fascicular Blocks

fascicular blocks are blocks of part of the left bundle, either the posterior or anterior division:

Figure 38: Divisions of the bundles.

Anterior fascicular block - the most common.

You will see left axis deviation (-30 to -90) and a small Q wave in lead I and an S in lead III (Q1S3). The QRS will be slightly prolonged (0.1 - 0.12 sec).

Figure 39: Anterior fascicular block.

Posterior fascicular block - less common.

You will see right axis deviation, an S in lead I and an Q in lead III (S1Q3). The QRS will be slightly prolonged (0.1 - 0.12 sec).

Figure 40: Posterior fascicular block.

Bifascicular block.

This means two (2) of the three (3) fascicles (in diagram) are blocked. The most important example is a right bundle branch block and a left anterior fascicular block. Watch out for this. Only one fascicle is left for conduction, and if that fasicle is intermittently blocked, the dangerous Mobitz 2 is set up!

Figure 41: Right bundle branch block and left anterior fascicular block.

"fascicular Blocks" may seem a bit complicated - simply remember that axis deviation is the clue. In your differential, consider posterior fascicular blocks with right axis deviation and consider anterior fascicular blocks with left axis deviation. Fascicular blocks cause axis deviations, like infarcts and hypertrophy. If you see a left or right axis deviation, first look for infarct or hypertrophy. If neither are present, the remaining diagnosis of fascicular block is usually correct. Review differential diagnosis of right and left axis deviation.

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