In Figure 7.4, you will see the pattern of LBBB, and will note that, indeed, it is the reverse (although not a mirror image) of RBBB, in the sense that it produces an upright "M" shaped pattern (this time on the left) in leads V5 and V6, and a deep, negative QS on the right in leads V1 and V2.
In LBBB, the impulse, of course, finds the left bundle blocked and goes down the right bundle (Figure 7.5). The septum, as usual, is the first part of
the ventricles to be activated, but because the left bundle is blocked, the septum is now activated by Purkinje fibers from the right bundle, producing a right-to-left vector across the septum. Thus, the first deflection of our QRS will be negative in lead V\ and positive in lead V6. Our normal initial R in lead Vj becomes a Q, and our normal initial Q in lead V6 becomes an R.
Next, the right ventricle is activated, but usually does not produce an R wave in Vj, possibly for two reasons. The first is that the vector of right ventricular depolarization traveling from endocardium to epicardium toward lead Vj is partially counterbalanced by the continuing right-to-left vector in the opposite direction through the septum with its greater muscle mass.
The second explanation is that upon normal activation of the right ventricle, a large slow vector immediately begins to spread into the larger mass of the left ventricle across muscle from right to left, also helping to counterbalance what would usually be our normal small R wave in lead Vj.
Nevertheless, it is worthwhile to note that, occasionally, the right ventricular forces spreading from endocardium to epicardium are still strong enough in LBBB to produce a tiny R wave in leads Vj and V2 (Figure 7.6).
Finally, the right ventricle finishes depolarizing before the slow wave of depolarization has finished spreading from right to left across the left ventricle. This leaves the vector spreading through the left ventricle unopposed and completes the wide, deep QS that we see in lead Vj in LBBB.
BBBs, ST Segments, and T Waves 47
While producing a deep QS in lead V1, these same right-to-left forces are producing a wide positive deflection in leads V5 and V6. Usually, the QRS in the left precordial leads (V5 and V6) is monophasic, as seen in Figure 7.7, meaning that it goes in only one direction (in this case upwardly without a negative deflection). However, sometimes the right ventricular forces will be strong enough to produce a small S in the middle of the QRS followed by an R', thus, again producing an M-shaped pattern on the left as in Figure 7.4.
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