Bifascicular Blocks – What You Need To Know

Anatomy of the Heart’s Electrical Conduction System



Ventricular depolarizaiton is facilitated by the heart’s electrical conduction system, sometimes referred to as the His/Purkinje system, which by convention is said to have three main fascicles or branches. There is a single fascicle in the right ventricle referred to as the right bundle branch. There are two fascicles in the left ventricle: the left anterior fascicle and the left posterior fascicle of the left bundle branch.

It reality, while it is clinically useful to think in terms of a left anterior fascicle and a left posterior fascicle, anatomically speaking the left bundle branch has many fascicles that fan into the the left ventricle as shown by these drawings by Sunao Tawara, the Japanese physician who discovered the AV node.

Let’s start by looking at what a single fascicle looks like when it is blocked. This is sometimes referred to as a hemiblock, or a hemifascicular block. To identify fascicular blocks you need to be able to identify a right or left axis deviation in the frontal plane. One way to do that is to use a cheat sheet like this one.

Left Anterior Fascicular Block (LAFB)

Rules for left anterior fascicular block:

Modest widening of the QRS complex

Left axis deviation

rS complexes in the inferior leads

qR complexes in the high lateral leads

You will also typically see a late transition in the precordial leads. In our example the QRS complex does not become equiphasic until lead V5 or V6. This is also sometimes referred to as a persistent S-wave.

Sinus rhythm at a rate of 69. The QRS duration is modestly prolonged at 108 ms. There is a left axis deivation with rS complexes in the inferior leads and qR complexes in the high lateral leads. There is a late transition (persistent S-wave) in the precordial leads.

Left Posterior Fascicular Block (LPFB)

Rules for left posterior fascicular block:

Modest widening of the QRS complex

Right axis deviation

qR complexes in the inferior leads

rS complexes in the high lateral leads

However, isolated left posterior fascicular block is very rare.

Sinus rhythm with a rate of 90. The QRS duration is modestly prolonged at 102 ms. There is a right axis deviation with qR complexes in the inferior leads and rS complexes in the high lateral leads. However, S1Q3T3 is also present along with anterior T-wave inversions. The patient was complaining of shortness of breath.

Although this ECG meets the criteria, this patient may have been suffering a pulmonary embolism. Left posterior fascicular block is a diagnosis of exclusion, meaning that we have to rule out right ventricular hypertrophy and acute right heart strain.

Right Bundle Branch Block (RBBB)

Rules for right bundle branch block:

Supraventricular rhythm

Wide QRS complex (≥ 120 ms)

Terminal R-wave in lead V1

Slurred S-wave in lead I

Sinus rhythm with a rate of 75. The QRS duration is wide at 146 ms. There is a rsR’ complex in lead V1 and a slurred S-wave in lead I.

Bifascicular Blocks

Now let’s combine these criteria together. When we have right bundle branch block with either left anterior fascicular block (common) or left posterior fascicular block (less common) we call it a bifascicular block.

The problem with bifascicular block is that the heart’s electrical conduction system is down to one fascicle. As such, the patient may be at risk for complete heart block (which is what would happen if all three fascicles were blocked).

Although many patients have chronic bifascicular block for years and do not develop concerning symptoms, we should be mindful of the fact that patients with syncope and bifascicular block may be experiencing transient episodes of third degree AV block or even asystole (third degree AV block with no escape rhythm).

Right Bundle Branch Block and Left Anterior Fascicular Block (RBBB/LAFB)

This is essentially right bundle branch block with a left axis deviation. Purists also require rS complexes in the inferior leads and the qR complexes in the high lateral leads because this pattern can also be caused by right bundle branch block and Q-waves from inferior myocardial infarction.

Sinus tachycardia with a rate of 109. The QRS duration is wide at 138. There is a rsR’ complex in lead V1. There is a left axis deviation with rS complexes in the inferior leads and qR complexes in the high lateral leads.

Right Bundle Branch Block and Left Posterior Fascicular Block (RBBB/LPFB)

This is essentially right bundle branch block with right axis deviation. Purists also require qR complexes in the inferior leads and rS complexes in the high lateral leads because this pattern can also be caused by right bundle branch block and Q-waves from high lateral myocardial infarction.

Sinus tachycardia with a rate of 113. The QRS is wide at 158 ms. There is a monophasic R-wave in lead V1. There is a right axis deviation with qR complexes in the inferior leads and rS complexes in the high lateral leads.

Trifascicular Block

Sometimes you hear the term “trifascicular block” but the term is not always used correctly.

Right Bundle branch Block, Left Anterior Fascicular Block, and First Degree AV Block

Although the combination of right bundle branch block, left anterior or posterior fascicular block, and first degree AV block is often called “trifascicular block” the term in this context is a bit of a misnomer. First degree AV block is not a true block because there is a 1:1 relationship between P-waves and QRS complexes.

