A left anterior fascicular block (LAFB) creates ECG vectors that readily suggest LVH. On the other hand, LVH can be difficult to detect in patients with a normal ECG and RBBB because the left and right vectors are in opposing directions. This confounds the findings and may cause false-negative results. The frontal QRS axis is variable in these patients.

The frontal QRS axis in patients with an LAFB shows a left axis deviation in precordial leads V1 through V6. One index for discerning LVH in the presence of LAFB evaluates the voltages of S_III + (R + S) _{maximal precordial}. If the index value is 30 mm or greater, the odds are high that the patient has LVH. This index has a sensitivity and specificity of about 80–90%, so it is quite good at accurately identifying patients who do or do not have LVH.¹³