Bundle Branch Block

When we talk about bundle branch block, we are referring to the Tawara conduction system, which divides into a right and a left Tawara limb.

Right bundle branch block refers to a blockage in conduction through the right Tawara limb, and left bundle branch block refers to a blockage in conduction through the left Tawara limb. In each case, the wave of depolarization must make a detour through the other Tawara limb in order to overcome the blockage.

Since the left Tawara limb splits into an anterior and a posterior fascicle, patients can also develop a left-anterior or a left-posterior fascicular block.

Because the Tawara conduction system has one right and two left branches, it is referred to as a trifascicular system, that is to say a system with three lanes of conduction.

It is important to remember that several fascicles may be blocked at the same time. For example, a right bundle branch block can occur together with one of the two fascicular blocks. This combination is called bifascicular block. Similarly, a total left bundle branch block refers to a bifascicular block of the left-anterior and the left-posterior fascicles.

If all branches are blocked, the excitation can no longer penetrate from the AV node into the ventricular myocardium. Functionally, there is a complete heart block.

Since the combination of a bifascicular block and an additional conductive disorder in the region of the AV node or His bundle increases the risk of a complete AV block, the term "trifascicular block" is occasionally used to describe this pattern in the literature.

The expression trifascicular block is also used a lot in ECG reports and hospital discharge letters. However, because this expression can be a little confusing, we recommend that you always explain what lesions were found when passing this information on to your colleagues.

In addition to the localization (left or right) the width of the QRS complex plays an important role.

In complete bundle branch block, the excitation is propagated exclusively over the other branch. A complete bundle branch block is defined by a QRS duration of 120 ms or above.

In an incomplete block, only some of the fibers are blocked, and so the widening of the QRS complex is less pronounced, with a QRS interval of 110–119 ms. In an adult ECG, intervals of between 100 and 110 milliseconds represent a gray area in terms of QRS duration interpretation. Some cardiologists argue that a QRS complex with a typical configuration and a duration of between 100 and 110 ms should be classified as an incomplete bundle branch block. However, it is more common to classify this as a nonspecific intraventricular conductive disorder.

If the configuration of the QRS complex is altered but the QRS complex is less than 100 ms long, this is NOT classified as a bundle branch block. In the ECG report, the QRS configuration, e.g., r - S - R '(dash) in V1 or V2, may be described as an expression of a right bundle branch block-like alteration. Configuration changes with a QRS duration of less than 100 ms are not considered pathological.

Bundle branch block may be permanent or intermittent. Intermittent block is always due to a change in the refractory period of the affected Tawara branch. The causes of this are manifold. A common finding is a bundle branch block which only occurs during tachycardia, for example during a stress test. Here, the conductive myocardial cells are refractory above a certain heart rate.

What do we see on the ECG of a patient with a left bundle branch block?

The most important changes in the QRS complex are:

1) The specifically widened R in 1, aVL, V5, and V6, which shows an M configuration, or the famous " broken-off sugar loaf";

2) The upper transition point in v6 is located more than 55 ms away from the beginning of the R wave; and

3) in the chest wall leads, the latest negative motion of the R wave will be found in V5 or V6.

How can you remember which configuration is typical for left and for right bundle branch block?

The beauty of the ECG is its simple underlying logic. You only need to understand one important principle: the spatial projection of the propagation of depolarization in relation to the ECG leads.

Consider once again the Cabrera circle or the camera analogy. In a left bundle branch block, depolarization begins on the right and spreads to the left. All the leads with a projection towards the left side of the heart; for example, lead I, aVL, V5, and V6, and to a lesser degree II and V4, will have a positive main vector. This is exactly where the typical M configuration lies.

In addition to the typical QRS morphology in leads 1, aVL, V5, and V6, there are other classical criteria for the diagnosis of “left bundle branch block”, such as where not only the excitation propagation, but also the repolarization is altered.

In the leads with M-configured QRS complexes (lead 1, aVL, V5, and V6), discordant T wave inversion is typically present. In the remaining precordial wall leads, in particular V1 and V2, a deep S wave is present.

In patients with complete left bundle branch block, evaluation of repolarization problems secondary to ischemia is difficult, since significant ST elevation often occur in leads V1 to V3. It is worth comparing the ECG with older recordings.

Because repolarization cannot be assessed in left bundle branch block, if you see a patient with acute chest pain and a new LBBB, the patient should be considered to have a ST-segment elevation myocardial infarction until proven otherwise.

What do we see on the ECG of a patient with a fascicular block?

