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LBBB with MI present?

by on March 1, 2011

Is this a STEMI or is this normal for a LBBB?

There are several criteria s to determine if a BBB is present and if so which one, Left or Right? First QRS has to equal to or be > .12mm wide. We can use the turn signal method In lead V1 to see which way the QRS terminates. If it terminates down it would be a LBBB and if it terminated up it would be a RBBB. If you choose, you can use the rabbit-ear method to determine Left or Right BBB. If you see rabbit-ear patten (RSR’) in Leads V1 and V2 it would then most likely be a RBBB. If you were to see rabbit-ear pattern (RSR’) in leads V5 and V6 then it is most likely a LBBB. Although V1 is not shown here this is indeed a LBBB.

Now that we know we have a LBBB we now need to try to determine if we have a MI with this LBBB as well. One thing we know is that a LBBB is a huge STEMI Imposter. This means that there will probably be ST elevations present. What do you think about these elevations?

From → Case File

  1. Acute STEMI. Lead V2 shows “excessive discordance”. The S-wave is approximately 25 mm deep. ST-elevation is “excessively discordant” when the ST/QRS ratio is > 0.2. Let’s do the math. 25 x 0.2 = 5 mm. Is the J-point elevated more than 5 mm above the isoelectric line? Yes. Lead V5 shows 1 mm of concordant ST-elevation which removes all doubt (if there was any).


    • Tom B, I am still learning how to do the math to figure out how many mm of discordant ST elevation is normal vs abnormal. I have read that usually 5mm or greater or ST elevation discordance is probably positive for STEMI. Thank you for sharing and I will now place that formula in my memory bank.

      • Discordant ST-elevation > 5 mm is one of Sgarbossa’s criteria.

        Concordant ST-elevation = or > 1 mm
        Concordant ST-depression in leads V1, V2 or V3
        Discordant ST-elevation = or > 5 mm

        However, it’s the weakest of Sgarbossa’s criteria because of the relatively poor specificity. The problem is that when the S-wave is deeper than 25 mm the ST-segment can be elevated beyond 5 mm.

        A more useful and accurate way to determine the acceptable amount of ST-elevation when the QRS is negatively deflected is to use a cut-off of 1/5 the depth of the S-wave (ST/QRS ratio of 0.2).

        It has a higher sensitivity and specificity!

        When the S-wave is only 10 mm deep, 3 mm of ST-elevation is unacceptable!


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