Cardiac conduction system disorders - overview

The atria, and their respective ventricles, are electrically insulated from each other by a fibrous tissue known as the atrioventricular (AV) junction. Impulses are transferred via a special bridge, called the His-Purkinje system (HPS). Some individuals, however, have additional bridges called accessory pathways. Irregular heartbeats resulting from impulses passing through accessory pathways are called supraventricular. Supraventricular tachycardias (SVT) may be atrial or AV junctional. In atrial tachycardia, the impulse arises from the superior portion of either atrium. This condition may arise when the tissues of the atria or pericardium, the membrane covering the heart, are affected by a chronic or acute inflammation, dilation or enlargement of an atrium, or formation of reparative tissue called fibrosis. Re-entry tachycardias result from congenital accessory pathways, and can be manifested at any age. AV nodal re-entry (AVNR) is the most common AV junctional tachycardia. The re-entry circuit is localized in the AV node, a small node involved in the conduction system of the heart. AV nodal re-entry results from differences in conducting fibers leading to divergence in impulses. Wolff-Parkinson-White syndrome is one of the most frequently encountered accessory pathway syndromes. The incidence is approximately 150 cases per year per 100,000 individuals. Electrical impulses bypass the AV node and travel along the bundle of Kent accessory pathway instead. This results in ventricular pre-excitation, because the impulse reaches the ventricle prematurely. Very rapid heartbeats, a complication of Wolff-Parkinson-White, can result in severe hypotension and syncope, or loss of consciousness.