Heart block refers to an impairment in impulse conduction. The block may occur at any point in the conduction system, although it is most commonly recognized at the level of the Av node or system.
The incidence of heart block increases with a and may result from fibrosis of the AV infarction of the AV node or His-Purkinje system, and calcification of the valvular annulus with impingement on the conduction system. Medications, either at therapeutic or toxic levels, may also result in various degrees of heart block. Heart block at the level of the AV node is divided into several types. First-degree AV block refers to a prolongation of AV conduction time(PR interval >200 msec) but with eventual conduction of the atrial impulse to the ventricles. Second-degree AV block refers to intermittent failure of the impulse to conduct from the atrium to the ventricles.
This can be further divided into Mobitz I second-degree heart block in which there is a gradual prolongation of the PR interval until a nonconucted P wave occurs, and Mobitz II second-degree heart block, in which there is a sudden nonconducted P wave, or series of non-conducted P waves, without prior PR interval prolongation. Mobitz I heart block usually reflects the effects of increased vagal tone on the AV node in the absence of structural disease of the node and is a benign rhythm that rarely causes symptoms. It commonly occurs at night, even in young patients with normal hearts. It may also occur in the setting of an acute inferior myocardial infarction, at which time it is usually transient, does not tend to progress to higher degrees of heart block, and rarely requires therapy.
Mobitz II heart block usually structural disease of the AV node or His-Purkinje system, may be associated with presyncope or syncope, and may progress to higher degrees of heart block. It may also occur in the setting of an acute myocardial infarction, at which time it reflects ischemia or infarction of the conduction system, is associated with a high mortality rate, and requires at least temporary pacing.