Bundle branch blocks
Recall that in the normal process of ventricular activation the electrical stimulus reaches the ventricles from the atria by way of the atrioventricular (AV) junction (see Chapter 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 is completed within about 0.1 second (100 msec). This is the reason the normal width of the QRS complex is less than or equal to 100 ms[*] (two and a half small boxes on the ECG graph paper). Any process that interferes with the normal stimulation of the ventricles may prolong the QRS width. This chapter focuses on the effects that blocks within the bundle branch system have on the QRS complex.
* Some references give the upper normal limits of QRS duration as 110 ms, especially when using computer-derived measurements based on multiple leads. 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.
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FIGURE 7-1 Step-by-step sequence of ventricular depolarization in right bundle branch block (see text). |
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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, since 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. ( Figure 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:
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Lead V1 shows an rSR' complex with a wide R'wave. |
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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 ).
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FIGURE 7-2 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 V3, II, III, and aVF, all of which show rSR'-type complexes. The abnormal ST-T changes in leads V4 and V5 are primary, however, because they are present in leads without an R' wave. AV, atrioventricular; LBB, left bundle branch; RBB, right bundle branch. |
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FIGURE 7-3 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. |
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Figures 7-2 and 7-3 [2] [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. The T wave inversions or ST depressions in other leads (V4 and V5), however, represent a primary change, perhaps resulting from ischemia or a drug effect.
COMPLETE AND INCOMPLETE RBBB
RBBB can be subdivided into complete and incomplete forms, depending on the width of the QRS complex. Complete RBBB is defined by a QRS that is 0.12 second or more in duration with an rSR' in lead V1 and a qRS in lead V6. Incomplete RBBB shows the same QRS patterns, but its duration is between 0.1 and 0.12 second.
CLINICAL SIGNIFICANCE
RBBB may be caused by a number of factors. First, some normal people have this finding without any underlying heart disorder. Therefore RBBB itself is not necessarily abnormal. In many people, however, RBBB is associated with organic heart disease. It may occur with virtually any condition that affects the right side of the heart, including atrial septal defect with left-to-right shunting of blood, chronic pulmonary disease with pulmonary artery hypertension, and valvular lesions such as pulmonic stenosis. In some people (particularly older individuals), RBBB is related to chronic degenerative changes in the conduction system. It may also occur with acute or chronic coronary artery disease.
Pulmonary embolism, which produces acute right-sided heart overload, may cause a right ventricular conduction delay. When RBBB occurs after coronary artery bypass graft surgery, it does not seem to have any special clinical implications.
RBBB may be permanent or transient. Sometimes it appears only when the heart rate exceeds a certain critical value (rate-related RBBB).
By itself, RBBB does not require any specific treatment. In patients with acute anterior wall infarction, however, a new RBBB may indicate an increased risk of complete heart block (see Chapter 17 ), particularly when the RBBB is associated with left anterior or posterior hemiblock.
LEFT BUNDLE BRANCH BLOCK
Left bundle branch block (LBBB) also produces a pattern with a widened QRS complex. The QRS complex with LBBB is very different, however, from that with RBBB. The major reason for this difference is that RBBB affects mainly the terminal phase of ventricular activation, whereas LBBB also affects the early phase.
Recall that the first phase of ventricular stimulation—depolarization of the left side of the septum—is started by a branch of the left bundle. LBBB therefore blocks this normal pattern. When LBBB is present, the septum depolarizes from right to left and not from left to right. Thus the first major ECG change produced by LBBB is a loss of the normal septal r wave in lead V1 and the normal septal q wave in lead V6 ( Fig. 7-4A ). Furthermore, the total time for left ventricular depolarization is prolonged with LBBB. As a result, the QRS complex is abnormally wide. Lead V6 shows a wide, entirely positive (R) wave ( Fig. 7-4B ). The right chest leads (e.g., V1) record a negative QRS (QS) complex because the left ventricle is still electrically predominant with LBBB and therefore produces greater voltages than the right ventricle.
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FIGURE 7-4 The sequence of ventricular depolarization in left bundle branch block produces a wide QS complex in lead V1 and a wide R wave in lead V6. LV, left ventricle; RV, right ventricle. |
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Thus with LBBB the entire process of ventricular stimulation is oriented toward the left chest leads; that is, the septum depolarizes from right to left, and stimulation of the electrically predominant left ventricle is prolonged. Figure 7-4 illustrates the sequence of ventricular activation in LBBB.[*]
With LBBB, the QS wave in lead V1 sometimes shows a small notching at its point, giving the wave a characteristic W shape. Similarly, the R wave in lead V6 may show a notching at its peak, giving it a distinctive M shape. (An example of an LBBB pattern is presented in Figure 7-5 .)
