Chapter 7 Ventricular Conduction Disturbances Bundle Branch Blocks and Related Abnormalities

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Recall that in the normal process of ventricular activation the electrical stimulus reaches the ventricles from the atria by way of the atrioventricular (AV) node and His-Purkinje system (see Chapters 1 and 4). The first part of the ventricles to be stimulated (depolarized) is the left side of the ventricular septum. Soon after, the depolarization spreads to the main mass of the left and right ventricles by way of the left and right bundle branches. Normally the entire process of ventricular depolarization in adults is completed within about 0.1 sec (100 msec). This is the reason the normal width of the QRS complex (measured by computer) from all 12 leads is less than or equal to 110 msec (about 2.5 small boxes on the ECG graph paper by eye). Any process that interferes with the physiologic, near simultaneous stimulation of the ventricles may prolong the QRS width or change the QRS axis. This chapter primarily focuses on a major topic: the effects that blocks or delays within the bundle branch system have on the QRS complex and ST-T waves.

ECG in Ventricular Conduction Disturbances: General Principles

A unifying principle in predicting what the ECG will show with a bundle branch or fascicular block is the following: The last (and usually dominant) component of the QRS vector will be shifted in the direction of the last part of the ventricles to be depolarized. In other words, the major QRS vector shifts toward the regions of the heart that are most delayed in being stimulated (Box 7-1).

BOX 7-1 QRS Vector Shifts in Bundle Branch and Fascicular Blocks

• Right bundle branch block (RBBB): late QRS forces point toward the right ventricle (positive in V₁ and negative in V₆).

• Left bundle branch block (LBBB): late QRS forces point toward the left ventricle (negative in V₁ and positive in V₆).

• Left anterior fascicular block (LAFB): late QRS forces point in a leftward and superior direction (negative in II and positive in I and aVL).

• Left posterior fascicular block (LPFB): late QRS forces point in an inferior and rightward direction (negative in I and positive in II and III).

Right Bundle Branch Block

Consider, first, the effect of cutting the right bundle branch, or markedly slowing conduction in this structure. Obviously, right ventricular stimulation will be delayed and the QRS complex will be widened. The shape of the QRS with a right bundle branch block (RBBB) can be predicted on the basis of some familiar principles.

Normally the first part of the ventricles to be depolarized is the interventricular septum (see Fig. 4-6A). The left side of the septum is stimulated first (by a branch of the left bundle). On the normal ECG, this septal depolarization produces a small septal r wave in lead V₁ and a small septal q wave in lead V₆ (Fig. 7-1A). Clearly, RBBB should not affect the septal phase of ventricular stimulation because the septum is stimulated by a part of the left bundle.

Figure 7-1 Step-by-step sequence of ventricular depolarization in right bundle branch block (see text).

The second phase of ventricular stimulation is the simultaneous depolarization of the left and right ventricles (see Fig. 4-6B). RBBB should not affect this phase either, because the left ventricle is normally electrically predominant, producing deep S waves in the right chest leads and tall R waves in the left chest leads (Fig. 7-1B). The change in the QRS complex produced by RBBB is a result of the delay in the total time needed for stimulation of the right ventricle. This means that after the left ventricle has completely depolarized, the right ventricle continues to depolarize.

This delayed right ventricular depolarization produces a third phase of ventricular stimulation. The electrical voltages in the third phase are directed to the right, reflecting the delayed depolarization and slow spread of the depolarization wave outward through the right ventricle. Therefore, a lead placed over the right side of the chest (e.g., lead V₁) records this phase of ventricular stimulation as a positive wide deflection (R′ wave). The rightward spread of the delayed and slow right ventricular depolarization voltages produces a wide negative (S wave) deflection in the left chest leads (e.g., lead V₆) (Fig. 7-1C).

Based on an understanding of this step-by-step process, the pattern seen in the chest leads with RBBB can be derived. With RBBB, lead V₁ typically shows an rSR′ complex with a broad R′ wave. Lead V₆ shows a qRS-type complex with a broad S wave. The tall wide R wave in the right chest leads and the deep terminal S wave in the left chest leads represent the same event viewed from opposite sides of the chest—the slow spread of delayed depolarization voltages through the right ventricle.

To make the initial diagnosis of RBBB, look at leads V₁ and V₆ in particular. The characteristic appearance of QRS complexes in these leads makes the diagnosis simple. (Fig. 7-1 shows how the delay in ventricular depolarization with RBBB produces the characteristic ECG patterns.)

In summary, the ventricular stimulation process in RBBB can be divided into three phases. The first two phases are normal septal and left ventricular depolarization. The third phase is delayed stimulation of the right ventricle. These three phases of ventricular stimulation with RBBB are represented on the ECG by the triphasic complexes seen in the chest leads:

• Lead V₁ shows an rSR′ complex with a wide R′ wave.

• Lead V₆ shows a qRS pattern with a wide S wave.

With an RBBB pattern the QRS complex in lead V₁ generally shows an rSR′ pattern (Fig. 7-2). Occasionally, however, the S wave never quite makes its way below the baseline. Consequently, the complex in lead V₁ has the appearance of a large notched R wave (Fig. 7-3).

Figure 7-2 Notice the wide rSR′ complex in lead V₁ and the qRS complex in lead V₆. Inverted T waves in the right precordial leads (in this case V₁ to V₃) are common with right bundle branch block and are called secondary T wave inversions. Note also the left atrial abnormality pattern (biphasic P in V₁ with prominent negative component) and prominent R waves in V₅, consistent with underlying left ventricular hypertrophy.

Figure 7-3 Instead of the classic rSR′ pattern with right bundle branch block, the right precordial leads sometimes show a wide notched R wave (seen here in leads V₁ to V₃). Notice the secondary T wave inversions in leads V₁ to V₂.

Figures 7-2 and 7-3 are typical examples of RBBB. Do you notice anything abnormal about the ST-T complexes in these tracings? If you look carefully, you can see that the T waves in the right chest leads are inverted. T wave inversions in the right chest leads are a characteristic finding with RBBB. These inversions are referred to as secondary changes because they reflect just the delay in ventricular stimulation. By contrast, primary T wave abnormalities reflect an actual change in repolarization, independent of any QRS change. Examples of primary T wave abnormalities include T wave inversions resulting from ischemia (see Chapters 8 and 9), hypokalemia and certain other electrolyte abnormalities (see Chapter 10), and drugs such as digitalis (see Chapter 10).

Some ECGs show both primary and secondary ST-T changes. In Figure 7-3 the T wave inversions in leads V₁ to V₃ and leads II, III, and aVF can be explained solely on the basis of the RBBB because the inversions occur in leads with an rSR′-type complex. However, the T wave inversions or ST segment depressions in other leads (V₄ and V₅) represent a primary change, perhaps resulting from ischemia or a drug effect.