The most striking ECG characteristics of left anterior fascicular block are (see Figure 2):

1. Left axis deviation in the frontal QRS axis between −30° and −90° in 90% of cases 
2. A single R wave in leads I and aVL 
3. A small q wave may be present, but an s wave is rare
4. Slurring of the R wave
5. An rS complex with a positive and asymmetric T wave in leads II, III, and aVF
6. A negative T wave in aVR
7. A positive, flat, or negative T wave in leads I and aVL

Other ECG characteristics in left anterior fascicular block:

1. A smooth QRS transition zone with an RS configuration in precordial leads V₁ through V6
2. May have an rS complex in V₁ through V₆ 
3. R and S amplitude may be equal in V₂ and V₃ and in V₄ through V₆
4. R rarely higher than S in V2 and V₃ or in V₅ and V₆
5. A small s wave in lead I and a mirror image of an R wave in lead III
6. Positive and asymmetric T wave in all precordial leads
7. Small q wave in V₃ through V₆  is a rare finding

Left axis deviation is seen in 90% of patients with left anterior fascicular block. The remaining 10% of patients may have anterior fascicular block due to inferior wall infarction, LVH, and anatomical anomalies in the thorax.

Left Anterior Fascicular Block With Vectors and Corresponding ECG Tracing ¹¹