Left anterior fascicular block

Impact on left ventricular hypertrophy assessment

When analyzing an electrocardiogram, an important step is the assessment of the electrical axis of the heart (EAH). Leftward axis deviation is traditionally considered one of the signs of left ventricular hypertrophy (LVH). However, this criterion has low specificity. The main condition under neuropathological axis deviation can be used to diagnose LVH is the strict absence of left anterior fascicular block.

If this block is excluded, the physician can rely on voltage criteria, which reflect the increased electrical potential of the enlarged myocardium:

• Sokolow-Lyon index: the sum of the amplitudes of the S wave in lead $V_1$ and the R wave in leads $V_5$ or $V_6$ exceeds 35 mm.
• Cornell voltage index: the sum of the amplitudes of R in aVL and S in $V_3$ is greater than 28 mm for men and greater than 20 mm for women.
• Isolated amplitude signs, such as the shift of the R wave peak from its normal position in lead $V_4$ to the left chest leads $V_5-V_6$.

Limitations in diagnosing right ventricular hypertrophy

Anterior fascicular block also significantly adjustments the assessment of the right heart chambers. When right ventricular hypertrophy (RVH) is suspected, specialists analyze amplitude changes in the left precordial leads ($V_5-V_6$). The main sign here is the predominance of the S wave depth over the R wave height (R/S amplitude ratio < 1.0).

The most important condition for the reliability of this sign: it works only in the absence of left anterior fascicular block and scarring changes (prior myocardial infarction) of the anterior wall.

Additionally, in RVH, secondary repolarization abnormalities are assessed in the right precordial ($V_1-V_2$) and inferior leads (III, aVF). They manifest as the formation of a negative asymmetric T wave and a specific domed ST segment depression.

Role of conduction in cardiac resynchronization therapy selection

It is extremely important to differentiate isolated anterior fascicular block from complete left bundle branch block (LBBB). This is crucial when planning cardiac resynchronization therapy (CRT) in patients with chronic heart failure.

CRT is prescribed to reduce mortality and alleviate symptoms that persist despite optimal medical therapy. The strict ECG criteria for patient selection are:

1. QRS complex duration $\ge$ 150 ms.
2. QRS complex morphology corresponding specifically to complete LBBB.

Isolated anterior fascicular block, second-degree atrioventricular block, or right bundle branch block (RBBB) are not indications for this treatment method. An additional mandatory condition for CRT is sinus rhythm and a left ventricular ejection fraction reduced to 35% or lower.

Conduction disorders in structural pathologies

Bundle branch blocks often accompany severe myocardial diseases, such as hypertrophic cardiomyopathy (HCM). This disease is characterized by a slow progression of outflow tract obstruction and a high risk of sudden cardiac death, which can occur even before the development of overt congestive heart failure.

ECG findings in HCM reveal signs of ventricular and atrial hypertrophy, deep Q waves (in leads I, aVL, $V_5-V_6$ due to interventricular septal hypertrophy), as well as ischemia-like changes: ST depression and negative T waves. Holter monitoring is mandatory to assess arrhythmia risks (ventricular tachycardia, atrial fibrillation).

The "gold standard" for confirming the diagnosis remains echocardiography, which visualizes septal thickening (1.8–2.2 cm or more) and a specific sign of obstruction—systolic anterior motion of the mitral valve (SAM syndrome).

Class and exam questions

What else to study on this topic

• Voltage ECG criteria for left ventricular hypertrophy (Sokolow-Lyon and Cornell indices)
• Secondary ST-T repolarization abnormalities in right ventricular hypertrophy
• Echocardiographic diagnosis of hypertrophic cardiomyopathy (SAM syndrome)
• Holter monitoring and sudden cardiac death risk assessment
• Patient selection criteria for cardiac resynchronization therapy (CRT)

Other topics in the "Cardiovascular system" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.