Review
Defining Left Bundle Branch Block in the Era of Cardiac Resynchronization Therapy

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Cardiac resynchronization therapy (CRT) has emerged as an attractive intervention to improve left ventricular mechanical function by changing the sequence of electrical activation. Unfortunately, many patients receiving CRT do not benefit but are subjected to device complications and costs. Thus, there is a need for better selection criteria. Current criteria for CRT eligibility include a QRS duration ≥120 ms. However, QRS morphology is not considered, although it can indicate the cause of delayed conduction. Recent studies have suggested that only patients with left bundle branch block (LBBB) benefit from CRT, and not patients with right bundle branch block or nonspecific intraventricular conduction delay. The authors review the pathophysiologic and clinical evidence supporting why only patients with complete LBBB benefit from CRT. Furthermore, they review how the threshold of 120 ms to define LBBB was derived subjectively at a time when criteria for LBBB and right bundle branch block were mistakenly reversed. Three key studies over the past 65 years have suggested that 1/3 of patients diagnosed with LBBB by conventional electrocardiographic criteria may not have true complete LBBB, but likely have a combination of left ventricular hypertrophy and left anterior fascicular block. On the basis of additional insights from computer simulations, the investigators propose stricter criteria for complete LBBB that include a QRS duration ≥140 ms for men and ≥130 ms for women, along with mid-QRS notching or slurring in ≥2 contiguous leads. Further studies are needed to reinvestigate the electrocardiographic criteria for complete LBBB and the implications of these criteria for selecting patients for CRT.

Section snippets

Cardiac Resynchronization Therapy

In the presence of complete LBBB, there is a significant delay between activation of the interventricular septum and activation of the left ventricular (LV) free wall (Figure 1).1 Thus, decreasing the delay by simultaneous pacing of the septum and LV free wall may resynchronize mechanical contraction. However, in the presence of QRS prolongation due to right bundle branch block (RBBB) or LVH, the LV endocardium is activated normally via the rapidly conducting Purkinje system.1

The major

Early History of Defining Bundle Branch Block

Eppinger and Rothberger9 created the first experimental bundle branch block in dogs in 1909 by injecting silver nitrate into the left ventricle. In 1914, Carter10 published the first series of patient electrocardiograms with bundle branch block and observed that RBBB was more common than LBBB, but he mistakenly switched the diagnosis of LBBB and RBBB, which was later attributed to differences in heart position between the dog and the human.11 In 1920, Wilson and Herrmann12 presented criteria

Challenges to the Definition of Left Bundle Branch Block

In 1956, Grant and Dodge16 studied 128 patients with QRS durations ≥120 ms and LV conduction delays who had an electrocardiograms showing normal conduction within the previous 2 years (<6 months in 80%). Their conclusion was that “the classical explanation of LBBB, which is based upon animal experimentation, is only partially accurate when applied to human LBBB,” and they proposed that 1/3 of electrocardiograms classified as LBBB by conventional criteria were incorrectly diagnosed.16 This was

Endocardial Mapping of Patients With Left Bundle Branch Block by Conventional Criteria

In 1984, Vassallo et al19 performed endocardial catheter mapping in 18 patients with LBBB by conventional ECG criteria. Twelve patients had 1 septal site of LV endocardial breakthrough. The other 6 had 2 LV endocardial breakthrough sites, often 1 septal and 1 at the superior base, consistent with 2 of the 3 sites of endocardial breakthrough in normal hearts. This suggests that 1/3 of patients classified by conventional criteria to have LBBB did not actually have complete LBBB, just as Grant and

Defining Complete Left Bundle Branch Block

Conventional criteria for LBBB that are used clinically include a QRS duration ≥120 ms, QS or rS in lead V1, and a monophasic R wave with no Q waves in leads V6 and I.23 The American Heart Association, American College of Cardiology Foundation, and Heart Rhythm Society recommendations do go beyond this to include “broad notched or slurred R wave in leads I, aVL, V5 and V6 and an occasional RS pattern in V5 and V6 attributed to displaced transition of the QRS complex.”24 We strongly support that

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    After completion of this report, Dr. Strauss moved to the United States Food and Drug Administration. The views expressed do not represent those of the Food and Drug Administration.

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