Blog #198 — Can VT Be Irregular (LBBB?)

The ECG in Figure-1 was obtained in the ED from a 60-year-old man who presented with increased dyspnea. He had a history of dilated cardiomyopathy — and was known to have complete LBBB (Left Bundle Branch Block) on previous ECGs, though no copy of any prior tracings was available at the time this patient was first seen. In view of this history — How would you interpret his ECG?

• What is the rhythm? Can you rule out VT (Ventricular Tachycardia) on the basis of this ECG?
• Is the ECG in Figure-1 typical for complete LBBB?
• Is there LVH?
• Are there hyperacute T waves suggestive of acute LAD occlusion in the anterior leads?

 

Figure-1: ECG obtained on a 60-year-old man with increased dyspnea. How would you interpret his initial ECG?

 

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NOTE #1: Some readers may prefer at this point to listen to the 5-minute ECG Audio before reading My Thoughts regarding the rhythm in Figure-1. Feel free at any time to review to My Thoughts (that appear below ECG MP-15).

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Today’s ECG Media PEARL #15 (5 minutes Audio) — Is Monomorphic VT a regular rhythm? — with attention to the 2 Caveats that emhasize how: i) Fast AFib may look regular; and, ii) Monomorphic VT is not always perfectly regular.

 

 

MY Approach to Assessing the ECG in Figure-1:

The ECG in Figure-1 is a WCT (Wide-Complex Tachycardia) rhythm.

• The rhythm is rapid — at times attaining a heart rate in the range of 160-180/minute.
• Although seemingly regular in places (See leads III and aVF) — caliper measurement suggests an irregular irregularity to the overall rhythm.
• The QRS complex is very wide (up to 0.16 second in duration in a number of leads).
• No P waves are seen.

IMPRESSION of the Rhythm: The finding of an irregularly irregular rhythm without clear sign of P waves strongly suggests that the rhythm in Figure-1 is AFib (Atrial Fibrillation) with a rapid ventricular response.

• PEARL #1: QRS morphology in the chest leads is perfectly consistent with complete LBBB (ie, predominantly negative QRS in anterior leads — with a monophasic upright R wave in lead V6). That said — it is important to appreciate that QRS morphology in the limb leads is very atypical for LBBB. This is because instead of seeing a monophasic upright R wave in lateral leads I and aVL — there is almost a null vector (ie, tiny QRS with an rsr’ configuration) in lead I — and, an rSR’ complex with small amplitude R wave in lead aVL.
• It would really help to see a prior tracing from this patient! There is significant variability in the way cardiologists interpret various conduction defects — and opinions are many as to whether QRS morphology in an ECG like the one in Figure-1 should be interpreted as “LBBB” — or — as some type of IVCD (IntraVentricular Conduction Defect). This is relevant to today’s case — because the question arises as to whether this WCT rhythm with an exceedingly wide QRS (ie, ~0.16 second) and QRS morphology atypical for LBBB might possibly be Ventricular Tachycardia?
• PEARL #2: It is important to appreciate that VT is not always a regular rhythm (this point discussed in more detail in the above ECG Media Pearl #15). That said — it would be rare for a WCT as fast, and as irregularly irregular as the rhythm in Figure-1 to be VT — so despite the atypical QRS morphology, the most likely rhythm diagnosis remains AFib with a rapid ventricular response.
• PEARL #3: Some patients with LBBB may manifest atypical QRS morphology for this conduction defect. This can be the result of: i) A relative imbalance within the left bundle branch conduction system, in which severely diseased but not completely nonfunctional hemifascicles may sometimes manifest a rightward frontal plane axis despite LBBB; and/or, ii) Scar from severe cardiomyopathy and/or prior infarction that alters the patient’s baseline ECG appearance.
• PEARL #4: Additional support that the rhythm in Figure-1 is supraventricular — is the very steep descent of anterior S waves in leads V2, V3 and V4 (albeit a bit less so in lead V1). While not definitive — the nearly straight-line descent of these anterior S waves is a common feature of LBBB conduction. In contrast — ventricular rhythms typically manifest more delay (and a significantly less steep initial descent) in anterior S waves.
• BOTTOM Line Regarding the Rhythm: I would estimate my comfort level at ~95% that the rhythm in Figure-1 is rapid AFib. I’d initiate treatment accordingly. Finding a prior tracing on this patient with identical QRS morphology could increase my confidence level to 100% that the rhythm was AFib.

 

Additional ECG Findings of Note in Figure-1:

• The usual ECG criteria for diagnosis of LVH (Left Ventricular Hypertrophy) do not hold true when there is LBBB. This is because the sequence of ventricular depolarization and repolarization is completely altered by this conduction defect. That said — virtually all patients with complete LBBB have at least some form of underlying structural heart disease. Statistically — the great majority of these patients have LVH. In the setting of LBBB — the finding of one or more very deep anterior S waves of ≥25-30 mm (in lead V1, V2 and/or V3) is strongly suggestive of LVH. In today's case — the S wave in lead V2 is >45 mm, and far exceeds this value. The S wave in lead V3 goes off the paper after 20 mm (and would doubtlessly also exceed 30 mm if the S wave was not cut off). 
• PEARL #5: Even without being told — the combination of LBBB + marked QRS widening (0.16 second) + huge anterior S wave amplitude — strongly suggests an underlying cardiomyopathy. With this in mind — the unusual QRS morphology that we see in leads I and aVL is not so unexpected.
• NOTE: The anterior T waves are huge! — attaining an amplitude of nearly 20 mm in lead V2. That said — considering how huge S waves are in these respective leads — I did not get a sense that anterior T waves were disproportionately tall. This is a judgment call. Objective assessment tools (such as modified Smith-Sgarbossa criteria) — are difficult to apply to the ECG in Figure-1, because the overly steep T wave ascent provides no hint of how much actual J-point ST elevation there might be. Finally — ST-T wave amplitude is further accentuated by the tachycardia. 

BOTTOM LINE: I suspected that the ST-T wave changes we see in the anterior leads of Figure-1 were not disproportionately increased given the huge S waves in these leads — and, I suspected these changes were unlikely to indicate an acute event (especially with no mention of chest pain). That said — I would not be ready to completely rule out an acute event on the basis of this ECG alone.

• KEY Principle: “Treat the Primary”. Regardless of whether the patient in today’s case was having an acute MI — the primary problem appears to be the rapid AFib. This should be treated first — with treatment addressing the cause of this patient’s increased dyspnea. The ECG can then be repeated — and a reassessment made.

 

Case Follow-Up: The patient’s prior ECG was found — and the conduction defect with the QRS morphology seen in Figure-1 was similar to the prior tracing. This confirmed AFib as the rhythm diagnosis. There was no acute MI.

 

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Additional Relevant ECG Blog Posts to Today’s Case: 

• ECG Blog #146 — for an example of LBBB with changes of acute STEMI.

I link to 3 additional illustrative Cases taken from Dr. Smith’s ECG Blog. For each of these posts — Please scroll down to the bottom of the page to see My Comment. These cases provide insight to assessment for acute OMI (Occlusion-based MI) in patients with LBBB (including reference when relevant to modified Smith-Sgarbossa Criteria):

• The December 16, 2019 post in Dr. Smith’s ECG Blog. 
• The April 7, 2019 post in Dr. Smith’s ECG Blog. 
• The May 24, 2019 post in Dr. Smith’s ECG Blog.