Left anterior fascicular block (LAFB) was previously known as left anterior hemiblock (LAH) .The earlier designation can be attributed to the widely accepted writings of Maurico Rosenbaum whose work ( 1967) seemed to indicate that in humans the left bundle divided into a left and a right branch.
This "trifascicular" concept (right bundle branch and the two branches of the left bundle) was widely accepted and persisted even though work in 1972 by Demoulin and Kulbertus make it clear than there was also frequently a septal branch of the left bundle and more importantly that the anatomy of left bundle is much more complex that simply consisting of two (or three) branchs, i.e. more of a spiderweb or fan.There is great variation in the interconnections and in few of the 49 careful dissections by Demoulin can a simple pattern of 2 or even 3 fascicles be seen. (the patterns can be seen on page 283 of reference 1, full text is available)
Realizing that all models are wrong but some are useful ( George Box, circa 1978),it may be that the LAFB model has some value.
LAFB is diagnosed on EKG when the frontal plan axis is between minus 30 and minus 90 with QRS duration less than 120 msec and the patient does not have an inferior wall myocardial infarction,LVH or WPW syndrome.So, typically LAD equals LAFB in the EKG reading context.
Mandyam et al ( 2) from UCSF studied long term outcomes in patients with LAFB who at the beginning of the data collection did not have evidence of heart disease. Using data from the Cardiovascular Health Study (CHS), they compared 39 subjects with LAFB with 1625 patients without LAFB over a 19 year period.The average age of the control groups with 71.4 and the age of LAFB group was 74.9
They report LAFB was significantly associated with atrial fibrillation (AF) Heart failure(CHF) and death with the following p values-AF .02,CHF .02, and death .001.
This was a small ( 39 patients),coarse grain study.The authors point out that although CHS tries to exclude preexisting disease on entry to the program hypertension or asymptomatic coronary artery disease may have been missed. The authors referenced no corroborating studies .
This UCSF article received considerable medical press coverage in part as it offered some suggestion that the previously accepted notion that LAFB was a "benign" finding might not be correct but within a year their findings were seeminly contradicted by research from Copenhagen.
JB Nielsen (4) studied the cardiovascular outcome of 222,227 subjects with a 5.7 year follow-up period. They did find a statistically significant correction between LAFB and AF, HF and both all cause and cardiovascular mortality. However, after adjusting for age and gender only the all cause risk retains statistical significance.
Quoting the authors: "current EKG definition of LAFB is not always clinically important marker of cardiovascular morbidity and mortality beyond what can be expected by age and sex."
What is the effect of LAFB on left ventricular function?
Leeters and colleagues from The Netherlands studied 28 patients with RBBB,LAFB and heart failure with 2D speckle tracking regional strain measurements as well as healthy controls and 28 LBBB patients. This is not a study of "lone LAFB" as the patients had RBBB and HF and a number of them had scars detected by CMR. Since the RBBB per se would not affect Left ventricular activation sequence it is reasonable to assume the delay in activation of the anterior LV is due to the LAFB.The study indicated wall motion abnormalities between the anterior wall and inferior wall of the LV analogous to the classic pattern of septal and lateral LV wall dyssynchrony characteristic of LBBB but apparently less disruptive of cardiac function.
1) Fisher,JH Hemiblocks and the fascicular system:myths and implications. J. Of Interventional cardiac electrophysiology 2018 52: 281-285
2)Mandyam, ML Long-term Outcomes of left anterior fascicular block in the absence of overt cardiovascular disease, JAMA 2014 309 (15)1587
3)Leeters IP, et al Left Ventricular regional contraction abnormalities by echocardiographic speckle tracking in combined right bundle branch block with left anterior fascicular block compared to left bundle branch block.J Electrocardiol 2016 49 (3) 353
4)Nielsen, JB et al Left anterior fascicular block and the risk of cardiovascular outcomes.
JAMA Int Med June 2014, 174(6),1001-1003