After I finished school, I was told that you couldn’t diagnose a STEMI on a 12 lead if the patient had a left bundle branch block (LBBB). After furthering my studies, I have learned that… yes… yes you can. When I hear that providers can’t diagnose a in a LBBB ECG, I feel like I am going to have a coronary on the spot. Now if that is you, no judgement here. Just don’t do it any more after reading this. You don’t want me to enter a hypertensive crisis or kick off my SVT do you?

*Fun fact, this also works for ventricularly paced rhythms*

Now to get things kicked off… ECG interpretation is all about pattern recognition and a little bit of math. And luckily for you it isn’t some advanced calculus; just simple division.

If you need a review on 12 lead interpretation, check out this post:

A Few Basics

A LBBB is when the left bundle is blocked and the impulses have difficulty depolarizing the left ventricle efficiently. The blocked left bundle, the impulse will slowly depolarize the left ventricle instead of going down the normal conduction pathway which makes the QRS wide. On a 12 lead, there are a few things to look out for:

1. A wide QRS (over 120 ms)
2. Dominant S wave (negative) noted in V1 and broad dominant R wave in V6 (lead I and aVL will have a similar appearance to V6)

*A tip to remember the difference between which way the QRS faces in a LBBB/RBBB is to think of a turn signal. If you want to turn LEFT, you pull the turn signal DOWN. So V1 will have a negative QRS complex in a LBBB. If you want to turn RIGHT, you push the turn signal UP. So V1 will have a positive QRS complex in a RBBB.*

In this example, you see the wide QRS. You also see the negative QRS (dominant S wave) in V1 and the prominent R wave in V6 that looks a little funny. Lead I and aVL look similar to V6 as well. This is your classic LBBB.

Now looking at this 12 lead, you may notice that when the QRS is negative, there is ST elevation. And when the QRS is positive, there is ST depression. That is called DISCORDANCE and is completely normal for a patient with a LBBB. When the QRS and T wave are travelling in the same direction, it is called CONCORDANCE. Because of this, people say you can’t diagnose a STEMI on a LBBB 12 lead.

Sgarbossa Criteria

Back in 1996, Dr. Sgarbossa created criteria for diagnosing a STEMI on a 12 lead in the presence of a LBBB. There was three criteria in that meant there was a STEMI:

1. Concordant ST depression that is greater than 1 mm in V1-V3.
2. Concordant ST elevation that is greater than 1 mm in any lead.
3. Greater than 5 mm of elevation in a discordant complex.

You can see the criteria below with examples of criteria 1 and 2:

This 12 lead is amazing because it shows the two criteria. You have the ST depression in V1-V3 and concordant elevation in leads II, V5, and V6!

Now these were a great start, but the criteria dealing with the elevation being over 5mm in any lead was leading to too many false cath lab activations. But a very intelligent physician named Dr. Smith came up with a new criteria that was based off of the Sgarbossa criteria and created the Smith-Sgarbossa criteria (modified Sgarbossa criteria). You may know that Dr. Smith has his own blog post which I will link here. It is a phenomenal site for free open access medical education that focuses on ECG interpretation (http://hqmeded-ecg.blogspot.com/). His criteria goes into proportionality instead of actual ST segment elevation height. We will hit on proportionality before we go into his criteria.

To put things simply, the T wave shouldn’t be massive in comparison to the QRS complex. The QRS should be larger than the T wave at all times. This concept goes into diagnosing an subtle STEMI or an occlusion MI (OMI) which I have already made a post on so review that to learn about proportionality.

An example of proportionality on a QRS is shown by dogs. We love dogs, so yes we used dogs to show proportionality.

Smith-Sgarbossa Criteria

Back to Dr. Smith and his criteria. He replaced the greater than 5mm of discordant elevation with a ratio:

1. Concordant ST depression that is greater than 1 mm in V1-V3.
2. Concordant ST elevation that is greater than 1 mm in any lead.
3. ST elevation is ≥ 25% (.25)

Now this may be a bit confusing so lets go over this with an example.

Now what do you think of this? It doesn’t meet the Sgarbossa criteria, but does it meet the Smith-Sgarbossa criteria? Well I am not putting this 12 lead in for no reason so yes it does. Look over at V3 and let’s blow it up.

You need to measure the ST elevation and divide that number by the mm of the S wave and it needs to be over 0.25. From the isoelectric line, you count the number of boxes up to the J point which is roughly 3.5 mm. For the S wave, you measure from the isoelectric line down to the bottom of the S wave which gives you around 12 mm. So, 3.5/12=.29 which means it does meet the criteria because it is over 0.25.

Barcelona Criteria

At the beginning of the year (2020), there was new criteria made called the Barcelona Criteria. I read the AHA journal several times and found several flaws in their criteria for an outcome among several areas. It was an interesting read, but I do not personally believe that this criteria is worth delving into now as it still has areas to improve on to be scientifically significant. I will link the article below for you to read it if you wish,

https://www.ahajournals.org/doi/10.1161/JAHA.119.015573

Conclusion

If you are going to remember one of the criteria it should be the Smith-Sgarbossa criteria. I have used this many times. If you are prehospital, it is important to know that a lot of cardiologists and ER physicians will not know about the Dr. Smith component to the formula as they are too busy learning about other stuff so try to be a huge patient advocate but understand they might just be blind to the information. Luckily, many providers have begun to learn the modified criteria.

This site is meant to be used for educational use only. We strive to push evidence based medicine with no bias to help you obtain all the important information. You should always follow your protocols that have been set in place.

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References

My Brain

Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012 Dec;60(6):766-76. doi: 10.1016/j.annemergmed.2012.07.119. Epub 2012 Aug 31. Erratum in: Ann Emerg Med. 2013 Oct;62(4):302. PMID: 22939607.