Third-Degree Atrioventricular Block
Synonym
Complete heart block (CHB)
Definition
1. PR interval and QRS duration
are age-dependent measures of atrioventricular (AV) conduction. Impaired
conduction is described as first-degree, second-degree, or third-degree heart
block (see Box 5-7).
2. No impulses from the atria
reach the ventricles in third-degree heart block.
Associated Clinical Features
1. Third-degree heart block is
rare in the pediatric age group.
2. Signs and symptoms in patients
with an otherwise normal heart include:
a. Usually asymptomatic
b. Older children may present
with syncope (syncope from a high-degree AV block not related to positional
changes or exertion is called a Stokes-Adams attack).
c. Older infants may present with
night terrors, irritability, or tiredness with frequent naps.
d. Acquired heart block is
frequently symptomatic, with syncope, congestive heart failure (CHF), shock, or
sudden death.
3. Prominent peripheral pulses
(secondary to large compensatory stroke volume)
4. Cardiomegaly (secondary to
increased diastolic ventricular filling)
Consultation
Cardiology
Emergency Department Treatment and Disposition
1. Patients presenting with
symptoms (e.g., syncope, CHF) or newly diagnosed patients require
hospitalization.
2. Symptomatic newborns (e.g.,
heart failure, evidence of hydrops) with CHB with ventricular rates 50 bpm require cardiac pacing. Adrenergic
agents (epinephrine or isoproterenol) or a vagolytic agent (atropine) may be
tried to increase the heart rate while awaiting placement of the pacemaker.
3. Cardiac pacing (transthoracic
[epicardial], transcutaneous, or transvenous) is also required in symptomatic
patients with CHB and congenital heart disease (CHD).
4. Temporary pacing may be
required in postoperative CHB following surgery for CHD.
Clinical Pearls: Third-Degree Heart Block
1. Autoimmune disease accounts
for 60 to 70% of all cases of congenital CHB.
2. About 25 to 33% of cases of
CHB occur in patients with associated structural heart disease.
3. Complete heart block may not
present at birth in infants born to mothers with SLE, and may develop within
the first 3 to 6 months after birth (Fig. 5-6). Unlike other manifestations of
neonatal lupus that resolve, CHB is permanent and patients often require
cardiac pacing.
4. An implantable pacemaker is
used to prevent sudden death in symptomatic patents with CHB.
Box 5-13. Third-Degree Atrioventricular Heart Block
Characteristic features:
Failure of conduction
of atrial impulses to the ventricles
AV dissociation (the
atria and ventricles beat completely independently and P waves and QRS
complexes have no constant relationship)
The ventricles are
paced by an escape pacemaker at a rate slower than the atrial rate.
The QRS duration may
be prolonged or may be normal if the heartbeat is initiated high in the bundle
of His (generally, the lower the location of the pacemaker within the
ventricular conduction system, the slower the heart rate and the wider the QRS
complexes).
Box 5-14. Etiology of Third-Degree Atrioventricular Block
Some examples of congenital or acquired diseases leading to
complete heart block include:
Infants born to
mothers with systemic lupus erythematosus (SLE), rheumatoid arthritis,
dermatomyositis, or Sjögren's syndrome (autoimmune destruction of AV tracts by
maternally-derived IgG antibodies)
Complex congenital
heart anomaly (e.g., common AV canal)
Abnormal embryonic
development of the conduction system
Postsurgical repair
of congenital heart disease involving the ventricular septum
Myocarditis
Long QT syndrome
Lyme disease (87%
incidence of AV block in patients with carditis with Lyme disease)
Digoxin toxicity
Myocardial tumors
Myocardial abscess
due to endocarditis
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