AV (and SA) conduction disturbances may result from physiologic, pharmacologic and/or pathologic causes. The physiologic causes refer primarily to an increase in vagal tone which will slow the sinus rate and slow conduction within the AV node.
The pharmacological causes refer to the drugs that affect conduction within and distal to the AV node by enhancing vagal tone and/or by acting directly on the AV node and on the more distal conduction system. For instance, digitalis and the beta adrenergic blocking drugs enhance vagal tone while the calcium channel blocking agents act directly on the AV node. Direct acting antiarrhythmic drugs, particularly the class 1 agents which block the rapid sodium inward current, may affect conduction in the AV node and more distal conducting system.
The pathological causes include a variety of congenital and acquired diseases that may involve the sinus and/or the AV node and/or the more distal conducting system.