In the setting of bifascicular block the conduction delay can be in the AV node (as you would normally expect) or it can be in the last remaining fascicle. Either can cause PR-prolongation in the setting of bifascicular block. So absent periods of documented third degree AV block, or alternating right and left bundle branch block, we should probably not refer to this ECG finding as “trifascicular block”.

Sinus rhythm with a rate of 77. There is first degree AV block with a PR-interval of 252 ms. The QRS duration is wide at 172 ms. There is a terminal R-wave in lead V1. There is a left axis deviation with rS complexes in the inferior leads and qR complexes in the high lateral leads.

Bifascicular Block and Third Degree AV Block

EMS was called to the residence of a 85 year old male who had lost consciousness while eating breakfast. The patient was found face-down on the kitchen table with snoring respirations. He was relocated to the floor. A carotid pulse was present but radial pulses were absent.

A 12-lead ECG was obtained.

Third degree AV block with ventricular escape rhythm at a rate of 24.

The patient suddenly regained consciousness. A rhythm change was noted on the monitor. Another 12-lead ECG was obtained.

Sinus rhythm with a rate of 60. First degree AV block with a PR interval of 208 ms. The QRS is wide at 152 ms. There is a terminal R-wave in lead V1. There is a right axis deviation with qR complexes in the inferior leads and rS complexes in the high lateral leads.

Bifascicular Block and Ventricular Asystole

Every once in a while patients with bifascicular block can give you a real scare. This was a patient who was being seen in the Emergency Department for syncope.

Undifferentiated wide complex tachycardia with a rate of 118. The QRS duration is 150. There is a terminal R-wave in lead V1 and left axis deviation with rS complexes in the inferior leads and qR complexes in the high lateral leads.

Suddenly the alarm started ringing at the nurses station.

Sudden ventricular asystole.

The patient suddenly converted to ventricular asystole!

P-waves are now visble with an atrial rate similar to the ventricular rate of the previous ECG so in hindsight we know this was a supraventricular rhythm. It may be an ectopic atrial tachycardia as the P-waves are upright in lead aVR and negative in lead II.

Just as nurses were about to begin CPR the patient’s symptoms spontaneously resolved. A transvenous pacer was placed as a precaution. I have no further information about the patient’s clinical course but there’s an excellent chance the patient ended up with a permanent pacemaker.

Bifascicular Block and STEMI

Although identifying acute STEMI in the presence of bifascicular block is a little bit more challenging, it can usually be accomplished.

Right Bundle Branch Block, Left Anterior Fascicular Block, and LAD Occlusion

Sinus rhythm with a rate of 94. The QRS is wide at 158 ms. There is a terminal R-wave in lead V1. There is a left axis deviation with rS complexes in the inferior leads and qR complexes in lead aVL. There is ST-segment elevation in leads V1-V3. The ST-segment elevation “crosses over” to the high lateral leads I and aVL. There is reciprocal ST-segment depression in leads II, III, and aVF. The patient was suffering LAD occlusion.

You can see an interesting case of bifascicular block with de Winter ST/T-waves here.

Right Bundle Branch Block, Left Posterior Fascicular Block, and Acute Inferior STEMI

Sinus rhythm with a rate of 99. The QRS is wide at 154 ms. There is a rSR’ complex in lead V1 and right axis deviation with qR complexes in lead III and rS complexes in lead I and aVL. ST-segment elevation is present in leads III and aVF with reciprocal ST-segment depression in leads I, aVL, and V2-V4 consistent with acute inferior-posterior STEMI.

Wide Complex Tachycardias with Bifascicular Patterns (Presumed to be Ventricular Tachycardia)

It’s worth remembering that the first rule when dealing with bifascicular blocks is to establish that you are dealing with a supraventricular rhythm. One of the reasons is that ventricular tachycardia often presents with a bifascicular pattern.

Bifascicular Pattern (RBBB/LAFB)

When VT originates in the left ventricle near the left posterior fascicle of the left bundle branch it will present with a RBBB/LAFB pattern.

Wide complex tachycardia with RBBB morphology in lead V1 and left axis deviation.

Bifascicular Pattern (RBBB/LPFB)

When VT originates in the left ventricle near the left anterior fascicle of the left bundle branch it will present with a RBBB/LPFB pattern.

Wide complex tachycardia with a rate of 141 with RBBB morphology in lead V1 and right axis deviation.

Wide and fast rhythm should be considered to be VT until proven otherwise! There is a difference between a bifascicular block and a bifascicular pattern. You will frequently see both.

References

Epstein A, DiMarco J, Ellenbogen K et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities. Journal of the American College of Cardiology. 2008;51(21):e1-e62. doi:10.1016/j.jacc.2008.02.032. (PDF)

Garcia T, Holtz N. 12 Lead ECG: The Art of Interpretation. Boston, Ma: Jones and Bartlett; 2001.

Surawicz B, Knilans T, Chou T. Chou’s Electrocardiography In Clinical Practice. Philadelphia: Saunders; 2001.