LBBB: Not just a typo from your cat sitting on your keyboard (Left Bundle Branch Block)

Smruti Desai, DO/MPH

It is one of your last shifts as an intern, and after a year of wondering if you will get any taller, you finally have the elusive factor known as a linear thought process. You confidently sit back in your chair with this thought, and take an all-too-large gulp of your hot coffee. As you sputter, cough, and try to blend in with the wall, your attending waves an ECG in front of you, and through watery eyes, you see this mess:

Image courtesy of lifeinthefastlane.com

You look at your attending, who says those dreaded words:

“What do you think about this ECG?”

LEFT BUNDLE BRANCH BLOCK

A left bundle branch block (LBBB) represents a conduction defect occurring in either the entire left bundle branch of the Bundle of His or from simultaneous conduction block in the left anterior and posterior fascicles after the bifurcation.² Recall that after reaching the bifurcation of the Bundle of His, the normally conducted impulse simultaneously travels through the right and left bundle branches. The left bundle further splits off into the left anterior fascicle (innervating the anterior and inferior walls of the heart), while the left posterior fascicle curves around to innervate the lateral aspect of the heart.

Left bundle branch block is an ECG finding that is unusual in asymptomatic, healthy individuals, and hence is one that we should not miss in the ED. The delayed conduction blunts mechanical function of the left ventricle; this triggers ventricular remodeling which consequently impairs perfusion and systolic/diastolic function.⁵ Furthermore, if a patient has a left bundle branch block in the context of chest pain and/or hemodynamic instability, a variety of conditions must be considered as possible causes.

What are some causes of LBBB?

Hyperkalemia, anterior MI, extensive coronary artery disease, aortic root manipulation (surgical or otherwise), dilated cardiomyopathy, structural damage of the conduction system.^1,2

What are the criteria defining LBBB?

1. QRS > 0.12 msec in adults
2. S waves in V1 and V2 (QS, rS)
3. Monophasic, broad, slurred or notched R waves in I, aVL, V5, V6

There are other findings in LBBB that are common but not necessary to meet criteria:

• LBBB is the most common cause of left axis deviation (LAD)
• There is often absence of a septal q wave in the precordial leads (V1, V2)
• Finally, ST-T wave discordance is common in LBBB

While these criteria may seem like yet another laundry list to memorize during residency, understanding why they appear in LBBB can save you plenty of nail-biting. Let’s go through each of these criteria conceptually:

1. QRS waves >0.12 msec: The QRS wave represents ventricular depolarization in the cardiac cycle. Any delay in conducting an impulse through the ventricular myocytes will cause a delay in the depolarization of the entire ventricle, which causes prolonging (and therefore widening) of the QRS complex.²

2. S waves in V1 and V2: This is where it gets tricky. A simple way to conceptualize S waves is to note that an S wave in a lead essentially indicates an impulse moving away from that lead. In LBBB, the ventricular myocytes depend on intraventricular spread of the impulse rather than the the faster bundle branch. It may help to imagine this in terms of time: the impulse now spends longer traveling from right-to-left through the myocyte network, causing the overall vector to travel away from the R-side leads, i.e. V1 and V2. As a result, we see S waves in these leads. These S waves may or may not be preceded by a small R wave.^3,5 This right-to-left depolarization can also explain left axis deviation in LBBB.

3. Monophasic, dominant, broad, notched R waves: R waves in a lead essentially indicate that an impulse is moving towards that lead. Since the overall direction is now right-to-left, we see R waves in lateral leads I, aVL, V5 and V6. Since the left ventricle has a conduction delay, it will depolarize after the right ventricle. This can manifest as a notched or broad R wave (think of two separate QRS complexes merging together), as seen in this image below:

Image courtesy of lifeinthefastlane.com

4. ST-T wave discordance, when present in LBBB, can be indicative of greater severity of disease and if associated with systolic heart failure, carries a worse prognosis.⁵ Recall that depolarization occurs from the endocardium to the epicardium, while repolarization occurs in the opposite physical direction (epicardium–>endocardium). This results in the normal ST-T wave concordance seen in healthy patients, because depolarization and repolarization are electrically opposite and are also traveling in physically opposite directions. The current theory explaining ST-T wave discordance proposes that in LBBB, a different transmural repolarization sequence may develop in which the epicardium recovers before the endocardium.⁵ Thus, repolarization occurs in the same physical direction as depolarization despite still being electrical opposites. This results in the QRS complex and T wave being in opposite directions, known as ST-T wave discordance.^5,6

5. Absence of septal q wave in V5 and V6: In normal conduction, the septum is depolarized in a L–>R direction, which causes a small septal q wave in left precordial leads. In LBBB, the direction is reversed to R–>L, and the q wave may not appear in leads V5 and V6 any longer.

With this information in mind, we are now ready to tackle these ECGs – that is, until we invent an ECG machine that can do this:

Image courtesy of Martindale & Brown, Rapid Interpretation of ECGs

References:

1. Mattu A, Brady W. ECGs for the Emergency Physician 2. Malden, MA: Blackwell Pub.; 2008.
2. Martindale JL, Brown DFM. Rapid interpretation of ECGs in emergency medicine: a visual guide. 1st ed. Philadelphia, PA: Wolters Kluwer/Lippincott Williams & Wilkins Health; 2012.
3. O’Keefe JH, Hammill SC, Freed MS. Complete guide to ECGs: a comprehensive study guide to improve ECG interpretation skills. Sudbury: Jones & Bartlett; 2017.
4. Dodd KW, Elm KD, Smith SW. Comparison of the QRS Complex, ST-Segment, and T-Wave Among Patients with Left Bundle Branch Block with and without Acute Myocardial Infarction. The Journal of Emergency Medicine 2016;51(1):1–8.
5. Khalil J, Bernard AS, Maurice K, et al. Discordant vs. concordant left bundle branch block: A potential clinical significance. Journal of Electrocardiology 2016;49(1):69–75.
6. Surawicz B, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society: endorsed by the International Society for Computerized Electrocardiology. Circulation. 2009;119:e235-e240

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