Bundle Branch Blocks

Look for signs of bundle branch block (BBB) in V1 and V6.

In the absence of BBB, passage of the depolarizing impulse down the His bundle and bundle branches is rapid and activation of the right and left ventricles is simultaneous and synchronous. The individual QRS complexes of the right and the left ventricles superimpose on each other and produce a composite QRS complex that is narrow in width (< 120 ms).
In BBB, irrespective of whether it is right or left, activation of the ventricles becomes asynchronous: Depolarization of the ventricle on the blocked side is delayed. This delay causes the individual QRS complex of the blocked ventricle to be wider than normal and appear after the individual QRS complex of the not-blocked ventricle. As a result, the composite QRS complex is > 120 ms wide and has RSRíŽ waves: the R wave belongs to the individual QRS of the not-blocked ventricle and the RíŽ wave to the individual QRS of the blocked ventricle.

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The S wave between the R and RíŽ waves may be deep and falls below the baseline; it may not be so deep and causes a notch between the R and RíŽ waves only; or it may be hardly visible such that the QRS complex is simply a tall and wide R wave.

 

In right bundle branch block (RBBB), the widened RRíŽ complex is seen most typically in V1. Secondary T wave inversion can also be seen in this lead because when depolarization is abnormal repolarization can also be expected to be abnormal. Since the late right ventricular depolarization current moves away from left chest leads, it shows up as wide S waves in V6. When there is an RRíŽ complex in V1 but it is less than 120 ms wide, the condition is called incomplete right bundle branch block.
In left bundle branch block (LBBB) the widened RRíŽ complex is seen most typically in V6. Secondary T wave inversion is present in the left but not right chest leads. Since the late left ventricular depolarization current moves away from the right chest leads, it shows up as deep and broad S waves in V1.

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Bundle branch block or ventricular rhythm?

The QRS complexes are wider than normal in both BBB and ventricular rhythm. But the rhythm in BBB is supraventricular in origin. There is a one-to-one P wave to QRS relationship in BBB:

In sinus rhythm with 3rd degree heart block, there are regular P waves that are totally asynchronous with the QRS complexes, which represent escape rhythm from a ventricular focus.

In ventricular rhythm with sinus arrest, only wide QRS complexes are seen and P waves are absent.

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