Left bundle branch block

Last revised by Henry Knipe on 13 Mar 2023

A form of interventricular conduction defect most often diagnosed on the electrocardiogram, the presence of a left bundle branch block (LBBB) disrupts the normal sequence of ventricular depolarization. 

Aberrant conduction in the left bundle branch producing a conduction block is most often associated with 1;

  • prolongation of QRS complex duration >0.12 seconds
  • broad, positive monophasic (or notched) R wave in the left sided leads (leads I, aVL, V5-6)
    • associated with discordant ST depression and T wave inversion
  • negative (typically QS or rS) QRS polarity in the right precordium
    • precordial leads V1-3

Transthoracic echocardiography is often utilized to evaluate cardiac structure and function, with the latter dependent on orderly electrical activation and efficient contraction to generate a cardiac output. Appreciation of the disordered activation of myocardial segments in a LBBB is predicated on an appreciation of a normal sequence of activation. The parasternal long axis is a common view to begin the assessment of regional wall motion, visualizing the anteroseptal and inferolateral left ventricular (LV) walls. M-mode may be a preferable modality to analyze regional wall motion in this context, as it offers excellent temporal resolution.

Normal left ventricular contraction occurs following electrical activation; after a brief lag period, simultaneous inward (in relation to the left ventricular cavity) excursion of the left ventricular anteroseptal and inferolateral walls occurs in conjunction with wall thickening. Relaxation, and outward excursion of the aforementioned walls, occurs after the electrocardiographic T wave in diastole.
The septum, which often has a minute inscription of its M mode tracing at the peak of its inward excursion, may be observed to move outward slightly before the corresponding movement of the anterolateral wall.

The characteristic wall motion abnormality, as viewed from a transthoracic parasternal long axis view, appears as follows 2;

  • during the ejection phase of ventricular systole, simultaneous anterior motion of the left ventricular inferolateral wall and the anteroseptal wall may be observed
    • this is "paradoxical" motion of the interventricular septum, which usually moves inward (posteriorly) during systole
  • a brief inward (posterior) excursion of the anteroseptal wall precedes the paradoxical anterior motion, producing the characteristic "beaking" of the M-mode tracing
    • this inward excursion is temporally related to the QRS complex on the ECG, occurring within 0.04 seconds of the R wave peak
  • systolic wall thickening is unaffected
    • allowing differentiation from myocardial ischemia, in which wall thickening is reduced or absent

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Cases and figures

  • Figure 1: cardiac conduction system
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  • Case 1: LBBB with dilated cardiomyopathy
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