Left Posterior Hemiblock

The story for LPH is essentially the reverse of LAH, as illustrated in Figure 6.4. The impulse travels down the anterior fascicle quite normally, but finds the posterior fascicle blocked. As a result, the anterosuperior wall of the

Left Posterior Hemiblock 37

Figure 6.4. Left posterior hemiblock. Schematic frontal section through the left ventricle illustrating sequences of depolarization in LPH. The impulse descends the left anterior fascicle normally, and then spreads by slow muscle-to-muscle conduction to the right and posteriorly toward the region of the ventricle normally served by the left posterior fascicle. The net result in the frontal plane is a left-to-right vector that produces RAD. Note that in posterior hemiblock the posterior fascicle can no longer depolarize the septum. Septal depolarization therefore takes place from right to left via Purkinje fibers from the right bundle branch (not depicted),

Figure 6.4. Left posterior hemiblock. Schematic frontal section through the left ventricle illustrating sequences of depolarization in LPH. The impulse descends the left anterior fascicle normally, and then spreads by slow muscle-to-muscle conduction to the right and posteriorly toward the region of the ventricle normally served by the left posterior fascicle. The net result in the frontal plane is a left-to-right vector that produces RAD. Note that in posterior hemiblock the posterior fascicle can no longer depolarize the septum. Septal depolarization therefore takes place from right to left via Purkinje fibers from the right bundle branch (not depicted),

Figure 6.5. Left posterior hemiblock. Marked right axis deviation of 170 degrees is present, along with a small R in lead I, and a small Q in lead III.

left ventricle is activated first, and then a slow muscle-to-muscle wave of depolarization spreads inferiorly and to the right in the direction of the posterior papillary muscle (Figure 6.4). The net result in this instance is prominent RAD with an electrical axis that may approach +120 degrees or more, again depending on the patient's normal axis before developing hemi-block. (Figure 6.5).

But this time, because the Purkinje fibers that supply the septum are a part of the posterior fascicle and are therefore blocked, septal depolarization no longer occurs from left to right. Instead, the septum is now depolarized by the right bundle, and the result is a right-to-left vector across the septum. Thus, with LPH, we now see in Figure 6.5 a small Q in lead III, and a small R in lead I- just the opposite of what we see with LAH. As before, QRS duration remains normal.

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