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Bundle Branch Blocks Study Guide

One of the more confusing topics for EMS students enrolled in an Electrophysiology course is that of Bundle Branch Blocks. They can be sneaky, they can mask things, and they can also be hard to diagnose if you don’t remember a few key things. This study guide will prepare you for the material you will be tested on and give you some tips and tricks for the field.

cardiac conduction

As you can recall, when an impulse is traveling normally, it is sent from the SA node, through the internodal pathways, to the AV node, down the bundle of His, to the right and left bundle branches, to the respective fascicles, and into the purkinje fibers. You may also recall, that the heart does not have two branches, it actually has three, due to the left branch dividing into two fascicles, where the right just has one.

What is a Bundle Branch Block?

  • A BBB is a conduction block in one of the bundle branches. When such a block exists, the ventricles fire more slowly as the impulse has to travel cell to cell through the muscle wall instead of through the conduction system.


  • Ischemic events
  • Myocardial infarctions
  • Deterioration from age
  • Prescription medications
  • Recreational drugs
  • An electrolyte imbalance

Bundle Branch Presentation

  • The QRS must be >.12 seconds or 3 small boxes.
  • You might ask, what happens if one of the QRS spikes is wider than another on my patient’s 12-lead ECG?
    • If that is the case, always use the widest QRS to interpret your rhythm. (It should be noted that if your local protocols say something different, ALWAYS FOLLOW YOUR PROTOCOLS!)

Right Bundle Branch Blocks:

  • Due to the slowing of the ventricular depolarization, the right and left ventricles fire out of phase. This ends up with a rabbit ear looking QRS because the monitor is combining the QRS spike from the right ventricle and the left ventricle. With this positive deflection, we see the RSR’ pattern in V1. This is explained further in the diagram below.

RBBB Explanation

  • Criteria for RBBB

    • QRS > 0.12 seconds
    • Rabbit ears in V1
    • The deflection before the J point will be in the positive direction, or the turning signal will be “Up and to the right”

Left Bundle Branch Blocks:

  • With a left bundle branch block, we see a general direction of the current in the heart moving away from V1. That is why we see the negative deflection just before the J point in LBBB’s. With this negative deflection, we see the QS pattern in V1.
  • Criteria for LBBB 

    • QRS > .12 seconds
    • Deflection just before the J point will be negative
    • Should see rabbit ears in V5 and V6 and even I.


Closing Points on Bundle Branch Blocks

  • We need to be able to quickly recognize these because they can mask other ECG changes. There is a saying in EMS that says, “treat the patient, not the monitor.” This is a perfect case of that. Just because you don’t see evidence of an MI in the 12-lead, treat your patient as such if they’re still presenting like an MI patient.
  • Make sure you ask if the patient has ever been told they have a BBB. A lot of times these patients will have cardiac history and are seeing a cardiologist for it. Ask them what prescription medications they’re on and if they’re following doctor’s orders.
  • Do not get in the habit of writing off the patient because they have a BBB. It is an important finding that requires more questioning and patient inquiry.
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