Left Anterior Fascicular Block - ECG

Left anterior fascicular block (LAFB) is also known as the left anterior hemiblock (LAHB). It is a very common random finding. It features positive main amplitude of QRS in lead I and negative main amplitudes of QRS complexes in leads II and III. The ECG axis is twisted to the left (horizontal position). 

 

Left Anterior Fascicular Block - ECGLAFB – We see negative QRS complexes in II and III (red) and positive in lead I (blue). The red ellipse highlights the relationship of aVL and aVF that indicates the horizontal ECG axis presence. 
 

Conclusion: The LAFB must be properly interpreted. It may occur in a myocardial infarction, but usually we find it absolutely randomly in an asymptomatic patient. However, it is necessary to take into account that the LAFB may be an unspecific sign of a still unrecognized cardiac pathology including silent ischemia, cardiomyopathy, degenerative processes, etc. Finding LAFB in a relatively young person should be followed by an echocardiography examination.