![]() ST-T abnormalities - Bundle branch blocks and pacemaker patterns are characteristically associated with secondary repolarization (ST-T) abnormalities. A pattern identical to that of LBBB, preceded by a sharp spike, is seen in most cases of electronic right ventricular pacing because of the relative delay in left ventricular activation. As a result, LBBB generates wide, predominantly negative (QS) complexes in lead V1 and entirely positive (wide R wave) complexes in lead V6 ( waveform 2 and table 2). However, the normal early left-to-right pattern of septal activation is disrupted so that septal depolarization proceeds from right-to-left as well. The late QRS vector is still directed to the left and posteriorly, since depolarization is delayed in the left ventricle. Left bundle branch block (LBBB) is different in that it alters both early and late phases of ventricular depolarization. Therefore, these changes are represented on the ECG by a late positive wave (rS Rʹ) in V1 and a late negative wave (qR S) in V6 ( waveform 1 and table 1). This vector points toward the positive axis of the anterior-posterior lead V1 and away from the positive axis of left-right lead V6. As a result, the terminal QRS vector is oriented anteriorly and to the right. However, right bundle branch block (RBBB) delays activation of the right ventricle to the late phase of depolarization. The major late QRS vector is normally directed to the left and posteriorly due to depolarization of the left ventricle. The QRS vector in bundle branch block is generally oriented in the direction of the myocardial region in which depolarization is delayed. With complete bundle branch blocks, the QRS interval is classically stated to be greater than or equal to 120 ms (0.12 s) in duration (three small box widths on standard ECG displays) with incomplete blocks, the QRS interval is defined between 100 (or 110 by computer) and 120 ms (0.10 to 0.12 s). The degree of prolongation of the QRS interval depends upon the severity of the impairment. Bradycardia or deceleration-dependent bundle branch blocks, in which conduction delay occurs when the rate falls below a certain level, are relatively rare.Įlectrocardiographic 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"). As an example, transient rate-related bundle branch blocks commonly occur when the heart rate exceeds some critical value (tachycardia- or acceleration-dependent). Bundle branch block can be chronic or intermittent. 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). 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). Some patients have a third subdivision, a median fascicle. The main left bundle bifurcates into two primary subdivisions: a left anterior fascicle and a left posterior fascicle. 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. Normal ventricular depolarization occurs after an impulse traverses the atrioventricular (AV) node and the bundle of His. (See "Left bundle branch block" and "Right bundle branch block" and "ECG tutorial: Intraventricular block".) ![]() More complete discussions of left and right bundle branch block are presented elsewhere. This topic will review the basic aspects of this problem. INTRODUCTION - Delayed intraventricular conduction is a common clinical abnormality detected on the electrocardiogram (ECG).
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