Kansas City University of Medicine & Biosciences
Department of Physiology
The Physiological Basis of the Abnormal ECG
Bundle Branch Blocks
Bundle Branch Blocks
The ventricles of the heart (either left or right) contain sufficient muscle cell mass that effective depolarization of all the cells requires that there be a specialized conduction pathway within the ventricle. The bundle branches (left and right) are the first division in the ventricular conduction system after the Bundle of His. Conduction blocks can occur in either of the two bundle branches. As noted above, these can occur as the result of infarction of the tissue, although a number of otherwise normal people have a bundle branch block due to the invasion of the conduction pathway with fibrous tissue. In fact, it is quite likely that one of more of your classmates will show a bundle branch block during the ECG labs. Luckily for us, the affected ventricle will continue to depolarize, but via a cell-to-cell interaction which is quite a bit slower than the normal pathway. Being quite the creative folks that we scientists are, these are referred to as "right bundle branch block" (RBBB) and "left bundle branch block" (LBBB).
Right Bundle Branch Block
In right bundle branch block, the right ventricle is activated via cell-to-cell transmission of the action potential. This is illustrated in the following animation:
Notice the delay in the time of right ventricular depolarization. This means that the right ventricle is activated after the left ventricle (which is depolarizing normally) and therefore contracts a "split second" after the left ventricle. Mechanically, this difference doesn't greatly influence the performance of the heart. However, it does produce some characteristic changes in the ECG.
Electrocardiographic changes in RBBB. As you might be able to guess from the animation, because the depolarization of the ventricle has been altered, the changes show up in the QRS complex and the T wave. The following pictures illustrate right bundle branch block in different leads.
|Appearance of three different leads in RBBB: From top to bottom: Lead I, Lead V1 and Lead V6. Due to the RBBB, the right ventricle is depolarizing after the left ventricle. In all three leads, there is a prolongation of the QRS complex due to the blockade. In leads I and V6 (which "see" the left ventricle best), the delayed depolarization of the right ventricle produces a late, negative going wave (S wave) that is very broad (there is no point to the wave) and prolonged. In lead V1 (which has a good view of the right ventricle, the delay in depolarizing the right ventricle leads to the aptly-named "bunny ears"- the QRS complex is composed of an initial positive wave (the R wave), followed by a negative deflection (the S wave) produced when the left ventricle depolarizes, and then a second positive wave (called R' (spoken "R prime")) as the right ventricle finally depolarizes. Because the normal conduction pathway was not followed for depolarization, the process of repolarization does not occur in the normal sequence leading to abnormalities in the T wave (seen best in V1).|
Summary of electrocardiographic changes in RBBB:
- P wave of normal morphology (shape).
- rate and rhythm normal.
- QRS complex is prolonged (> 0.12 seconds).
- QRS complex is bizarrely shaped in leads with "good" views of the right ventricle, such as V1 and V2.
- The appearance of "bunny ears" is a hallmark of a bundle branch.
- The amplitude of the individual waves in the QRS complex is usually increased because the depolarization in the left and the right ventricles are no longer simultaneous (when they are simultaneous, they cancel each other out to some extent).
- The T wave is abnormal in appearance because of alterations in the repolarization of the heart.
Left Bundle Branch block
Left Bundle branch block (LBBB) is very similar to right bundle branch block, as the following animation illustrates.
As with the RBBB, the major change is that the left ventricle is now depolarized due to the spread of the action potential via cell-to-cell conduction. This results in the left ventricle depolarizes (and therefore contracting) a "split-second" after the right ventricle does. As with RBBB, the mechanical effects of this on the heart are minimal.
Electrocardiographic changes in LBBB:
The following pictures illustrate left bundle branch block as seen in three different leads:
|The appearance of LBBB in three different leads: As with right bundle branch block, the major change is in the shape and duration of the QRS complex (since it is ventricular depolarization that has been altered). All of the leads show a prolonged QRS complex due to the time it takes the left ventricle to depolarize. Leads I and V6 record long R waves (lead I shows a small initial R wave (you might be mistaking it for a P wave) and a larger R' leading to a somewhat odd pair of "bunny ears") because the depolarization is heading "at" the lead for a prolonged period of time as the ventricle depolarizes. Lead V1 (on the right side of the sternum sees an initial very small positive wave as the faster right ventricle depolarizes, followed by a prolonged S wave produced by the left ventricular depolarization (note that the S wave also has two peaks - an inverted pair of "bunny ears").|
Summary of electrocardiographic changes in LBBB:
- P wave of normal morphology (Note: like a lot of things in this world, the ECG tracing shown above for LBBB has more than one thing wrong with it - no P waves are visible because this person is in atrial fibrillation).
- Normal rate and rhythm.
- Normal PR interval
- QRS complex is prolonged and bizarrely shaped.
- Bunny ears
- other leads may show QRS complexes that aren't "pointy", instead they are rounded at the top/bottom.
- The amplitude of the QRS complex peaks are often increased, due to the failure of the left and right ventricle to depolarize simultaneously.
- The T wave is abnormal in morphology because repolarization cannot occur normally if depolarization did not.