Bundle Branch Block

A BBB develops when there is either a functional or pathological block in one of the major branches of the intraventricular conduction system. As conduction through one bundle is blocked, the impulse travels along the unaffected bundle and activates one ventricle normally. The impulse is delayed in reaching the other ventricle because it travels outside of the normal conducting fibers. The right and left ventricles are thus depolarized sequentially instead of simultaneously. The abnormal activation produces a wide QRS complex, representing the increased time it takes for ventricular depolarization (Fig. 17-34). The broad QRS complex has two peaks (RSR′), indicating that depolarization of the two ventricles was not simultaneous.

An RBBB and LBBB are diagnosed on the 12-lead ECG but can also be identified on the bedside monitor using a V1 or MCL1 tracing and a V6 or MCL6 tracing. To identify the presence of a BBB, the QRS complex duration must be prolonged to 0.12 second or longer, representing the delay in conduction through the ventricles. An RBBB alters the configuration of the QRS complex in the right-sided chest leads, V1 and V2. Normally, these leads have a small, single-peaked R-wave and deep S-wave configuration. With an RBBB, depolarization of the right ventricle
is delayed, and the ECG pattern changes. An RBBB is evidenced by an RSR′ configuration in V1. If the initial peak of the QRS complex is smaller than the second peak, the pattern would be described as rSR′. An “r” is used to describe the first, smaller peak, and an “R” is used to describe the second, taller peak. Likewise, if the initial peak of the QRS complex is taller than the second peak, the pattern is described as an RSr’. Whenever ventricular depolarization is abnormal, so is ventricular repolarization. As a result, ST-segment and T-wave abnormalities may be seen in leads V1 and V2 for patients with an RBBB.

An LBBB changes the QRS complex pattern in the leftsided chest leads, V5 and V6. Normally, these leads have a tall, single-peaked R wave and a small or absent S wave. Instead, the double-peaked RSR′ pattern is noted.

In addition, V1 shows a small R wave with a widened S wave, indicating delayed conduction through the ventricles. Like RBBB, the ST segments and T waves may be abnormal in the leftsided chest leads V5 and V6 when the patient has an LBBB (see Fig. 17-34).

The most common causes of BBB are MI, hypertension, heart failure, and cardiomyopathy. RBBB may be found in healthy people with no clinical evidence of heart disease. Congenital lesions involving the septum and right ventricular hypertrophy (RVH) are other causes of RBBB.

LBBB is usually associated with some type of underlying heart disease. Longterm CVD in the older patient is a common cause of LBBB.

BBB signifies underlying disease of the intraventricular conduction system. Patients should be monitored for involvement of the other bundles or fascicles or for progression to complete heart block. Progression of block may be very slow or rapid, depending on the underlying cause. A new-onset LBBB in conjunction with an acute MI is associated with a higher mortality rate.

The underlying heart disease determines treatment and prognosis. Patients with an MI and new-onset BBB are closely monitored for progression to a type of complete heart block. A temporary pacemaker may be inserted.