Hypertrophy of One Ventricle. When one ventricle greatly hypertrophies, the axis of the heart shifts toward the hypertrophied ventricle for two reasons. First, a far greater quantity of muscle exists on the hypertrophied

Figure 12-12

Left axis deviation in a hypertensive heart (hypertrophic left ventricle). Note the slightly prolonged QRS complex as well.

ii iii iii

Figure 12-13

High-voltage electrocardiogram in congenital pulmonary valve stenosis with right ventricular hypertrophy. Intense right axis deviation and a slightly prolonged QRS complex also are seen.

111 degrees to the right of the normal 59-degree mean ventricular QRS axis. The right axis deviation demonstrated in this figure was caused by hypertrophy of the right ventricle as a result of congenital pulmonary valve stenosis. Right axis deviation also can occur in other congenital heart conditions that cause hypertrophy of the right ventricle, such as tetralogy of Fallot and interventricular septal defect.

Bundle Branch Block Causes Axis Deviation. Ordinarily, the lateral walls of the two ventricles depolarize at almost the same instant because both the left and the right bundle branches of the Purkinje system transmit the cardiac impulse to the two ventricular walls at almost the same instant. As a result, the potentials generated by the two ventricles (on the two opposite sides of the heart) almost neutralize each other. But if only one of the major bundle branches is blocked, the cardiac impulse spreads through the normal ventricle long before it spreads through the other. Therefore, depolarization of the two ventricles does not occur even nearly simultaneously, and the depolarization potentials do not neutralize each other. As a result, axis deviation occurs as follows.

Vectorial Analysis of Left Axis Deviation in Left Bundle Branch Block. When the left bundle branch is

Figure 12-14

Left axis deviation caused by left bundle branch block. Note also the greatly prolonged QRS complex.

blocked, cardiac depolarization spreads through the right ventricle two to three times as rapidly as through the left ventricle. Consequently, much of the left ventricle remains polarized for as long as 0.1 second after the right ventricle has become totally depolarized. Thus, the right ventricle becomes electronegative, whereas the left ventricle remains electropositive during most of the depolarization process, and a strong vector projects from the right ventricle toward the left ventricle. In other words, there is intense left axis deviation of about -50 degrees because the positive end of the vector points toward the left ventricle. This is demonstrated in Figure 12-14, which shows typical left axis deviation resulting from left bundle branch block.

Because of slowness of impulse conduction when the Purkinje system is blocked, in addition to axis deviation, the duration of the QRS complex is greatly prolonged because of extreme slowness of depolarization in the affected side of the heart. One can see this by observing the excessive widths of the QRS waves in Figure 12-14. This is discussed in greater detail later in the chapter. This extremely prolonged QRS complex differentiates bundle branch block from axis deviation caused by hypertrophy.

Vectorial Analysis of Right Axis Deviation in Right Bundle Branch Block. When the right bundle branch is blocked, the left ventricle depolarizes far more rapidly than the right ventricle, so that the left side of the ventricles becomes electronegative as long as

0.1 second before the right. Therefore, a strong vector develops, with its negative end toward the left ventricle and its positive end toward the right ventricle. In other words, intense right axis deviation occurs. Right axis deviation caused by right bundle branch block is demonstrated, and its vector is analyzed, in Figure 12-15, which shows an axis of about 105 degrees instead of the normal 59 degrees and a prolonged QRS complex because of slow conduction.

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Essentials of Human Physiology

Essentials of Human Physiology

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