Electrovectorcardiographic Diagnosis of Left Septal Fascicular Block: Anatomic and Clinical Considerations

Abstract

Several publications considering anatomical, histological, pathological, electrocardiographic, vectorcardiographic, and electrophysiologic studies have shown that the left bundle branch splits into three fascicles or in a "fan-like interconnected network" in the vast majority of human hearts. The left His system is trifascicular with a left anterior, a left posterior, and a left septal fascicle (LSF). Consequently, the classic term "hemiblock," to describe the block of one of the fascicles, established several decades ago by the Rosembaum's school, should be updated. Electrovectorcardiographic changes resulting from conduction abnormalities of the left anterior and left posterior fascicles are commonly diagnosed, mainly by their changes in the frontal plane. However, the existence of conduction defects of the LSF remains controversial. The ECG/VCG hallmark of LSF block is prominent anterior QRS forces (PAF) on the horizontal plane. This ECG/VCG phenomena should be distinguished from other conditions that also produce anterior QRS shift in the HP as: normal variants, right ventricular enlargement, misplaced precordial leads, lateral myocardial infarction, right bundle branch block, Wolff-Parkinson-White, obstructive and nonobstructive forms of hypertrophic cardiomyopahty, diastolic left ventricular enlargement, endomiocardial fibrosis, Duchenne muscular dystrophy, and dextroposition. The two highly frequent etiologies of LSFB are ischemia (coronary artery disease (CAD) with critical proximal obstruction of the left anterior descending coronary artery) and, in Latin America, Chagas' cardiomyopathy. The aims of this review are to revise the evidence of the existence of a trifascicular left Hissian system and to help in the ECG/VCG recognition of the LSFB.

Unusual Conduction Disorder: Left Posterior Fascicular Block + Left Septal Fascicular Block

“…In the absence of RVH/RVE, latero-basal myocardial infarction, right bundle branch block, type A Wolff-Parkinson-White pattern, obstructive and nonobstructive forms of hypertrophic cardiomyopahty, endomiocardial fibrosis, muscular dystrophy, and dextroposition; it strongly suggests left septal fascicular block (LSFB). 1,2 The combination of LPFB + LSFB has not been properly documented yet and this is the first report that confirms this association. 3 The postoperative ECG (Fig.…”

Electrocardiographic manifestation of the middle fibers/septal fascicle block: a consensus report

“…This was extensively outlined in a recent review. 1 As said before, some of the coauthors of this manuscript prefer to name it middle fibers block because it better reflects the true nature of the problem accepting that only in around 33% of cases a true fascicle can be isolated and also because we would like to separate this type of block from the well-established blocks of the SA and IP fascicles described by Rosenbaum School in 1968. However, most of the articles that have been publishing about this issue prefer to use the term SF block.…”

“…[11][12][13][14][15] Currently, it is firmly supported by the Brazilian School. 1,11 Recently, major and minor diagnostic criteria of this type of block have been described. 1 The only major criterion, according to this hypothesis, was the presence of intermittent PAF, which is manifested in the ECG as prominent R wave in lead V 1 and especially in lead V 2 with progressive increasing of R voltage across the mid precordial leads and decreasing from leads V 5 to V 6 .…”

Quadrifascicular?

“…However, several articles and even reviews indicate that either loss of the normal septal Q wave or an exaggeration of anterior QRS forces may result from "middle septal fiber delay or block." [9][10][11][12] Thus, prominent R waves in the anterior precordial leads with anterior QRS displacement in the VCG may be seen. The term anterior conduction delay has been proposed for such ECG/VCGs.…”