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FIGURE 7-5 Notice the characteristic wide QS complex in lead V1 and the wide R wave in lead V6 with slight notching at the peak. The inverted T waves in leads V5 and V6 (secondary T wave inversions) are also characteristic of left bundle branch block. AV, atrioventricular; LBB, left bundle branch; RBB, right bundle branch. |
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Just as secondary T wave inversions occur with RBBB, they also occur with LBBB. As Figure 7-5 shows, the T waves in the leads with tall R waves (e.g., the left precordial leads) are inverted; this is characteristic of LBBB. T wave inversions in the right precordial leads cannot, however, be explained solely on the basis of LBBB. If present, these T wave inversions reflect some primary abnormality such as ischemia (see Fig. 8-21 ).
In summary, the diagnosis of complete LBBB pattern can be made simply by inspection of leads V1 and V6:
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Lead V1 usually shows a wide, entirely negative QS complex (rarely, a wide rS complex). |
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Lead V6 shows a tall wide R wave without a q wave. |
You should have no problem differentiating LBBB and RBBB patterns ( Fig. 7-6 ). Occasionally, an ECG shows wide QRS complexes that are not typical of an RBBB or LBBB pattern. In such cases, the general term intraventricular delay is used ( Fig. 7-7 ).
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FIGURE 7-6 Comparison of leads V1 and V6, with normal conduction, right bundle branch block (RBBB), and left bundle branch block (LBBB). Normally, lead V1 shows an rS complex and lead V6 shows a qR complex. With RBBB, lead V1 shows a wider rSR' complex and lead V6 shows a qRS complex. With LBBB, lead V1 shows a wide QS complex and lead V6 shows a wide R wave. |
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FIGURE 7-7 In nonspecific intraventricular conduction delay, the QRS complex is abnormally wide (0.11 sec). Such a pattern is not typical of left or right bundle branch block, however. In this patient, the pattern was caused by an anterolateral wall myocardial infarction (see Chapter 8 ). |
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* A variation of this pattern sometimes occurs: Lead V1 may show an rS complex with a very small r wave and a wide S wave. This superficially suggests that the septum is being stimulated normally from left to right. Lead V6, however, shows an abnormally wide and notched R wave without an initial q wave.
COMPLETE AND INCOMPLETE LBBB
LBBB, like RBBB, has complete and incomplete forms. With complete LBBB, the QRS complex has the characteristic appearance described previously and is 0.12 second or wider. With incomplete LBBB, the QRS is between 0.1 and 0.12 second wide.
CLINICAL SIGNIFICANCE
Unlike RBBB, which is occasionally seen without evident cardiac disease, LBBB is usually a sign of organic heart disease. LBBB may develop in patients with long-standing hypertensive heart disease, a valvular lesion (e.g., calcification of the mitral annulus, aortic stenosis, or aortic regurgitation), or different types of cardiomyopathy (see Chapter 11 ). It is also seen in patients with coronary artery disease and often correlates with impaired left ventricular function. Most patients with LBBB have underlying left ventricular hypertrophy (see Chapter 6 ). Degenerative changes in the conduction system may lead to LBBB, particularly in the elderly. Often, more than one contributing factor may be identified (e.g., hypertension and coronary artery disease). Rarely, otherwise normal individuals have an LBBB pattern without evidence of organic heart disease by examination, echocardiogram, or even invasive studies.
LBBB, like RBBB, may be permanent or transient. It may also appear only when the heart rate exceeds a certain critical value (rate- or acceleration-dependent LBBB).[†]
Important clinical consideration: LBBB may be the first clue to four previously undiagnosed but clinically important abnormalities. These are advanced coronary artery disease, valvular heart disease, hypertensive heart disease, and cardiomyopathy.
† Less commonly, LBBB occurs only when the heart decelerates below some critical value.
PACEMAKER PATTERNS
Pacemakers are battery-operated devices designed to stimulate the heart electrically. A pacemaker is used primarily when a patient's own heart rate is not adequate (e.g., in complete heart block or sick sinus syndrome). In most cases, the pacemaker electrode is inserted into the right ventricle. Therefore the ECG shows an LBBB pattern, which reflects delayed activation of the left ventricle.
Figure 7-8 is an example of a pacemaker tracing. The vertical spike preceding each QRS complex is the pacemaker spike. This spike is followed by a wide QRS complex with an LBBB morphology (QS in lead V1 with a wide R wave in lead V6). (Pacemakers are discussed in greater detail in Chapter 21 .)
