Heart Block

1^st Degree

1st degree heart block is diagnosed when the PR interval is greater than normal (220 ms in example on right). It is due to intranodal conduction delay. It is associated with rheumatic fever, diphtheria, rubella, mumps, the muscular dystrophies and myocardial infarction. It may be induced by drugs eg. digoxin, β blockers. If itself it has no clinical implications for the pediatric patient.

1st degree block

2^nd Degree - Mobitz type 1

This is otherwise known as Wenckebach phenomenon. The PR interval progressively increases until the P wave is no longer transmitted & a beat is dropped. It is a frequent incidental finding on 24 hour tape recordings - particular during sleep. It is usually of no clinical significance and rarely progresses to more severe forms of AV block.

Mobitz Type 1 (Wenkeback)

2^nd Degree - Mobitz type 2

In Mobitz type 2 block there is loss of AV conduction without an increase in the PR interval. This is more malign and it may progress to cause syncope, sudden death or complete heart block.

2:1 Block 4:1 Mobitz type 2 Block
4:1 Block

3^rd Degree - Complete Heart Block

Complete Heart Block

Congenital Complete Heart Block

Congenital complete heart block is usually caused by maternal autoimmune disease - SLE, Sjogren's syndrome & rheumatoid arthritis. Indeed 95% of infants with isolated complete heart block have mothers who are Anti-Ro and/or anti-La positive. These antibodies cross the placenta and cross react with the fetal conducting system. Complete heart block is well tolerated provided the rate is sufficient to meet the cardiac output requirements however when symptoms present in the 1st year of life then the outcome is poorer and even with pacing this group has a significant mortality rate (Pediatrics 2000;106:86). A study in adults shows that the majority require pacing and that as the risk factors for sudden death are not well defined pacing should be recommended for teenagers onwards Circulation 1995;92:442.

Acquired Complete Heart Block

This is most commonly due to open-heart surgery and may be due to oedema of the myocardium in which case it is usually recoverable or to disruption by sutures in which case it may be permanent. If there is no recovery is seen after two weeks then a permanent system is implanted.

Some structural cardiac lesions are also associated with complete heart block - eg. congenitally corrected transposition of the great arteries and the isomerisms.

Inflammation and infection (eg myocarditis, Borrelia) may also cause complete heart block and may require temporary pacing. Usually if the patient recovers so does the conducting system and it is uncommon for them to require permanent pacing.

Management

When a child presents acutely with complete heart block and is symptomatic then it is important to increase the cardiac rate as quickly as possible to maintain the cardiac output. Atropine is sometimes useful but an isoprenaline infusion is a better short term measure. If this fails then temporary pacing is necessary.

For those in a stable condition then pacing is indicated if the rate is < 50bpm, there are symptoms or significant pauses on Holter monitoring.

Bundle Branch Block

There are times when one or more of the His Purkinje fibers fail to conduct properly. These cause specific ECG patterns rather than an arrhythmias and are addressed in the section on the electrocardiogram.