The electrical conduction system of the heart has two branches for each of the two lower chambers of the heart – right bundle branch and the left bundle branch. They are branches of the bundle of His. Right bundle branch supplies the right ventricle (lower muscular chamber of the heart). and the left bundle branch supplies the left ventricle. Conduction delay in the right bundle branch can be either complete or incomplete. In complete right bundle branch block the QRS complex in the ECG (electrocardiogram) is wide (120 milliseconds or more), while in incomplete right bundle branch block, it is less than 120 milliseconds. Incomplete right bundle branch block (IRBBB)occurs in conditions in which the right ventricle is dilated (enlarged) as in atrial septal defect (defect in the wall separating two upper chambers of the heart). It can also occur in other causes of right ventricular enlargement as in pulmonary embolism (blocking of blood vessels to the lung due to clots carried in the bloodstream, usually from clots in the veins of the legs). Incomplete right bundle branch block can also occur without any other significant heart defect. In such cases there is no great significance for an IRBBB. Significance is mainly for the underlying heart problem than for the conduction delay itself.
IRBBB manifests as an rSr’ pattern in the precordial lead V1, with a QRS width less than 120 msec. A slurred S wave may be seen in lead I. rSr’ pattern in V1 can also be seen in Brugada syndrome (a life threatening disease due defects in cardiac ion channels). But in Brugada syndrome there is associated ST segment elevation, which is not seen with IRBBB.
IRBBB was noted in 3% of about 3.3 million ECGs of healthy outpatient children in a recent study . It was noted that patients with secundum atrial septal defect were not more likely to have IRBBB than general population.