Second-Degree Atrioventricular Block
Second-degree AV block is defined as intermittent failure of conduction of some atrial depolarizations to the ventricles. Second-degree AV block is subdivided into Mobitz type I (Wenckebach) and Mobitz type II AV block. In Mobitz type I (Wenckebach) AV block there is progressive delay in A-V conduction before a single atrial beat fails to conduct. Mobitz type I AV block is generally caused by factors similar to those causing first degree A-V block; it is usually well tolerated, and rarely causes symptomatic bradycardia. Mobitz Type II AV block is recognized by the intermittent failure of atrioventricular conduction, without an associated prolongation of the P-R interval. There is often a fixed ratio of atrial to ventricular depolarizations, although this ratio may vary over time. Asymptomatic patients with isolated Mobitz Type II AV block do not require a permanent pacemaker, but should receive close follow-up.
Third-degree (Complete) Atrioventricular Block
Third-degree (complete) atrioventricular (AV) block is defined by the absence of conduction of atrial impulses to the ventricles. The ventricular rhythm may arise from the AV node or His bundle (junctional escape rhythm), in which case there is usually a narrow QRS complex. Alternatively, the ventricular rhythm may arise from the ventricles, resulting in a wide QRS complex. The ventricular rate is related to the origin of the escape rhythm, with AV junctional escape rhythms generally being faster than idioventricular rhythms.
Congenital complete heart block is frequently recognized in utero. It is estimated to occur in 1 of 20,000 live
births. In approximately 50% of infants with congenital heart block, there is an associated cardiovascular malformation (e.g., l-transposition of the great arteries, heterotaxy syndrome, endocardial cushion defect). In the absence of structural heart disease, the heart block is usually related to maternal autoantibodies (anti-Ro and/or anti-La) that cross the placenta and interact with the developing conduction system (114). These antibodies are associated with maternal connective tissue disease, particularly lupus erythematosis and Sjögren syndrome. In newborns with heart block and no structural cardiac abnormalities, testing of their mothers for these antibodies is indicated because they may have no signs or symptoms of connective tissue disease.
births. In approximately 50% of infants with congenital heart block, there is an associated cardiovascular malformation (e.g., l-transposition of the great arteries, heterotaxy syndrome, endocardial cushion defect). In the absence of structural heart disease, the heart block is usually related to maternal autoantibodies (anti-Ro and/or anti-La) that cross the placenta and interact with the developing conduction system (114). These antibodies are associated with maternal connective tissue disease, particularly lupus erythematosis and Sjögren syndrome. In newborns with heart block and no structural cardiac abnormalities, testing of their mothers for these antibodies is indicated because they may have no signs or symptoms of connective tissue disease.
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