Palpitations of unusual etiology

Written by Pendell Meyers, with edits by Steve Smith

A male in his 60s with history of HTN and previous complaint of palpitations but with a negative holter monitor workup, presented to our ED with palpitations for the past hour, associated with lightheadedness and presyncope.

He was hemodynamically stable and well appearing, but was symptomatic with palpitations and lightheadedness.

Here is his 12-lead on arrival:

What do you think?

There is a regular, seemingly wide complex tachycardia at 224 beats per minute. The computer QRS duration is calculated at 178ms, but I believe the true QRS duration is much shorter, and in most leads no greater than 100ms. The QRS morphology is similar to RBBB with LPFB. However, the RBBB morphology in V1 is not the classic rSR’ [first wave (r-wave) smaller than the second (R’-wave)]. Instead, the first R-wave in V1 is taller than the second (the R’-wave).  This “sign” has been described as an indicator of ventricular origin. However, the initial deflection of the QRS complex has undeniably rapid and organized (steep slope) conduction, strongly suggesting that the conduction pattern is utilizing the intrinsic conduction system of the Purkinje fibers.

For a QRS complex to have narrow QRS duration, organized initial conduction, and morphology consistent with RBBB + LPFB, it must either:

1) be supraventricular with RBBB and LPFB


2) originate in the left anterior fascicle itself

The differential at this point includes SVT with RBBB and LPFB, anterior fascicular VT, and good old regular VT, all plus or minus hyperkalemia to be safe.

Here are some helpful ECG core content diagrams for review of the differential of a wide complex QRS, as well as tachycardias in general:

Back to the case:

The treating physicians recognized the morphology as likely fascicular VT, and suspected that it may be one of the verapamil sensitive variants. As a review, posterior fascicle VT is thought to be fairly consistently responsive to verapamil; anterior fascicle VT is thought to be a similar entity to posterior fascicle VT but it seems less consistently responsive to verapamil. The working diagnosis of anterior fascicle VT thus prompted consideration of verapamil. Vagal maneuvers and/or trial of adenosine also would have been acceptable choices at this point.

The team first confirmed good LV function with bedside US (this is important before giving verapamil, as it is contraindicated by our guidelines in the setting of structural heart disease and heart failure). They then gave 10 mg verapamil IV. There was no response. They they tried 12 mg adenosine with no response. The patient was then sedated with etomidate and cardioverted with 200J, with immediate return to sinus rhythm.

Here is the post-cardioversion ECG:

This shows sinus rhythm with RBBB. There is no LPFB present on this ECG. So the RBBB seen on the presentation ECG is already present at baseline, however the LPFB was not. This feature does not help distinguish between SVT with RBBB and LPFB vs. fascicular VT originating in the left anterior fascicle, because it is still possible to have a rate-related LPFB in the setting of SVT with preexisting RBBB.

The patient underwent an electrophysiology study, during which Bundle Branch Reentrant Ventricular Tachycardia (BBRVT) was reproduced and successfully ablated. He received an AICD, also underwent a cath showing non obstructive disease and cardiac MRI without abnormalities.

Bundle Branch Reentrant Ventricular Tachycardia is a rare arrhythmia involving components of the infra-His conduction system (including the three fascicles themselves) as necessary components of a reentrant pathway. As you would expect, there are different ways to combine these circuit limbs, all of which would create different reentrant loops and different QRS morphologies.

Tchou and Mehdirad have been credited with describing three categories of BBRVT (see diagram below). Because our patient’s QRS morphology includes RBBB and LPFB, this may be consistent with Type B, which appears to use one of the LBB fascicles anterogradely and the other retrogradely to comprise the reentry loop. If the circuit were to progress anterogradely down the LAF and retrogradely up the LPF, it would theoretically have the appearance of RBBB with LPFB, matching our patient.

There are other matching possibilities, however, including Type C with additional rate-related LPFB. At some point, enumerating these possibilities becomes purely academic because they do not have implications for prospective clinical management.

