Complete RBBB

Now let's look at some specifics. You will note in Figure 7.3 that RBBB reverses our normal pattern of a predominantly negative QRS in lead V1. Instead, we see an upright QRS with an RSR'. Now let's see if we can figure out what produces this pattern. In RBBB, the impulse goes down the left bundle quite normally and activates the septum and then the left ventricle. Because the septum is depolarized normally, we can expect our initial QRS deflection to be quite normal and to produce an initial R wave in lead V1 and a normal initial Q wave in leads V5 and V6.

Next, we see the beginning of an S wave in lead V1 and an R wave in lead V6. This reflects normal depolarization of the left ventricle. However, at this point (which is actually quite late in the QRS), our wave of depolarization in the left ventricle begins to slowly travel across muscle into the right ventricle and, therefore, toward lead V1. This produces a second positive deflection in lead V1 (the R') and a late S wave in leads V5 and V6. Note that the slowness of this process is reflected by the fact that the R' in lead V1 and the S wave in leads V5 and V6 are quite wide and account for the increase in our QRS duration to 0.12 s or greater.

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Figure 7.3. Right bundle branch block showing the characteristic RSR' pattern in the right precordial leads (V1 and V2), and a prominent wide S wave in the lateral precordial leads (V5 and V6).

Figure 7.3. Right bundle branch block showing the characteristic RSR' pattern in the right precordial leads (V1 and V2), and a prominent wide S wave in the lateral precordial leads (V5 and V6).

Complete LBBB 45

Figure 7.4. LBBB showing a deep QS in the right precordial leads (V, and V2) and the typical RSR' in the left precordial leads.

We can summarize the pattern of RBBB, then, by saying that it produces an RSR' (an upright M-shaped pattern) on the right side in lead V1 and a prominent, wide S on the left side in leads V5 and V6. You will see that LBBB produces a very similar pattern, but in reverse.

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