Incomplete BBB
Sometimes either the right or the left bundle branch conducts the electrical impulse more slowly than normal, but is not completely blocked. When this occurs, on the side of the slow conduction, the electrical impulse arrives in the ventricle slightly later than normal. As a result, the QRS complex is slightly wider than normal, but not as wide as it would be with a complete BBB. This slight widening of the QRS is often called incomplete BBB. (Another name for it is an “intraventricular conduction defect," or an IVCD.)Incomplete bundle branch block sometimes indicates underlying heart disease. But, especially when it occurs on the right side (i.e., incomplete RBBB,) it often has no significance at all. So in general, incomplete bundle branch block should trigger a non-invasive search for underlying heart disease. If none is found, no further tests or treatment are needed.
The Hemiblocks
The left bundle branch itself divides into two major branches – the anterior and the posterior fascicles. Sometimes block can occur in just one of these fascicles. Such blockage is referred to as a hemiblock, or a fascicular block. (Note that the right bundle branch does not divide into discrete fascicles.)When blockage is in the anterior fascicle, left anterior hemiblock (or left anterior fascicular block) is said to be present; whereas if the blockage occurs in the posterior fascicle, then left posterior hemiblock (or left posterior fascicular block) is said to be present.
Hemiblock generally does not widen the QRS complex, but instead changes the pattern of the QRS complex. (It alters the “axis” of the QRS complex. The axis is calculated by measuring the pattern of the QRS complex on several of the 12 leads acquired on a standard ECG.)
In general, finding a hemiblock should trigger a search for underlying heart disease. Often none is found, in which case the hemiblock can be ignored.
Treating BBB
As we have noted, the heart depends on the bundle branches. Without them, the electrical impulse is not delivered to the ventricles. Block in both bundle branches, therefore (a condition called complete heart block,) can be fatal.Fortunately, it is quite uncommon for stable RBBB or LBBB to progress to complete heart block. Thus, despite the fact that BBB is a common finding on routine ECGs, it is an uncommon reason for implanting pacemakers.
Still, there are a few conditions in which people with bundle branch block require pacemakers:
1) When “conduction system disease” is bilateral, and is associated with a heart attack:We have previously noted that RBBB plus left anterior hemiblock should not be particularly alarming. This is true except in this one instance – where the block occurs acutely with a heart attack. Here, the conduction system disease tends to be unstable, and can progress to complete heart block. These patients often need pacemakers.
2) When bundle branch block is associated with syncope (loss of consciousness):When a patient with bundle branch block experiences syncope, in general an electrophysiology study should be considered to test for impending complete heart block. A permanent pacemaker eliminates the problem.
3) In certain patients with dilated cardiomyopathy:In patients who have dilated cardiomyopathy and either complete or incomplete bundle branch block, a new form of pacing – called cardiac resynchronization pacing or CRT – has now been shown to improve symptoms and to prolong life. CRT should now be strongly considered in any patient with heart failure and bundle branch block. Click here to read more about CRT.
As we have seen, bundle branch block causes the ventricles to beat sequentially (one after another) instead of simultaneously. This discoordination of the normal pattern of ventricular contraction diminishes the efficiency of the heart beat. In a person with a normal heart, the loss of efficiency is inconsequential. But in a person with dilated cardiomyopathy this loss of efficiency can be critical, and can contribute greatly to symptoms of heart failure. Resynchronization pacing restores much of this efficiency by pacing both the right and left ventricles simultaneously. In patients who have heart failure from dilated cardiomyopathy and bundle branch block, resynchronization pacing has become an important component to therapy.