As you know, the left tawara branch is split into an anterior and a posterior fascicle. If one of these is blocked, we speak of a so-called fascicular block or more specifically of a left-anterior or left-posterior fascicular block. If both fascicles are blocked, there is again a complete left bundle branch block. It is important not to confuse a fascicular block with an incomplete bundle branch block. The ECG criteria are completely different!

Left-anterior fascicular block is significantly more common than left-posterior fascicular block, since the anterior fascicle is smaller and more vulnerable. The typical ECG changes of the fascicular block are caused by an alteration in the pathway of myocardial excitation. Modification of the depolarization pathways is reflected primarily by a vector change for the R wave.

In the left-anterior fascicular block, the main vector rotates to the left, because the excitation of the left ventricle relies completely on the left-posterior fascicle and moves from right to left. The definition therefore includes an extreme left axis deviation. Other LAH criteria are a persistent S wave in the chest wall leads up to V6 and a QRS duration of less than 110 ms.

If the QRS complex is wider than 110 ms, this indicates a different or additional incomplete or complete form of bundle branch block or a complex intraventricular conduction disorder.

In leads 2, 3, and aVF, no Q-waves should be visible. If Q waves are visible in these leads, an older, inferior wall myocardial infarction is more likely than left-anterior fascicular block.

In your daily clinical work, extreme left axis deviation in addition to S-persistence in v6 are of relevance in terms of diagnosis.

Left-posterior fascicular block, or LPH, occurs very rarely. In this case, the main vector rotates in the opposite direction to that found in LAH, namely to a right. We therefore see an extreme right axis deviation. Correspondingly, a deep S wave is found in leads I and aVL, and a qR configuration in leads II, III, and aVF. An additional criterion is a 45 ms delay in the upper transition point in aVF. The QRS duration is less than 110 ms.

What do we see on the ECG of a patient with a right bandle branch block?

In right bundle branch block, in short, RBBB, the excitation spreads through the left Tawara branch and into the right ventricular myocardium. The specific M-configuration can be found in V1 and V2.

In this case, the time up to the upper transition point is at least 30 ms, and the latest down stroke of the R wave occurs in V1 and V2. Typically, discordant T wave inversions occur in those leads where there is the typical M configuration of RBBB.

Now, how can you remember which configuration is typical for left and which for right bundle branch block?

The typical signs can be found in the chest wall leads. Just keep in mind that v1 and v2 are located directly over the right heart, and v5 and v6 are located over the apex of the left ventricle. This makes it very easy to remember that an M configuration of the QRS complex in v1 and v2 is typical for right bundle branch block, while an M configuration in v5 and v6 is typical for a left bundle branch block.

The clinical significance of right bundle branch block must always be assessed in context. Incomplete RBBB is often found in healthy subjects, especially in young athletes. However, RBBB may be a sign of acute right heart strain, for example in pulmonary embolism or right coronary artery myocardial infarction.

As we have already discussed, in bifascicular block two of the three fascicles are affected.

Three scenarios are possible:

1)        the right Tawara fascicle in combination with the left-anterior fascicle

2)        the right Tawara fascicle in combination with the left-posterior fascicle

3)        the left-anterior in combination with the left-posterior Tawara fascicle. This particular pattern of bifascicular block cannot be differentiated from LBBB in the ECG.

Both bifascicular blocks which include the right Tawara fascicle naturally show the ECG signs of a right bundle branch block. Therefore, if right bundle branch block is present in combination with extreme left axis deviation, this points to an additional left-anterior fascicular block. If right bundle branch block is present in combination with extreme right axis deviation, this points to an additional left posterior fascicular block.

As always, it is important to look for significant Q waves in II, III, and aVF, in order to rule out an inferior myocardial infarction as the cause of the deviation of the normal axis.

These two bifascicular blocks have a very different prognosis. While a right bundle branch block in combination with a left-anterior fascicular block rarely progresses to a trifascicular block, the probability is significantly higher for a RBBB plus left-posterior fascicular block. This is due to the greater vulnerability of the left anterior fascicle, which would be the only one remaining. A prophylactic pacemaker implant should be considered when diagnosing a right bundle branch block in combination with a left-posterior fascicular block. The threatening trifascicular block generally has the same effect as a third-degree AV block. More on this in the chapter "Bradycardia". In case of an acute occurrence of a trifascicular block, asystole or a haemodynamically relevant bradycardia often occurs, since the tertiary pacemaker center is usually located in the ventricular myocardium and is thus unstable and with an extremely slow heart rate.