Clinical Competencies – 12 Lead Interpretation

Those of you reading in the Massachusetts area know about the recent administrative requirement put fourth by the Office of Emergency Medical Services. The requirement states that all ALS providers in the Commonwealth must demonstrate a set of core competencies prior to December 31 2011. The memo can be read here. It is annoying. If you’re not from the Massachusetts area, feel free to read the memo from the Commonwealth’s OEMS. Let me know what you think. It would be cool to get some opinions from across the country. We all know we need to be able to interpret 12-Lead EKG’s. With that being said, I figured I’d take a few minutes to talk about a specific type of EKG’s that always have given me a problem.


There are two types of bundle branch blocks. You have your Right Bundle Branch Block (RBBB) and your (LBBB). It is a relatively simple concept to understand. When you have a bundle branch block, there is a delay in the conduction down the nerve pathways in the heart. We all know that stereotypical wide-complex QRS that is indicative of a BBB.

Views of V1 and V6 with suspected LBBB

Left Bundle Branch Block.

The Left Bundle Branch Block is when conduction is delayed in the Left Bundle, thus causing delayed depolarization. A new onset of LBBB can be indicative of a possible AMI. In order to diagnose a LBBB you must meet the following diagnostic criteria:

  • QRS > 0.12 seconds.
  • Broad monomorphic R Waves in I and V6 with no Q waves.
  • Broad monomorphic S Waves in V1 and may have a small R wave.

In simple terms, the left ventricle depolarizes later than the right ventricle.

Left Bundle Branch Blocks, can also be indicative of AMI. Patients with a questionable new onset LBBB should be treated as AMI until proven otherwise. Of course, this is just the opinion of one paramedic. This blog should not take the place of previously established standard treatment protocols.

Also, LBBB is a classified as an ACS imitator. You want to make sure that you get serial EKG’s in any STEMI patient to monitor for trending. However, this is specifically important in those patients with suspected LBBB. According to (a subsidiary of Wikipedia), there is a scoring system that you can use to diagnose STEMI from a 12 Lead. However, this is not practical to the EMS environment due to time constraints and other factors. If you’re a geek like me, then you will defintaely find it interesting.

The Right Bundle Branch Block

In the RBBB the conduction in the bundle to the right ventricle is slow. as the right ventricle depolarizes, the left ventricle is often halfway finished and few counteracting electrical activity is left. The last electrical activity is to the right or towards lead V1. In RBBB the QRS complex in V1 is always markedly positive. In order to diagnose a RBBB you would need to look at lead V1 again, and have the following diagnostic criteria:

– QRS > 0.12 seconds.
– Slurred S wave in lead I and Lead V6.
– RSR’ pattern in V1 where R1 > R.

According to, STEMI Diagnosis in a RBBB is not as difficult. RBBB usually  affects re-polarization in leads V1-V3, these are usually not enough to diagnose ischemia.

That is just my two cents on a topic that I don’t feel was touched on enough when I was in Paramedic school. I am going to direct you to another blog, by clicking here. Its all EMS 12 Leads, all the time. It is a great resource for EMS case review when it comes to cardiology.

Until next time folks, stay safe out there!


One response to “Clinical Competencies – 12 Lead Interpretation

  1. Good stuff!

    I would modify that RBBB definition as only including rSR’ for the morphology in V1 misses the actual morphology definition:

    – Terminal R wave in V1

    So in V1 if you have anything “ending in an R” (e.g. qR, rR’, or RSR’) these all potentially candidates for RBBB.

    This is easy to picture because the right ventricle depolarizes last in RBBB (and slowly). We would expect the terminal axis (“second half of the QRS”) to then shift rightward and anteriorly. The slurred S in I/V6 show this late shift to the right and the terminal R wave in V1 shows the late anterior shift.

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