If these reentrant tachycardias are particularly sensitive to any medications, I am not yet aware of it. I cannot find any evidence stating that BBRVTs typically respond to any particular medication with any reliability. The few publications that do comment on pharmacologic therapy in the acute arrhythmic phase seem to agree that pharmacologic therapy is usually ineffective. Because they exist in the conduction system below the AV node, I would not expect that they should respond to AV nodal blockade such as adenosine.

This is in contrast to posterior fascicular VT, which is verapamil responsive, and to right ventricular outflow tract VT, which is responsive to adenosine.

Apparently BBRVT is highly associated with structural heart disease and cardiomyopathy in most cases, but more rarely has been described in the absence of structural heart disease (which our patient seems to fall into, except for his baseline RBBB). This further highlights the importance of considering bedside US for LV function before considering verapamil in these cases.

Image obtained from:

See these other cases of fascicular VT to compare and contrast with this case:

Posterior Fascicle VT
Another Posterior Fascicle VT
Originating in the posterior fascicle, therefore shows morphology of RBBB + LAFB
Usually verapamil sensitive

Anterior Fascicle VT
Originating in the anterior fascicle, therefore shows morphology of RBBB + LPFB
Sometimes verapamil sensitive, somewhat less so than posterior fascicle VT

Right Ventricular Outflow Tract VT
Originating in the RV outflow tract, therefore shows morphology similar to LBBB with inferior frontal plan axis (positive QRS complexes in inferior leads)
Usually adenosine responsive

Unfortunately, as EM physicians we rarely know the exact electrophysiologic diagnosis prospectively. What we can see prospectively is whether the QRS morphology matches one of the established patterns:

RBBB + LAFB Morphology: 
DDx includes classic VT vs. SVT+RBBB+LAFB vs. posterior fascicle VT vs. BBRVT vs. (probably other even more rare and obscure rhythms); if you believe it is one of the ventricular causes but not classic VT, then posterior fascicle VT seems to be one of the most common, and it is typically verapamil sensitive. Electricity works.

RBBB + LPFB Morphology: 
DDx includes classic VT vs. SVT+RBBB+LPFB vs. anterior fascicle VT vs. BBRVT vs. (who knows what else); if you believe it is one of the ventricular causes but not classic VT, then anterior fascicle VT is possible and may be verapamil sensitive, but it may not be; the pharmacologic solutions are unknown in this category. Electricity works.

LBBB + inferior axis (positive in inferior leads) Morphology: 
DDx includes classic VT vs. SVT+LBBB vs. RVOT VT vs. BBRVT vs. (probably others). If you believe it is one of the ventricular causes but not classic VT, then RVOT VT is possible and is typically adenosine sensitive (and adenosine is already indicated for a stable patient with monomorphic wide complex tachycardia). Electricity works.


Bundle Branch Re-entry Ventricular Tachycardia Storm During the Recovery Phase of Transient Complete Heart Block 

Bundle branch reentry ventricular tachycardia.     1995 Jul;18(7):1427-37. 

Learning Points:

For learners, please remember that one should assume classic VT until proven otherwise by a combination of clinical experience with ECG and clinical findings.

With experience and training, one can recognize wide QRS complexes which are likely to represent VT originating within the conduction system itself. Unfortunately, this category still includes a wide variety of arrhythmias which differ in their mechanisms and effective medications. Perhaps the only universal truth is that they are all susceptible to electrical cardioversion.

Posterior fascicle VT is typically thought to respond to verapamil.

Anterior fascicle VT may respond to verapamil, but seems less responsive than posterior fascicle VT.

RVOT is typically thought to respond to adenosine.

BBRVT is a rare arrhythmia which originates inside the ventricular conduction system and acts similarly to other fascicular VTs we have discussed on this blog. However, there do not appear to be any clear patterns of response to pharmacologic therapy.

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