Bundle Branch Block
In a normal heart beat, the impulse travels from the SA node, through the atria, the AV node (where AV blocks occur as described above) and then through the Bundle of His, the bundle branches and the Purkinje fibers to the ventricles. There are two main bundle branches, the right and left, with a further subdivision of the left bundle branch into two minor branches. The determination that a bundle branch block (often referred to as BBB) exists is made by reviewing a patient’s EKG. In the presence of a bundle branch block, the QRS is wider than its normal value of .04-.10 seconds and will typically be .12-.16 seconds. In bundle branch block, only one of the ventricles is directly caused to contract by the impulse from the atria. The other ventricle is actually caused to contract by the impulse traveling through the ventricles heart tissue itself. Since this results in one ventricle contracting before the other, the QRS width is increased. Both right and left bundle branch blocks can occur (with additional classifications due to the fact that there are two minor branches of the left bundle). These are referred to as right and left bundle branch blocks or RBBB and LBBB.
In the example above, the rate is 78 beats per minute, the PR interval is .18 seconds(normal) and the QRS width is .14 seconds (abnormal). The significance of a bundle branch block varies greatly. Some people can be born with them, while in others, they can develop slowly as people get older. When a complete bundle branch block does not exist, but some degradation has occured, the QRS may vary from .10 to .11 seconds. This is often referred to as an IVCD (intraventricular conduction defect). In other people, the BBB may be a new finding which suggests that some underlying heart disease may exist or the patient may have had some type of recent cardiac event.
In many cases of bundle branch block no treatment is required. In other cases, the bundle branch block may indicate the existence of some underlying heart disease and that underlying heart disease may need to be treated.