An accessory pathway is an additional route for impulse conduction between the atrium and the ventricle that generally does not involve the atrioventricular node (AV node). Conduction through accessory pathways usually exhibit electrophysiological properties different from the AV node. From an anatomical viewpoint, there are at least five connections that bypass the AV node.

• Accessory atrioventricular muscle bundles (bundle of Kent) are atrioventricular bypass found outside the normal conducting system connecting the atria directly with the ventricle.
• Nodoventricular accessory connections (a variety of Mahaim fibers) mediate the atrioventricular bypass through the normal conducting system connecting AV node to ventricular myocardium.
• Fasciculoventricular connections (another variety of Mahaim fibers) also mediate the atrioventricular bypass through the normal conducting system connecting the bundle of His (distal part of junctional area) to ventricular myocardium.
• Atrioventricular bypass fibers (James fibers) connect the atrial fibers to bundle of His by penetrating through the AV node, again a bypass through the normal conducting system. They may not be considered as an accessory pathway since they never completely bypass the AV node and are found to exist in all hearts.
• Atrio-Hisian fibers (Brechenmacher fibers) are atrial fibers that bypass the AV node and connect to the upper margin of bundle of His.

• Wolff-Parkinson-White syndrome (WPW)
• Lown-Ganong-Levine syndrome (LGL)

• There is a shortened PR interval to < 0.12 seconds.
• There is a wide QRS with a delta wave.
• The QRS is 0.11 second or longer.

• There is a shortened PR interval to < 0.12 seconds.
• The QRS complex is of normal duration.

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