Rhythm Nation: Trifascicular Block

As you recall, the electricity in a well-functioning heart marches orderly from the SA node through the gap junctions of cardiac myocytes, into the AV node, down the Bundles of His, and into the Left and Right Bundles. And if you’re really astute, you may even divide the Bundles into Fascicles, the Right Bundle as one and the Left Bundle as two: Anterior and Posterior branch.

Just like pre-school, let’s start stacking, it’s easy at first, but basics let you build skyscrapers (it is New York…). Prior to the bundles and fascicles, there are degrees of AV Block (slow conduction in the AV node). First Degree is just “longer than a big block” on the ECG (PR > 200 milliseconds). Second Degree is when “da beat drops” and has two flavors, Mobitz I and II. One flavor “winks back at you” before it drops (Mobitz I, Wenckebach, PR prolongation prior to non-conducted QRS) and the other “drops whenever” (Mobitz II, non-conducted QRS without PR prolongation). Lastly, Third Degree block occurs when communication between Atria and Ventricles breaks down and they “do their own thing” (AV dissociation).

Our friend, William Marrow, taught us early in medical school that if the Right Bundle is blocked, the QRS will be prolonged (>120 msec) with a “bunny ear pattern” in V1 or V2 (rSR, rsR or RR) and a “dip” in V6 (Terminal S Wave). And that Orange Book in the library said if the Left Bundle is blocked, the QRS will be prolonged (>120 msec) with “a dip” in V1 (QS or rS) and an “up down” in V6 (slurred R wave) with no “dips in the V’s” (absent septal Q waves in precordial leads).

Once we got fancy, we realized the Left Bundle split into two branches, an Anterior and a Posterior branch. Dubin said that when these fascicles were blocked, we’d see “little dips in front of the tall thing” (LAFB: rS in II, III, aVF and qR in I, aVL vs. LPFB: qR in III, aVF and rS in I), and “a quick axis check” would tell us which was which (LAFB: LAD vs. LPFB: RAD).

Knowing this, we can easily recognize blocks in the Right Bundle, Left Bundle or the Left Anterior or Left Posterior Fascicle. 

What if more than one of the three fascicles is blocked (RBBB, LAFB, LPFB) at the same time? This could lead to a Bifascicular Block (2 Fascicles) or a Trifascicular Block (3 Fascicles). If all three of these fascicles are blocked (Complete Heart Block) no signals from the atria will reach the ventricles, and the pacing ventricular myocytes will begin to pace the ventricles very slowly (Ventricular Escape Rhythms). Until you place a pacemaker, this may lead to decreased perfusion in key organs (e.g. the brain causing syncope) – a clearly dangerous situation.

Before Hipsters roamed Brooklyn, and beards ruled the land, Dutch anatomist, Karl Frederik Wenckebach was sporting a killer moustache in medical wards in the Netherlands. He studied the heart’s communication system, and the tract that connects the SA and AV node
(Wenckebach’s bundle) bears his namesake. Interestingly enough, his father is credited with developing the Netherland’s first telegraph. Let’s just say the family communicated well. Heart touching, really

Bifascicular Block is relatively straightforward. It’s a block in either the (1) Right Bundle and Left Anterior Fascicle, (2) Right Bundle and Left Posterior Fascicle, or (3) in the Left Anterior and Left Posterior Fascicle. Interestingly, if both fascicles of the Left Bundle are blocked, the ECG would show a Left Bundle Branch Block, meaning one can think of Left Bundle Branch Block as a type of Bifascicular block (though in some cases the block may occur in the Bundle above the fascicles). In Bifascicular Block, there still remains a pathway for the atria to communicate with the ventricles. Clinically, this means Cardiology can see the patient on an outpatient basis, and emergent pacemaker placement is not always indicated.

Which leads us to…

Trifascicular Block indicates a conduction problem in the Right Bundle as well as the Left Bundle or both of its fascicles. It comes in two flavors: Definite (all three fascicles contain disease) and Possible (all three fascicles likely contain disease).
Definite Trifascicular Block exists in three cases:

1) Alternating Bundle Branch Block: If the ECG shows a Left Bundle (2 fascicles) at one moment and a Right Bundle (1 fascicle) the next, then all three fascicles must have disease.

2) If the ECG always show a Right Bundle (1 fascicle) but alternates between a Left Anterior and Left Posterior Fascicular Block (2 fascicles), all three fascicles must have disease.

3) Mobitz II blocks usually occur past the Bundle of His in the fascicles, so if this exists (1 fascicle) with a Left Bundle Branch Block (2 fascicles), all three fascicles are definitely diseased.
Possible Trifascicular Block exists in two cases:

1) Mobitz I (and 1^st Degree AVB) may occur before the Bundle of His (0 fascicles) or past it (1 fascicle), if either of these exist with a Bifascicular Block (2 fascicles), it is possible that all three fascicles are diseased.

2) Complete AV Block exists in many ways (in the AV node itself, in the bundles below it, or in the fascicles below those) and can be categorized as possible Trifascicular Block.

Armed with our new understanding of blocks, we see that this ECG shows a Right Bundle Branch Block, a Left Anterior Fascicle Block as well as a Mobitz I (Wenckebach) Second Degree AV Block. Since Mobitz I may occur past the Bundle of His (in this case possibly in the Left Posterior Fascicle), this can be classified as a Possible Trifascicular Block. Given the increased risk of progression to complete AV block (and resultant unstable escape bradydysrhythmias or possible triggered tachydysrhythmias), cardiology consultation for pacemaker placement is warranted.