The electrocardiographic diagnosis of intraventricular blocks coexisting with ventricular preexcitation

The electrocardiographic diagnosis of intraventricular conduction disturbances may be hindered by the coexistence of ventricular preexcitation. In fact, the premature depolarization of ventricular myocardium through an accessory pathway tends to conceal any electrocardiographic manifestation of a bundle-branch block. However, there are several conditions favoring the diagnosis of bundle-branch block associated with ventricular preexcitation: intermittency of ventricular preexcitation and/or bundle-branch block, fast atrioventricular (AV) nodal impulse propagation, slow conduction over the accessory pathway or between its ventricular insertion site and the remaining myocardium, and presence of atrioventricular junctional ectopic beats exposing the intraventricular conduction disturbance. This article reexamines the available data on preexcitation in patients with intraventricular blocks and presents clinical examples to emphasize the importance of a thorough examination of the electrocardiogram to attain the correct diagnosis of this association.