Congenital atrioventricular block.

Congenital atrioventricular block is defined (Yater) as a documented conduc tion defect in a young subject with unquestionable bradycardia in the absen ce of a history of infection which might have caused the condition after bi rth. It is a rare condition (1 out of 20 000 births) and may be isolated or associated with another congenital malformation. Four cardiac malformation s are classically associated : endomyocardial fibrosis, morphological abnor malities close to the conduction system : corrected transposition, left iso merism; patent ductus arteriosus and atrial septal defect apparently unrela ted but the most common; the association of mitral regurgitation in adults, although the significance is not very clear. Isolated block is often obser ved in patients with mothers suffering from autoimmune disease, often clini cally latent. Anti Ro/SS-A and La/SS-B antibodies cross into the foetal circulation and c ause inflammation of the conduction tissues but the causal mechanism is not known. The diagnosis of the conduction defect is sometimes made during foe tal life by echocardiography. After birth, the diagnosis is made by electro cardiography 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. Longterm follow-up studies have revised t he previously considered good prognosis of isolated congenital atrioventric ular block but advances in cardiac pacing provide satisfactory treatment. I n the foetus, isolated atrioventricular block is usually associated with an escape rhythm > 60/min and enables normal vaginal delivery; a low heart ra te < 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 Hotter monitoring, exercise testing and echocardiography is justified in children and adolescents; the patients may become symptomatic at any ag e. Pacing is essential in symptomatic cases (malaise, ventricular dysfuncti on) and useful in cases with long QT intervals, frequent ventricular extras ystoles 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 introduc ed via the subclavian vein and small pacemakers implanted in a sub or prepe ctoral site. A pacing mode which restores the normal atrioventricular seque nce is theoretically superior to single ventricular pacing even if rate res ponsive frequency.