LBBB was a STEMI equivalent.

Turns out, neither of those things is necessarily true.

The 2013 STEMI Guidelines endorsed by the AHA state that new or presumed new LBBB is no longer considered an indicator or equivalent of STEMI, and there is indeed a method to diagnose STEMI in the presence of a left bundle branch block.

In 1996, Dr. Elena Sgarbossa's developed criteria for recognizing ischemia in the presence of LBBB. That criteria is now widely accepted in clinical practice, and may yield valuable clues allowing the astute clinician to call for timely activation of the cath lab.

Sgarbossa's Criteria, in simplest terms, rely on the presence of inappropriate concordance or excessive discordance.

Normally, in LBBB, we'd expect the J-point or ST segment to move in opposite direction of the QRS complex. In other words, in those leads where the QRS complex is normally a positive deflection, the J-point and ST-segment should be slightly below the isoelectric line, and in those leads where the QRS is negative (primarily V1-V3), the J-point and ST-segment should rise slightly above the isoelectric line.

This is known as discordance, and a small amount of discordance is appropriate and expected in LBBB, as demonstrated in the illustration below:

Now, when the EKG shows inappropriate concordance (J-point and ST-segment elevation in positive leads, and J-point and ST-segment depression in negative leads), there is a 90% likelihood that the patient is having a STEMI. 

In the beat on the left, you'll note inappropriate concordance in a lead with a positive deflection of the QRS (Sgarbossa Criteria A). In the beat on the right, you'll note inappropriate concordance in a lead with a negative QRS (Sgarbossa Criteria B). Inappropriate concordance of ≥ 1 mm is highly predictive of STEMI.

The middle beat illustrates Sgarbossa Criteria C, ≥ 5 mm excessive discordance. Criteria C is the weakest criteria of the three, and may indicate a STEMI only 50% of the time. However, even lower sensitivity is still better than throwing up your hands and saying that it can't be done.

For a more in-depth discussion of Sgarbossa's Criteria and other techniques to recognize ischemia in the presence of LBBB, check out Tom's blog, EMS 12 Lead, or this excellent video tutorial by Dr. Amal Mattu.

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