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, according to the severity of the conduction disturbance.
- 1.
First-degree AV block is a simple prolongation of the PR interval but all P waves are conducted to the ventricle.
- 2.
In second-degree AV block, some atrial impulses are not conducted into the ventricle.
- 3.
In third-degree AV block (or complete heart block), none of the atrial impulses is conducted into the ventricle ( Fig. 17.1 ).
Fig. 17.1
Atrioventricular (AV) block.
From Park, M. K., & Guntheroth, W. G. (2006). How to read pediatric ECGs (4th ed.). Philadelphia: Mosby.
I. First-Degree AV Block
- 1.
Description: There is a prolongation of the PR interval beyond the upper limits of normal (see Table 2.3) due to an abnormal delay in conduction through the AV node (see Fig. 17.1 ).
- 2.
Causes
- a.
In otherwise healthy children and young adults, particularly athletes, mediated through excessive parasympathetic tone.
- b.
CHDs (such as endocardial cushion defect, ASD, Ebstein anomaly).
- c.
Other causes, including infectious disease, inflammatory conditions (rheumatic fever), cardiac surgery, and certain drugs (such as digitalis, calcium channel blockers).
- a.
- 3.
Significance
- a.
Usually no hemodynamic disturbance results. Exercise, both recreational and during stress testing, induces parasympathetic withdrawal resulting in normalization of AV conduction and the PR interval.
- b.
Sometimes it may progress to a more advanced AV block.
- a.
- 4.
Treatment: No treatment is indicated except in digitalis toxicity.
II. Second-Degree AV Block
In second-degree AV block, some but not all P waves are followed by QRS complexes (dropped beats). There are three types: Mobitz type I (Wenckebach phenomenon), Mobitz type II, and high-grade (or advanced) second-degree AV block.
A. Mobitz Type I (Wenckebach Phenomenon)
- 1.
Description: The PR interval becomes progressively prolonged until one QRS complex is dropped completely (see Fig. 17.1 ).
- 2.
Causes: In otherwise healthy children, myocarditis, cardiomyopathy, myocardial infarction, CHD, cardiac surgery, and digitalis toxicity.
- 3.
Significance
- a.
The block is at the level of the AV node (with prolonged atrio-His interval).
- b.
It occurs in individuals with vagal dominance.
- c.
It usually does not progress to complete heart block.
- a.
- 4.
Treatment: The underlying cause is treated.
B. Mobitz Type II
- 1.
Description: The AV conduction is “all or none.” AV conduction is either normal or completely blocked (see Fig. 17.1 ).
- 2.
Causes: Same as for Mobitz type I.
- 3.
Significance
- a.
The block usually occurs below the AV node (at the level of the bundle of His).
- b.
It is more serious than type I block because it may progress to complete heart block, resulting in Stokes-Adams attacks.
- a.
- 4.
Treatment: The underlying cause is treated. Prophylactic pacemaker therapy may be indicated.
C. Two-To-One (OR Higher) AV Block
- 1.
Description
- a.
A QRS complex follows every second (third or fourth) P wave, resulting in 2:1 (3:1 or 4:1, respectively) AV block (see Fig. 17.1 ). 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.
- b.
When two or more consecutive P waves are nonconducted, the rhythm is called advanced or high-grade second-degree AV block.
- a.
- 2.
Causes: Similar to those of other second-degree AV blocks.
- 3.
Significance
- a.
The block is usually at the bundle of His, alone or in combination with the AV nodal block.
- b.
It may occasionally progress to complete heart block.
- c.
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.
- a.
- 4.
Treatment: The underlying cause is treated. Electrophysiologic studies may be necessary to determine the level of the block. Pacemaker therapy is indicated for symptomatic advanced second-degree AV block.
III. Third-Degree AV Block (Complete Heart Block)
- 1.
Description
- a.
In third-degree AV block, the atrial and ventricular activities are entirely independent of each other (see Fig. 17.1 ). The P waves are regular (with regular PP interval) with a rate comparable to the heart rate of the patient’s age. The QRS complexes are also quite regular (with regular RR interval) but with a rate much slower than the P rate.
- b.
The third-degree AV block is either congenital or acquired.
