Incomplete left bundle-branch block. A definite electrocardiographic entity

Progression of incomplete toward complete left bundle branch block: A clinical and electrocardiographic analysis

Background

Complete left bundle branch block (cLBBB) is associated with increased cardiovascular mortality and heart failure. On the contrary, the clinical relevance of incomplete left bundle branch block (iLBBB) is less known. This study investigated the profile and outcome of iLBBB patients and assessed the risk of progression to cLBBB.

Methods

Patients diagnosed with iLBBB between July 2013 and April 2018 were retrospectively included. Subsequently, echo‐ and electrocardiographic examinations at time of iLBBB diagnosis and during follow‐up, as well as progression to non‐strict cLBBB and strict cLBBB, were evaluated.

Results

The study enrolled 321 patients (33% female, age 74 ± 11 years). During the follow‐up of 21 (8;34) months, 33% of iLBBB patients evolved to non‐strict cLBBB and 27% to strict cLBBB. iLBBB patients who evolved to non‐strict or strict cLBBB were older, had more frequently reduced left ventricular ejection fraction, and had more often QRS notching/slurring in the lateral leads and inferior leads, compared to patients without progression to cLBBB. In multivariate analysis, only QRS notching/slurring in the lateral leads was independently associated with progression to non‐strict cLBBB (odds ratio 4.64, p < .001) and strict cLBBB (odds ratio 9.6, p < .001). iLBBB patients with QRS notching/slurring had a progression rate to non‐strict cLBBB of 52% and 49% to strict cLBBB.

Conclusion

Among patients with iLBBB, up to one third of the patients progress to cLBBB within a period of 2 years. The presence of QRS notching/slurring in the lateral leads during iLBBB was the strongest predictor for progression toward cLBBB.

Electrocardiographic manifestations: bundle branch blocks and fascicular blocks

Intraventricular conduction block is the general name given to a varied group of electrocardiographic entities. All share a common finding of some degree of delay in ventricular activation; recognition of these blocks hinges upon analysis of the QRS complex, as well as the ST-T changes associated with them. Bundle branch block (right or left), and fascicular block (left anterior or left posterior) are all examples of intraventricular conduction block. Causation of intraventricular conduction block may be cardiac or noncardiac; early recognition of the etiology may be of clinical importance. This article reviews the basic anatomy and physiology related to intraventricular conduction blocks, and then examines each in terms of electrocardiographic definition and clinical correlation.

Criteria for intraventricular conduction disturbances and pre-excitation. World Health Organizational/International Society and Federation for Cardiology Task Force Ad Hoc

In an effort to standardize terminology and criteria for clinical electrocardiography, and as a follow-up of its work on definitions of terms related to cardiac rhythm, an Ad Hoc Working Group established by the World Health Organization and the International Society and Federation of Cardiology reviewed criteria for the diagnosis of conduction disturbances and pre-excitation. Recommendations resulting from these discussions are summarized for the diagnosis of complete and incomplete right and left bundle branch block, left anterior and left posterior fascicular block, nonspecific intraventricular block, Wolff-Parkinson-White syndrome and related pre-excitation patterns. Criteria for intraatrial conduction disturbances are also briefly reviewed. The criteria are described in clinical terms. A concise description of the criteria using formal Boolean logic is given in the Appendix. For the incorporation into computer electrocardiographic analysis programs, the limits of some interval measurements may need to be adjusted.

Electrocardiographic criteria of left ventricular hypertrophy in left bundle-branch block

In order to determine whether the electrocardiographic criteria of left ventricular hypertrophy apply in the presence of left bundle-branch block we studied 79 cases of intermittent left bundle-branch block and compared the QRS voltage and axis before and after its onset. Cases of incomplete left bundle-branch block were excluded. There was a statistically significant correlation between pre- and post-left bundle-branch block values of R or S wave voltage in leads I, V1, V2, V5, and V6, the Sokolow index (R V5 or V6 + S V1), and the QRS axis. There was a statistically significant reduction in R wave voltage in leads I, V5, and V6, an increase in S wave voltage in V1 and V2, and leftward shift of QRS axis, but the Sokolow index remained unchanged, after the onset of left bundle-branch block. The Sokolow criteria for left ventricular hypertrophy apply satisfactorily even in the presence of left bundle-branch block, though specificity is low, but QRS axis is unhelpful.

Reassessment of Q waves in left bundle branch block

This study disputes a number of recent reports claiming that abnormal Q waves or a QS configuration in inferior leads (II, III and AVF) coexisting with left bundle branch block is highly suggestive of, and indeed specific for, myocardial infarction. Five patients reported herein demonstrate disappearance of Q waves in inferior leads on spontaneous reversal of LBBB to normal conduction. This necessitates the conclusion that these Q waves represent a postdivisional conduction variant most closely equivalent to left anterior fascicular block coexisting with predivisional LBBB. Absence of inferior R waves in the five patients demonstrating LBBB is explicable by as little as a 20 msec conduction delay in the posterior fascicle coexisting with a higher grade conduction defect in the anterior fascicle. It is concluded that LBBB with a QS configuration in II, III and AVF cannot be considered diagnostic of inferior wall infarction since it regularly results from impaired conduction of the left anterior and possibly the left posterior fascicle (to a lesser extent), which may be reversible.