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Let’s again review how the normal sinus impulse reaches the ventricles. The SA node depolarization first conducts through the atria, the AV node, and the bundle of His. Next, the stimulus proceeds through the right bundle branch and the left bundle branch and its divisions. The left common bundle branch divides into two major fascicles called the left anterior-superior fascicle and left posterior-inferior fascicle, as well as a smaller mid-septal fascicle (Figure 8-1). Remember that this is a simplified description of the left bundle branch because the two major divisions and one minor branch are arranged more like a fan-shaped array rather than distinct, isolated fibers. The right and left bundle branches arborize widely, dividing into an extensive network of Purkinje fibers that interconnects with the ventricular myocytes. Transmission of the sinus impulse through the atria and the remainder of the specialized conduction system (AV node and His-Purkinje system) is recorded on the ECG as the PR interval. Electrical transmission through the His-Purkinje system is extremely rapid, with the majority of the duration of the PR interval representing conduction through the atria and AV node (Figure 8-2). The QRS complex records depolarization of the ventricular myocardium, a process that is normally completed within 0.10 seconds. Recall from Chapter 6 that we can simplify ventricular depolarization into three stages, each represented by a vector (Figure 8-3).

  1. The left side of the interventricular septum.

  2. The right and left ventricular free walls.

  3. The posterobasal portions of the left ventricle and interventricular septum.

Figure 8-1.

Conduction system of the heart (see text for details).

Figure 8-2.

ECG timing relationship of transmission through the specialized conduction system.

Figure 8-3.

Depolarization of the ventricular myocardium can be simplified into three vectors representing (1) the left side of the interventricular septum, (2) the right and left ventricular free walls, and (3) the posterobasal portions of the left ventricle and interventricular septum. Normal vector loops are shown in the frontal and horizontal plane. Numbered points on the ECG correspond to the vector orientations.

The first portion of the ventricle to be stimulated is the left side of the interventricular septum, which receives the electrical signal via a division of the left bundle branch (septal fascicle). This corresponds to vector 1, which is directed left-to-right, anterior, and (usually) superior. On the ECG it produces an initial Q wave in leftward-looking leads (I, aVL, V5, V6) and an initial R wave in rightward-looking leads (V1, V2, aVR).

The next portions of the heart to depolarize are the free walls of the right and left ventricles, which correspond to vector 2. Remember that ...

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