- i.
In congenital complete heart block, the duration of the QRS complex is normal, since the pacemaker for the QRS complex is at a level higher than the bifurcation of the bundle of His. The ventricular rate is faster (50 to 80 beats/min) than in the acquired type.
- ii.
In surgically induced or acquired (from postmyocardial infarction) complete heart block, the QRS duration is prolonged, and the ventricular rate is in the range of 40 to 50 beats/min (idioventricular rhythm). The pacemaker for the wide QRS complex is at a level below the bifurcation of the bundle of His.
- i.
- a.
- 2.
Causes
- a.
Congenital heart block
- i.
In 60% to 90% of cases, it is caused by neonatal lupus erythematosus. Maternal antibodies to the intracellular ribonucleoprotein Ro (SS-A) and La (SS-B) for autoimmune connective tissue diseases cross the placenta to the fetus causing the heart block.
- ii.
In 25% to 33%, it is associated with CHDs, most commonly with l -TGA, single ventricle, or polysplenia syndrome.
- iii.
Neonatal myocarditis and several genetic disorders such as familial ASD and Kearns-Sayre syndrome have been identified.
- i.
- b.
The acquired type
- i.
It occurs as a complication of cardiac surgery in children.
- ii.
Rarely, severe myocarditis, Lyme carditis, acute rheumatic fever, mumps, diphtheria, cardiomyopathies, tumors in the conduction system, or overdose of certain drugs causes the block.
- iii.
It may also follow myocardial infarction.
- iv.
These causes produce either temporary or permanent heart block.
- i.
- a.
- 3.
Significance
- a.
Complete heart block can be diagnosed by fetal bradycardia during fetal echo study between 18 and 28 weeks of gestation. Complications in utero may include hydrops fetalis, myocarditis, and fetal death.
- b.
CHF may develop in infancy, particularly when there are associated CHDs.
- c.
Patients with isolated congenital heart block are usually asymptomatic during childhood and achieve normal growth and development.
- d.
Syncopal attacks (Stokes-Adams attack) or sudden death may occur with the heart rate below 40 to 45 beats/min.
- a.
- 4.
Treatment
- a.
When detected in utero , steroid therapy may be applied if associated with anti-Ro/SS-A and anti-La/SS-B.
- b.
Asymptomatic children with congenital complete heart block with acceptable heart rate, narrow QRS complex, and normal ventricular function may not need to be treated.
- c.
In symptomatic children and adults, atropine or isoproterenol is indicated until temporary ventricular pacing is secured.
- d.
A temporary transvenous or epicardial ventricular pacemaker is indicated in patients with heart block, or it may be given prophylactically in patients who might develop heart block.
- e.
Permanent pacemaker therapy is indicated in patients with congenital heart block under the following situations (see Box 18.1 for detailed indications).
- i.
If the patient is symptomatic or develops CHF.
- ii.
Dizziness or lightheadedness may be an early warning sign of the need for a pacemaker.
- iii.
If an infant has a ventricular rate <50 to 55 beats/min or if the infant has a CHD with a ventricular rate <70 beats/min.
- iv.
If the patient has a wide QRS escape rhythm, complex ventricular ectopy, or ventricular dysfunction.
- v.
In patients with surgically induced heart block that is not expected to resolve or that persist at least 7 day after cardiac surgery.
- i.
- f.
A variety of problems may arise after a pacemaker is placed in children. Stress placed on the lead system by the linear growth of the child, fracture of the lead system in a physically active child, electrode malfunction (scarring of the myocardium around the electrode, especially in infants), and the limited life span of the pulse generator require follow-up of children with artificial pacemakers.
- a.
IV. Atrioventricular Dissociation
AV dissociation should not be confused with complete heart block (third-degree AV block).
- 1.
AV dissociation results from a marked slowing of the sinus node activity, atrial bradycardia, or acceleration of the AV node.
- 2.
In AV dissociation the atrial rate is slower than the ventricular rate, whereas in complete heart block the ventricular rate is usually slower than the atrial rate.
- 3.
In AV dissociation an atrial impulse may conduct to the AV node if it comes at the right time ( Fig. 17.2 ).
