In Figure 5, the top image shows a normal QRS complex. The middle figure shows RBBB, and the bottom figure shows LBBB. Note that with both kinds of bundle branch block, the QRS is wide and misshapen. The characteristic shapes of the QRS complex allow doctors to determine whether the right or the left bundle branch is blocked.
Why does the QRS complex change in BBB? The QRS represents the spread of the heart’s electrical impulse across the right and left ventricles. Because with BBB the ventricles receive the electrical impulse one after another instead of at the same time, it takes longer to form the QRS complex on the ECG. The QRS complex is said to “widen.” Also, since the pattern of the spreading of the electrical impulse is abnormal in BBB, the pattern of the QRS complex is also abnormal.
What is the significance of bundle branch block?
BBB is quite common, and occurs in a variety of medical conditions. RBBB occurs in medical conditions that affect the right side of the heart or the lungs, so a finding of RBBB on the ECG ought to trigger a screening exam for such conditions. These include blood clots to the lung (pulmonary embolus), chronic lung disease, cardiomyopathy, and atrial and ventricular septal defects. However, RBBB also commonly occurs in normal, healthy individuals, and the screening exam therefore often turns up no medical problems. In these cases, the RBBB has no apparent medical significance, and can be written off as a “normal variant,” and safely ignored.In contrast, LBBB usually indicates underlying cardiac pathology. It is seen in dilated cardiomyopathy, hypertrophic cardiomyopathy, hypertension, aortic valve disease, coronary artery disease, and a variety of other cardiac conditions. While occasionally LBBB occurs in apparently healthy people, its appearance should trigger a thorough search (as opposed to a simple screening) for underlying cardiac problems.
If both the right and left bundle branches are completely blocked (i.e., "bilateral" BBB), the electrical impulse cannot reach the ventricles. In this case, unless spontaneous electrical impulses arise within the ventricles (a so-called “escape rhythm”), the ventricles stop beating and death occurs. Fortunately, this form of “complete heart block” is relatively uncommon.
In summary, the major significance of BBB is that it may indicate the presence of previously unknown underlying cardiovascular disease. When BBB is found, therefore, a search for such underlying disease ought to be carried out. Since RBBB often occurs in normal individuals while LBBB usually indicates underlying disease, the search for underlying disease generally should be more aggressive with LBBB than with RBBB.
