[Congenital atrioventricular block].

Abstract

Congenital atrioventricular block is defined (Yater) as a documented conduction defect in a young subject with unquestionable bradycardia in the absence of a history of infection which might have caused the condition after birth. It is a rare condition (1 out of 20,000 births) and may be isolated or associated with another congenital malformation. Four cardiac malformations are classically associated: endomyocardial fibrosis, morphological abnormalities close to the conduction system: corrected transposition, left isomerism; patent ductus arteriosus and atrial septal defect apparently unrelated but the most common; the association of mitral regurgitation in adults, although the significance is not very clear. Isolated block is often observed in patients with mothers suffering from autoimmune disease, often clinically latent. Anti Ro/SS-A and La/SS-B antibodies cross into the foetal circulation and cause inflammation of the conduction tissues but the causal mechanism is not known. The diagnosis of the conduction defect is sometimes made during foetal life by echocardiography. After birth, the diagnosis is made by electrocardiography but the block is not always complete or permanent, its degree often increasing with time. In addition, in advanced degrees of block, the escape rhythm tends to slow down. Long-term follow-up studies have revised the previously considered good prognosis of isolated congenital atrioventricular block but advances in cardiac pacing provide satisfactory treatment. In the foetus, isolated atrioventricular block is usually associated with an escape rhythm > 60/min and enables normal vaginal delivery; a low heart rate < 55/min and anasarca carry a poor prognosis. In the neonate, pacing is indicated in babies with cardiac failure and a heart rate < 55/min. Follow-up by Holter monitoring, exercise testing and echocardiography is justified in children and adolescents; the patients may become symptomatic at any age. Pacing is essential in symptomatic cases (malaise, ventricular dysfunction) and useful in cases with long QT intervals, frequent ventricular extrasystoles and wide ventriculogrammes. Pacing is not always easy in children. Epicardial pacing by thoracotomy or an epigastric approach is possible but endocavitary pacing is to be preferred using thin pacing catheters introduced via the subclavian vein and small pacemakers implanted in a sub or prepectoral site. A pacing mode which restores the normal atrioventricular sequence is theoretically superior to single ventricular pacing even if rate responsive frequency.