Effect of Hemiblock on Sequence of Activation Left Anterior Hemiblock

As you know from Chapter 5, any change in the sequence of activation of the ventricles will change the axis or direction of vector 3, which is our main vector of ventricular depolarization. To illustrate the impact of hemiblocks, let's begin by considering LAH.

Effect of Hemiblock on Sequence of Activation 35

Figure 6.1. Schematic section through the left ventricle as viewed from the patient's left (posterior axillary view) showing how the anterior and posterior fascicles of the left bundle run toward their respective papillary muscles.

Effect of Hemiblock on Sequence of Activation 35

In LAH, the impulse comes down the main left bundle and the left posterior fascicle quite normally (Figure 6.2). However, the impulse finds the left anterior fascicle blocked. As a result, the posteroinferior section of the ventricle is activated first. A wave of depolarization then spreads by slow conduction from muscle fiber to muscle fiber into the portion of the ventricle that is normally served by the anterosuperior fascicle. The net result is a vector spreading superiorly through the wall of the left ventricle and shifting the axis of main ventricular depolarization counterclockwise to the left. This axis shift is so pronounced that LAH usually produces marked LAD, frequently approaching -60 degrees (Figure 6.3). Exactly how extreme the LAD is depends upon what the patient's normal axis was before developing the hemiblock.

Although vector 3 is dramatically shifted to the left, it should be readily apparent that septal depolarization is still occurring normally because the posterior fascicle is still intact, and it is the posterior fascicle that predominantly supplies the septum with Purkinje fibers. Therefore, as you will note in Figure 6.3, although lead III is now predominantly negative, there is still a small R wave present,reflecting normal septal depolarization. Note that there is still a small Q present in lead I, also, of course, reflecting normal septal depolarization.

Criteria for LAH

The diagnostic criteria, then, for isolated LAH include the following:

1. Marked LAD, frequently approaching -60 degrees

2. A small R in lead III

Figure 6.2. Left anterior hemiblock. Schematic frontal section through the anterior wall of the left ventricle, just anterior to the anterior papillary muscle. The right ventricle is not seen because it lies anterior to the left ventricle and is cut away. In this illustration, the anterior fascicle of the left bundle is blocked. As a result, depolarization is initiated by the Purkinje system of the posterior fascicle behind the posterior papillary muscle. A vector then spreads to the patient's left and superiorly into the region served by the anterosuperior fascicle, as illustrated by the arrow, producing LAD. Note that the septum will still be depolarized normally from left to right by the intact posterior fascicle.

Figure 6.3. Left anterior hemiblock. Marked LAD of -60 degrees is present, as quickly confirmed by the equally biphasic QRS in lead aVR. In addition, there is a small R in lead III, and a small Q in lead I. The QRS duration is normal,

4. Normal QRS duration

Why normal QRS duration? Although LAH certainly falls within the technical category of a delay in intraventricular conduction because it can increase QRS duration by 0.01 to 0.02 s, note in Figure 6.3 that the QRS duration is still normal. This is because, although activation of the left superior ventricle is somewhat delayed, it is a small enough area with a short enough transit time required for the wave of muscle-to-muscle depolarization that no significant prolongation of the QRS to 0.10 s or greater results.

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