Bifascicular and Trifascicular Block

Now that you have completed study of both the hemiblocks and the BBBs, it may have occurred to you that there would be situations in which there could be combinations of BBB and hemiblock. Indeed, this is the case.

If we consider that there are normally three major final or distal routes for conduction of an impulse to the ventricles (the right bundle branch plus the two left bundle fascicles), we can easily envision that there could be block present in two of the three, or in all three pathways (fascicles).

Bifascicular and Trifascicular Block 53

Figure 7.17. Right bundle branch block and LAH. Tracing displays RBBB with extreme left axis deviation (-55 degrees), a small Q in lead I, and a small R in lead III.

Figure 7.17 shows a tracing with not only RBBB but also with extreme LAD, a small Q in lead I, and a small R in lead III. This patient has both RBBB and LAH.

Figure 7.18 again shows a tracing with typical RBBB in the precordial leads, but, in this case, with extreme RAD, with a small R in lead I and a small

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Figure 7.18. Right bundle branch block and LPH. Tracing displays RBBB with extreme right axis deviation (-175 degrees), a small R in lead I, and a very small Q in lead III.

Figure 7.18. Right bundle branch block and LPH. Tracing displays RBBB with extreme right axis deviation (-175 degrees), a small R in lead I, and a very small Q in lead III.

Q in lead III. This represents the combination of RBBB and LPH. In actuality, this is the most common presentation of LPH. Isolated LPH without accompanying RBBB is very rare.

Thus, both tracings display a block of two of the three pathways, or fascicles, for impulse conduction to the ventricles, and both fall into the category of what some people call bifascicular block. Notice, however, that both tracings have a normal PR interval, which indicates that the impulse is getting down the remaining functioning fascicle on time. Left BBB alone is also considered to be an example of bifas-cicular block because both the anterior and posterior left fascicles are blocked.

As you have undoubtedly surmised by now, the term trifascicular block indicates either partial or complete block in all three major fascicles. Obviously, if all three fascicles are completely blocked, no impulse will reach the ventricles and, in such instances, complete AV block (third-degree block) will be present.

On the other hand, if two of the three fascicles are completely blocked, but the remaining fascicle is only partially blocked, or if there is a delay in the AV node, then we could expect to see any of our examples of bifascicular block and, in addition, a prolonged PR interval (first-degree block). Thus, if Figure 7J7 showed a PR interval of 0.20s or greater, in addition to RBBB and LAH, we would have an example of trifas-cicular block. Complete LBBB with a prolonged PR interval would also be an example of trifascicular block because there would be delay in the impulse coming down the remaining functioning right bundle branch.

In summary, then, trifascicular block is present whenever we see a bifas-cicular block in addition to first-degree or higher AV block, which is summarized as follows:

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