Fascicular blocks (hemiblocks)
See brief interpretation of 12 lead EKG above
– RBBB configuration in V1 (rSR’)
– left axis deviation with absence of other causes to left axis deviation
– borderline first degree AV block (PR interval around 0.22 sec) – so the EKG shows bifascicular or trifascicular block
Fascicular blocks (hemiblocks) are present when one of the fascicles of the left bundle branch is blocked.
Above, to the very left the normal conduction of the heart and the electrical vector (gray arrow) are illustrated. Left anterior fascicular block (left anterior hemiblock, center illustration) is characterized by a block of the anterior fascicle of the left bundle branch. Left posterior fascicular block (left posterior hemiblock, right illustration) is characterized by a block of the posterior fascicle of the left bundle branch.
When a fascicle is blocked the overall electrical vector (gray arrows) of the heart is changed. And when the vector is changed it is reflected by axis deviation in the EKG. This is the key to EKG interpretation of fascicular blocks.
Hence, as the anterior fascicle of the left bundle branch is blocked the axis shift to the left. And as the posterior fascicle of the left bundle branch is blocked the axis shift to the right.
However, of course, other causes of axis deviation must be considered and ruled out.
Left anterior hemiblock is much more common than left posterior hemiblock as the posterior fascicle has dual blood supply leaving it less vulnerable to ischemic changes. Left hemiblocks alone is without symptoms and require no treatment.
The combination of a hemiblock and right bundle branch block is called bifascicular heart block (left illustration below). The combination of a hemiblock, right bundle branch block and first degree AV block is called trifascicular heart block (right illustration below). The latter will often require pacemaker implantation.
Now, look at the 12 lead EKG above. What type of block is present?