Bradycardia with Alternating Bundle Branch Block

A 75yo patient is admitted for dizziness. On telemetry, the rate suddenly dropped. What is your interpretation?

Figure 1 - ECG case

 Figure 2 -ECG case marked

The rhythm is sinus at a rate of about 60 bpm (P waves marked with black lines). The computer computed ventricular rate is about 41 bpm. The initial few complexes have a 2:1 conduction with a left bundle branch block (LBBB) morphology. The latter part has 3:2 conduction with alternating LBBB and right bundle branch (RBBB) morphology. Examining closely, the LBBB QRS has a PRI of about 0.24 sec and the RBBB QRS has a PRI of 0.28. So, when asked how to describe the strip, it should be:

2:1 and 3:2 conduction in the setting of alternating bundle branch block with changing PRI

A highly skilled electrocardiographer can grasp what is happening once they hear this description which we will see later.

A simplistic interpretation would be second degree AV block type I because there is PRI prolongation then there is a non-conducted P wave. The LBBB can possibly be explained by rate-related aberrancy (bradycardia-dependent). However, there is an "alternative unifying explanation".

A Unifying Explanation

In ECG cases that looked bizarre, odd, funky or a WFR (what a f*&*#g rhythm), there is a single unifying explanation. There is no such thing as WFR. There is a logical explanation for every rhythm. In this case, the unifying explanation is a bilateral bundle branch block.

Prolonged PR Interval… not only AV nodal problem
In our case, there is a prolonged PRI or first degree AV block. Basic electrocardiography taught us that the basic problem in first degree AV block is in the AV node. However, the prolongation in the PRI can be due to conduction delay in the atrium, AV node, intra-His, infra-His or bundle branches. Lepeschkin came up with an illustration demonstrating the effects of blocks in the bundle branches (Figure 3).

Figure 3 – Schematic Representation of the Effect of Various Degrees of First Degree Bilateral Bundle Branch Block on the ECG in Lead I

If you cannot understand it with the first reading, then you are not alone. It took me several times to grasp the concept. Anyway, the purpose of the illustration is to show that problems in the bundle branches can prolong the PRI.
Aside from first degree, second degree and third degree block can also happen in the bundle branches.

The Ladder Diagram
Proposing the mechanism for a bilateral bundle block is difficult. For this ECG case, I created a ladder diagram (Figure 4) of the possible mechanism.

Figure 4 - Ladder Diagram. The right bundle branch is on the reader's left and the left bundle branch is on the reader's right. 0 – blocked, + conducted.

In the bottom of ladder diagram, there is an annotation on the conduction of the bundle branches. The first 5 beats are blocked in the LEFT BUNDLE BRANCH (marked 0 at the bottom) and are conducted to the RIGHT BUNDLE BRANCH with a 2:1 pattern. When there is a LBBB pattern, conduction is through the RIGHT BUNDLE and then spreads to the LEFT BUNDLE and vice versa. Thus, even though the LEFT BUNDLE BRANCH is blocked it was still depolarized from the RIGHT BUNDLE BRANCH (little delayed). This explains the initial 2:1 LBBB pattern.

Later, there is a 3:2 pattern (LBBB/RBBB/BLOCKED). This is can be due to an asynchronous 3:1 block in both branches. This is best understood by looking at the codes at the bottom of the ladder diagram. There is a 3:1 block in the RIGHT BUNDLE (0 0 +) which means that for every 3 attempted conduction there is only one is successful beat. The same is true on the LEFT BUNDLE but the occurrence is not synchronous with the RIGHT BUNDLE. Thus, creating the 3:2 conduction with alternating bundle branch block.

What Happened Later?

The patient ventricular rate dropped further and manifested with high-grade AV block on the surface ECG (Figure 5). The patient was transferred to the ICU and a pacemaker was eventually inserted.

Figure 5 – High grade AV Block

Alternating Branch Block

According to Dr. Mark Josephson, "spontaneous alternating bundle branch block, particularly when associated with a change in P-R interval, represents the most ominous sign for progression to A-V block. Beat-to-beat alternation is the most ominous, whereas a change in bundle branch block noted on different days is somewhat less ominous. In either case, this finding portends the development of A-V block. This phenomenon implies instability of the His-Purkinje system and a disease process involving either both bundle branches, the His bundle, or the main trunk".

Take Home Message: Simplistic Pattern Recognition

It is difficult to grasp the concept of bilateral branch block but to simplify things for most of us involved in ECG pattern recognition:


Final Interpretation: Bilateral Bundle Branch Block / alternating bundle branch block presenting as 2:1, 3:2 conduction


Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.

Josephson, M. 2008. Clinical Cardiac Electrophysiology: Techniques and Interpretations, 4th Edition Lippincott Williams & Wilkins

Lepeschkin E. 1964. The Electrocardiographic Diagnosis of Bilateral Bundle Branch Block in Relation to Heart Block. Progress in CV Disease Vol 6 # 5, 445-471

Ranganathan N et al. 1972. His Bundle Electrogram in Bundle-Branch Block. Circulation XLV
Schloff L et al. 1967. Bilateral Bundle-Branch Block Clinical and Electrocardiographic Aspects. Circulation Vol XXXV 790-801


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