Delayed intraventricular conduction is a common clinical abnormality detected on the electrocardiogram (ECG). This topic will review the basic aspects of this problem [1,2]. More complete discussions of left and right bundle branch block are presented elsewhere. (See "Left bundle branch block" and "Right bundle branch block" and "ECG tutorial: Intraventricular block".)
Normal ventricular depolarization occurs after an impulse traverses the atrioventricular (AV) node and the bundle of His. This bundle of specialized conducting tissue splits into two main branches, the right and the left bundles, that rapidly transmit depolarization impulses to the right and left ventricular myocardium, respectively, via the Purkinje fibers. The main left bundle bifurcates into two primary subdivisions: a left anterior fascicle and a left posterior fascicle. Some patients have a third subdivision, a median fascicle. The depolarization wavefronts spread through the ventricular wall, from endocardium (inner layer) to epicardium (outer layer), triggering intracellular calcium release and myofilament contraction (electromechanical coupling).
RIGHT AND LEFT BUNDLE BRANCH BLOCKS
Right and left bundle branch blocks usually reflect intrinsic impairment of conduction in either the right or left bundle system, respectively (intraventricular conduction disturbances). Bundle branch block can be chronic or intermittent. As an example, transient rate-related bundle branch blocks can occur when the heart rate exceeds some critical value (tachycardia- or acceleration-dependent). Bradycardia or deceleration-dependent bundle branch blocks, in which conduction delay occurs when the rate falls below a certain level, are relatively rare.
Electrocardiographic changes — Bundle branch block leads to prolongation of the QRS interval and sometimes to alterations in the QRS vector. (See "Basic principles of electrocardiographic interpretation".) The degree of prolongation of the QRS interval depends upon the severity of the impairment. With complete bundle branch blocks, the QRS interval is classically stated to be greater than or equal to 120 ms (0.12 sec) in duration (three small boxes on electrocardiogram [ECG] paper); with incomplete blocks, the QRS interval is defined between 100 and 120 ms (0.10 to 0.12 sec).
The QRS vector in bundle branch block is generally oriented in the direction of the myocardial region in which depolarization is delayed. The major late QRS vector is normally directed to the left and posteriorly due to depolarization of the left ventricle. However, right bundle branch block (RBBB) delays activation of the right ventricle to the late phase of depolarization (movie 1). As a result, the terminal QRS vector is oriented anteriorly and to the right. This vector points toward the positive axis of the anterior-posterior lead V1 and away from the positive axis of left-right lead V6. Therefore, these changes are represented on the ECG by a late positive wave (rSR) in V1 and a late negative wave (qRS) in V6 (waveform 1 and table 1).