Case: A 70 yo male with a history of hypertension, CKD, and diabetes presents after 4 syncopal episodes over the last 3 weeks. These episodes are preceded by mild dizziness and a “funny” sensation in his chest, as if his heart was “skipping a beat”.

His initial ECG is shown:

Computer interpretation: sinus rhythm with first degree AV block (PR interval 232 ms), HR of 94 bpm, right bundle branch block.

Official physician interpretation: sinus rhythm with first degree AV block, right bundle branch block (RBBB), left axis deviation, and left anterior fascicular block (LAFB).

Why this ECG: This is an important finding on ECG that often gets overlooked. It is commonly referred to as a “trifascicular block.”

The heart’s electrical conduction system that facilitates ventricular depolarization, known as the His/Purkinje system, consists of three main fascicles. There is a single fascicle in the right ventricle (the right bundle branch) and two fascicles in the left ventricle (the left anterior fascicle and left posterior fascicle, which together form the left bundle branch), as shown in the figure.

A RBBB can be identified by the QRS >120 msec, RSR’ pattern in V1-V3, and a wide, slurred S wave in the lateral leads (I, aVL, V5-V6), all of which are seen in this ECG. LAFB can be identified by rS complexes in II, III, aVF (seen on the ECG) and qR in I and aVL

The term “trifascicular block” is commonly used clinically to describe conduction disease of two of the three fascicles with prolongation of the PR interval (ie, first or second degree AV block). This PR interval prolongation may represent delayed conduction through either the AV node or the remaining third fascicle if the delay is below the bundle of His (hence the name, “trifascicular block”). Of course in the emergency department, we do not know whether the delay is at the AV node or below the bundle of His without an electrophysiology study.

In this patient, we have conduction disease of two fascicles (the right bundle branch and the left anterior fascicle), with a first degree AV block, which may represent either conduction delay of the AV node, or of the remaining third fascicle (in this case, the left posterior fascicle). It is an important finding because these patients are at higher risk of progression to complete heart block, with one study reporting a rate of progression of 1% per year (1). Patients with this finding who present with syncope or presyncope should be admitted for monitoring and likely pacemaker insertion.

This term, however, while commonly used in clinical practice, is a misnomer, as the PR interval prolongation represents conduction delay that may either occur in the AV node or in the remaining third fascicle. A true “trifascicular block” would involve a block of the right bundle branch and both fascicles of the left bundle branch, manifesting as a third degree (complete) heart block, instead of a first or second degree heart block.

What happened next?

The patient subsequently became bradycardic, with heart rates in the 30s and 40s bpm. Pacer pads were immediately placed with atropine placed at the bedside. Labs demonstrated a normal potassium of 4.8 and a negative troponin, indicating no obvious reversible causes of bradycardia.

Repeat ECG is shown:

Physician interpretation: sinus bradycardia (~43 bpm) with second degree AV block, Mobitz type 1, right bundle branch block, left anterior fascicular block, and left axis deviation. The majority of the ECG rhythm demonstrates second degree AV block with 2:1 conduction. It is difficult to be certain whether these second degree 2:1 rhythms are Mobitz type I or Mobitz type II. However in this case, there is a single area in the mid portion of the ECG (beats 4-5 seen in the rhythm strip of lead II) in which 3:2 conduction is present, and the PR interval prolongs. This confirms the diagnosis of Mobitz type I.

Bedside ultrasound showed grossly normal ejection fraction with no regional wall motion abnormalities. Cardiology was consulted, and the patient was admitted to the CCU. He had a permanent pacemaker placed the next day.

The Heart of the Matter:

Recognize the “trifascicular block” pattern on ECG
Patients with a “trifascicular block” are at higher risk of progression to complete heart block
When associated with syncope or presyncope, these patients warrant cardiology consultation for pacemaker insertion
Whenever there is high grade AV block and bradycardia, look for and correct reversible causes, such as ischemia, medications, and hyperkalemia

For more reading

Life in the fast lane: https://litfl.com/trifascicular-block-ecg-library/
Life in the fast lane: https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/
Dr Smith’s ECG Blog: http://hqmeded-ecg.blogspot.com/2016/07/symptomatic-bradycardia-so-called.html

Resources

Natural history of “high-risk” bundle-branch block: final report of a prospective study. McAnulty JH, Rahimtoola SH, Murphy E, DeMots H, Ritzmann L, Kanarek PE, Kauffman S. N Engl J Med. 1982;307(3):137.

Written by:

Rayyan Kadi, MD

Emergency Medicine

Cook County Health, PGY-3

Twitter: @RayyanKadi

Edited by:

Tarlan Hedayati, MD

Chair of Education

Cook County Health

Twitter: @HedayatiMD

the County Consult