In the normal heart a dense band of fibrous tissue forms early in embryogenesis to anatomically and electrically insulate the atria from the ventricles. The only natural electrical link between the atria and the ventricles is via the AV node and His-Purkinje system. Ventricular preexcitation is said to occur when all or part of the ventricular myocardium is activated by the atrial impulse earlier than would be expected if the stimulus were to reach the ventricles by way of the normal AV conduction system. This premature activation can occur only if there is an alternate route that allows the electrical stimulus to bypass the normal physiologic delay of the AV node. Atrioventricular bypass pathways are embryologic remnants of muscle bundles that bridge the heart’s fibrous skeleton, providing the means for abnormal electrical conduction between the atrium and ventricle (Figure 9-1). Transmission of the electrical impulse over such an accessory pathway produces characteristic findings on the ECG and may provide a substrate for cardiac arrhythmias.
Conduction system of the heart showing accessory AV pathways in blue.
PATHWAYS, CONNECTIONS, AND TRACTS, OH MY!
Accessory pathways are anomalous electrical conduits that are not part of the normal specialized conduction system. Although often used interchangeably, the terms pathway, connection, and tract should be used more precisely. In my experience, this ambiguity is a source of confusion for those trying to understand the anatomy and physiology behind preexcitation. We’ll use “pathway” to describe any anomalous electrical route. A “connection” will describe an accessory conduction pathway that inserts into the working myocardium, regardless of its origin. A “tract” will define an atypical pathway of any origin that inserts into specialized conduction tissue. As we will see shortly, using these terms properly will help you understand how electrical transmission over these pathways affects their ECG appearance.
Accessory pathways between the atria and ventricles may take a number of anatomical forms. The most common and clinically important of these are atrioventricular bypass connections, responsible for the classic Wolff-Parkinson-White pattern and syndrome (see following discussion). Rarely, preexcitation is due to accessory pathways that link the atria, AV node, bundle branches (fascicles), and the ventricles.
FOR HISTORY BUFFS:
You will encounter various eponyms to describe accessory pathways. Atrioventricular bypass connections are also called bundles of Kent. The unusual atriofascicular, nodofascicular, nodoventricular, and fasciculoventricular pathways are collectively known as Mahaim fibers. James fibers have been used to describe perinodal (also called atrionodal or intranodal) bypass tracts, but these pathways are disputed and of uncertain clinical significance. I discourage the use of these eponyms in favor of the more precise anatomical description.
Transmission of the normal sinus impulse ...