Atrioventricular (AV) block is a disturbance in conduction between the normal sinus impulse and the eventual ventricular response. The block is assigned to one of three classes, depending on the severity of the conduction disturbance. First-degree AV block is a simple prolongation of the PR interval, but all P waves are conducted to the ventricle. In second-degree AV block, some atrial impulses are not conducted into the ventricle. In third-degree AV block (or complete heart block), none of the atrial impulses is conducted into the ventricle ( Fig. 25-1 ). Holter monitoring often reveals patterns not apparent in the relatively short electrocardiogram.
First-Degree Atrioventricular Block
Description. The PR interval is prolonged beyond the upper limits of normal for the patient’s age and heart rate (see Table 3-2 and Fig. 25-1 ). The PR interval includes the time required for depolarization of the atrial myocardium (PA interval), the delay of conduction in the AV node (AH interval), conduction through the bundle of His, and the time of onset of ventricular depolarization (HV interval).
Causes. First-degree AV block can appear in otherwise healthy children and young adults, particularly in athletes, mediated through excessive parasympathetic tone. Other causes include congenital heart diseases (CHDs) (e.g., endocardial cushion defect, atrial septal defect [ASD], Ebstein’s anomaly), infectious disease, inflammatory conditions (rheumatic fever), cardiac surgery, and certain drugs (e.g., digitalis, calcium channel blockers).
Significance. Slow intraatrial or AV nodal conjunction is almost always the mechanism for first-degree AV block. First-degree AV block does not produce hemodynamic disturbance. Exercise, both recreational and during stress testing, induces parasympathetic withdrawal, resulting in normalization of AV conduction and the PR interval. The PR interval can be very long, but in the absence of heart disease, it usually does not progress.
Management . No treatment is indicated except when the block is caused by drugs.
Second-Degree Atrioventricular Block
Some, but not all, P waves are followed by QRS complex (dropped beats). There are three types: Mobitz type I (Wenckebach phenomenon), Mobitz type II, and high-grade (or advanced) second-degree AV block.
Mobitz Type I (Wenckebach)
Description. The PR interval becomes progressively prolonged until one QRS complex is dropped completely (see Fig. 25-1 ).
Causes. Mobitz type I AV block appears in otherwise healthy children. Other causes include myocarditis, cardiomyopathy, myocardial infarction, congenital heart defect, cardiac surgery, and digitalis toxicity.
Significance. The block is at the level of the AV node (with prolonged AH interval). It usually does not progress to complete heart block. It occurs in individuals with vagal dominance.
Management. The underlying causes are treated.
Mobitz Type II
Description. The AV conduction is “all or none.” AV conduction is either normal or completely blocked (see Fig. 25-1 ).
Causes. Causes are the same as for Mobitz type I.
Significance. The block usually occurs below the AV node (at the level of the bundle of His). It is more serious than type I block because it may progress to complete heart block, resulting in Stokes-Adams attack.
Management. The underlying causes are treated. Prophylactic pacemaker therapy may be indicated.
Two to One (or Higher) Atrioventricular Block
Description. A QRS complex follows every second, third, or fourth P wave, resulting in 2:1, 3:1, or 4:1 AV block (see Fig. 25-1 ). When two or more consecutive P waves are nonconducted, the rhythm is called advanced or high-grade second-degree AV block. In contrast to third-degree complete AV block, some P waves continue to be conducted to the ventricle, and the PR interval of conducted beats is constant.
Causes. Causes are similar to those of other second-degree AV blocks.
Significance. The block is usually at the bundle of His, alone or in combination with the AV nodal block. It may progress to complete heart block. Higher grade second-degree AV block should always be regarded as abnormal. The implications of high-grade AV block appear to be similar to those of complete AV block.
Management. The underlying causes are treated. Electrophysiologic studies may be necessary to determine the level of the block. Symptomatic second-degree AV block, although uncommon, can be acutely treated with atropine, isoproterenol, and temporary pacing. Pacemaker therapy is indicated for symptomatic advanced second-degree AV block.
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