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FIGURE 7-8 A pacemaker inserted in the right ventricle generally produces a pattern resembling that of left bundle branch block, with a wide QS in lead V1 and a wide R wave in lead V6. This pattern is caused by delayed depolarization of the left ventricle. Notice the pacemaker spike in each lead preceding the QRS complex. In some leads (e.g., II), the spike (S) is positive; in others (V1 to V6), it is negative. |
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FASCICULAR BLOCKS (HEMIBLOCKS)
Fascicular blocks, or hemiblocks, are a slightly more complex but important topic. To this point, the chapter has described the left bundle branch system as if it were a single pathway. Actually, this system has been known for many years to be subdivided into an anterior fascicle and a posterior fascicle (from the Latin fasciculus, meaning “small bundle”). The right bundle branch, by contrast, is a single pathway and consists of just one main fascicle or bundle. This revised concept of the bundle branch system as a trifascicular highway (one right lane and two left lanes) is illustrated in Figure 7-9 .
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FIGURE 7-9 With a trifascicular conduction system, notice that the left bundle branch subdivides into a left anterior fascicle and a left posterior fascicle. This diagram is a more detailed version of the original depiction of the conduction system (see Fig. 1-1 ). AV, atrioventricular; SA, sinoatrial. |
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In summary, the bundle of His divides into a right bundle branch and a left main bundle branch. The left main bundle branch then subdivides into an anterior and a posterior fascicle.
It makes sense to suppose that a block can occur at any single point (or at multiple points) in this trifascicular system. The ECG pattern with RBBB has already been presented (see Figs. 7-2 and 7-3 [2] [3]). The pattern of LBBB can occur in one of two ways: by a block in the left main bundle before it divides or by blocks in both subdivisions (anterior and posterior fascicles).
What happens if a block occurs in just the anterior or just the posterior fascicle of the left bundle? A block in either fascicle of the left bundle branch system is called a hemiblock or fascicular block.Recognition of fascicular blocks on the ECG is intimately related to the subject of axis deviation (see Chapter 5 ). Somewhat surprisingly, a fascicular block (unlike a full LBBB or RBBB) does not markedly widen the QRS complex. Experiments have shown that the main effect of cutting these fascicles is a change in the QRS axis. Specifically, left anterior fascicular block results in marked left axis deviation (-45° or more); left posterior fascicular block produces marked right axis deviation (+120° or more).[*]
In summary, fascicular blocks are partial blocks in the left bundle branch system and involve either the anterior or posterior fascicle. The diagnosis of a fascicular block is made from the mean QRS axis in the limb (frontal plane) leads. This is in contrast to the diagnosis of complete (or incomplete) RBBB or LBBB, which is made primarily from the QRS patterns in the chest (horizontal plane) leads. Complete bundle branch blocks, unlike fascicular blocks, do not cause a characteristic shift in the mean QRS axis.
* Left anterior fascicular block shifts the QRS axis to the left by delaying activation of the more superior and leftward portions of the left ventricle. Left posterior fascicular block shifts it to the right by delaying activation of the more inferior and rightward portions of the left ventricle. In both cases, the QRS axis therefore is shifted toward the direction of delayed activation.
LEFT ANTERIOR FASCICULAR BLOCK
Isolated left anterior fascicular block (LAFB) is diagnosed by finding a mean QRS axis of -45° or more negative and a QRS width of less than 0.12 second. A mean QRS axis of -45° or more negative can be easily recognized because the S wave in lead aVF equals or exceeds the R wave in lead I[*] ( Fig. 7-10 ). Lead aVL usually shows a qR complex, with rS complexes in leads II, III, and aVF (or QS waves if an inferior myocardial infarction is also present).
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FIGURE 7-10 Left anterior fascicular block (hemiblock). Notice the marked left axis deviation without significant widening of the QRS duration. Compare this with Figure 9-8 B, which shows left posterior fascicular (hemiblock). |
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* Some authors use an axis of -30° or more negative. The original description of this pattern used a cutoff of -45° or more negative.
LEFT POSTERIOR FASCICULAR BLOCK
Isolated left fascicular block (LPFB) (see Fig. 9-8B ) is diagnosed by finding a mean QRS axis of +120° or more, with a QRS width of less than 0.12 second. Usually, an rS complex is seen in lead I, and a qR complex is seen in leads II, III, and aVF. The diagnosis of left posterior fascicular block can be considered, however, only after other, more common causes of right axis deviation (RAD) have been excluded (see Chapter 24 ). These causes can include right ventricular hypertrophy (RVH), normal variant, emphysema, lateral wall infarction (see Fig. 8-11 ), and acute pulmonary embolism (or other causes of acute right ventricular overload).
