INTRODUCTION

Normal electrical activation of the right and left ventricles during the cardiac cycle follows a precisely defined pattern. After an impulse emerges from the atrioventricular (AV) node, it traverses the His bundle, and propagates down the bundle branches and the fascicles of the bundle branches to the terminal Purkinje fibers and ultimately the ventricular myocardium.

Conduction abnormalities are generally due to asynchronous activation of the ventricles. This problem can be induced by one of two mechanisms: delayed activation of an area of the ventricles or early activation (preexcitation) of an area of the ventricles.

This topic will present an overview of asynchronous activation and preexcitation. The clinical manifestations of abnormal activation or preexcitation are discussed in detail separately. (See "Right bundle branch block" and "Left bundle branch block" and "Left anterior fascicular block" and "Left posterior fascicular block" and "Atrioventricular reentrant tachycardia (AVRT) associated with an accessory pathway".)

DELAYED ACTIVATION

Delayed activation producing asynchronous activation may be a result of anatomic abnormalities or of physiologic properties of the cardiac tissues. Activation delay may occur between the ventricles or portions of the ventricles (interventricular delay), within the terminal Purkinje fibers and/or ventricular myocardium (intraventricular delay) or between layers of the heart (intramural delay) [1].

Clinically, conduction delay causing asynchronous ventricular activation is usually manifest as an abnormal QRS complex on the electrocardiogram (ECG). Specific ECG patterns that are clinically distinguished include right bundle branch block (RBBB), left bundle branch block (LBBB), a prolonged QRS complex without specific features of LBBB or RBBB (usually called "intraventricular conduction delay" or IVCD), left anterior hemiblock, and left posterior hemiblock. While all of these conduction patterns are sometimes referred to collectively as forms of intraventricular conduction delay, the precise location of the conduction disturbance cannot be reliably determined from the surface ECG. In the case of RBBB and LBBB, it is probably more accurate to refer to these entities as forms of "interventricular conduction delay."