Bundle branch block

In this condition there is abnormal conduction through the right or left bundle branches and a consequent delay in the depolarisation of part of the ventricular muscle. When the heart is in sinus rhythm, ventricular conduction along a slower, abnormal pathway results in a QRS complex of greater than 0.12 sec but with a constant PR interval.

A block of both bundles is equivalent to third degree, or complete, heart block.

Thus three types of bundle branch block may occur:

• bundle branch block involving both branches
• a right bundle branch block
• a left bundle branch block

Right bundle branch block:

• principle indicator is "M" in V1 and V2.
• also, S wave: prominent in V6, slurred in I.

Left bundle branch block:

• principle indicator is "M" in V5 and V6.
• also, tall and very broad in I and aVL.