Although left anterior fascicular block is relatively common, isolated left posterior fascicular block is quite rare. Most often it occurs with RBBB, as shown in Figure 17-8 . In general, the finding of isolated left anterior fascicular block is not of much clinical significance. Bifascicular and trifascicular blocks are considered further in the discussion of complete heart block in Chapter 17 .
ECG PATTERNS IN VENTRICULAR CONDUCTION DISTURBANCES: GENERAL PRINCIPLES
A unifying theme to predicting what the ECG will show with a bundle branch block or hemiblock 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:
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RBBB: late QRS forces point toward the right ventricle (positive in V1 and negative in V6). |
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LBBB: late QRS forces point toward the left ventricle (negative in V1 and positive in V6). |
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LAFB: late QRS forces point in a leftward and superior direction (negative in II and positive in I and aVL). |
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LPFB: late QRS forces point in a rightward and inferior direction (negative in I and positive in II and III). |
DIAGNOSIS OF HYPERTROPHY IN THE PRESENCE OF BUNDLE BRANCH BLOCKS
The ECG diagnosis of hypertrophy (see Chapter 6 ) in the presence of bundle branch blocks may pose special problems. A few general guidelines are helpful.
When RVH occurs with RBBB, RAD is often present. A tall peaked P wave with RBBB should also suggest underlying RVH.
The usual voltage criteria for left ventricular hypertrophy (LVH) can be used in the presence of RBBB. Unfortunately, RBBB often masks these typical voltage increases. The presence of LAA with RBBB suggests underlying LVH.
The finding of LBBB, regardless of the QRS voltage, is highly suggestive of underlying LVH. Finding LBBB with prominent QRS voltages and evidence of left atrial abnormality virtually ensures the diagnosis of LVH (see Chapter 6 ).
Finally, it should be emphasized that the echocardiogram is much more accurate than the ECG in the diagnosis of cardiac enlargement (see Chapter 6 ).
DIAGNOSIS OF MYOCARDIAL INFARCTION IN THE PRESENCE OF BUNDLE BRANCH BLOCKS
The ECG diagnosis of myocardial infarction in the presence of bundle branch blocks is discussed in Chapter 8 .
REVIEW
Right bundle branch block (RBBB) shows the following characteristic patterns: an rSR' with a prominent wide final R' wave in lead V1, a qRS with a wide final S wave in lead V6, and a QRS width of 0.12 second (three small time boxes) or more. Incomplete RBBB shows the same chest lead patterns, but the QRS width is between 0.1 and 0.12 second.
Left bundle branch block (LBBB) shows the following characteristic patterns: a deep wide QS (occasionally an rS with a wide S wave) in lead V1, a prominent (often notched) R wave without a preceding q wave in lead V6, and a QRS width of 0.12 second or more. Incomplete LBBB shows the same chest lead patterns as LBBB, but the QRS width is between 0.1 and 0.12 second.
Pacemaker patterns produced by an electrode in the right ventricle generally resemble LBBB but have a pacemaker spike before each QRS complex.
Fascicular blocks (hemiblocks) can occur because the left bundle splits into two subdivisions (fascicles): the left anterior fascicle and the left posterior fascicle. Conduction through either or both of these bundles can be blocked.
Left anterior fascicular block or hemiblock is characterized by a mean QRS axis of -45° or more negative. (When the mean QRS axis is -45°, left axis deviation is present and the height of the R wave in lead I (RI) is equal to the depth of the S wave in lead aVF (SaVF). When the mean QRS axis is more negative than -45°, SaVF becomes larger than RI.)
Left posterior fascicular block or hemiblock is characterized by marked right axis deviation (RAD). Before the diagnosis of left posterior hemiblock is made, however, other more common causes of RAD must be excluded, including lead reversal (left/right arm electrodes), normal variants, right ventricular overload syndromes (including chronic lung disease), and lateral wall infarction (see Chapter 24 ).
QUESTIONS
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Draw the shape of the QRS complexes in leads V1 and V6 that would be expected with right and left bundle branch blocks. |
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Examine the chest leads shown below and then answer these questions:
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Examine the following 12-lead ECG and lead II rhythm strip carefully. Can you identify the major conduction abnormality?
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Define the terms primary and secondary T wave abnormality. |
True or false (Questions 5 to 8):
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Left anterior hemiblock does not markedly widen the QRS complex. |
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Left bundle branch block is generally seen in patients with organic heart disease. |
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Bundle branch blocks may occur transiently. |
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An electronic pacemaker stimulating the left ventricle will produce a left branch block pattern. |
