Incomplete BBB

Sometimes, the blocked bundle branch is not really totally blocked, but merely conducts the impulse more slowly than usual. This delays depolarization of the affected ventricle long enough for a wave of depolarization to begin spreading over muscle across from the unaffected side. In the meantime, the impulse also finally travels down the partially blocked bundle branch and reaches the Purkinje fibers on the affected side. The result is that part of the muscle on the affected side is activated by the conduction system and part is activated by the slow wave of depolarization spreading over muscle from the normal, or unaffected, side (Figure 7.2). This produces a QRS that is not quite as wide as in complete BBB, but is wider than normal. In other words, QRS duration in incomplete BBB is between 0.10 and 0.12s.

Figure 7.2. Incomplete RBBB. The impulse comes down the left bundle normally, initiating normal depolarization of the left ventricle (Event 1). Next, a slow vector of muscle-to-muscle depolarization begins to spread into the right ventricle (Event 2). Finally, the delayed impulse coming down the right bundle reaches the Purkinje fibers (Event 3). The net result is that the right ventricle is depolarized by both the slow vector spreading from the left ventricle and the delayed impulse coming down the right bundle.This produces a QRS that is intermediate in duration; i.e., between 0.10 and 0.12 s.

Figure 7.2. Incomplete RBBB. The impulse comes down the left bundle normally, initiating normal depolarization of the left ventricle (Event 1). Next, a slow vector of muscle-to-muscle depolarization begins to spread into the right ventricle (Event 2). Finally, the delayed impulse coming down the right bundle reaches the Purkinje fibers (Event 3). The net result is that the right ventricle is depolarized by both the slow vector spreading from the left ventricle and the delayed impulse coming down the right bundle.This produces a QRS that is intermediate in duration; i.e., between 0.10 and 0.12 s.

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