Arrhythmias

In CHB, there is complete dissociation between atrial and ventricular contractions. The ventricular rate is regular but slow (40 to 90 beats per minute) and is not exactly half that of the atrial beat. Complete heart block can occur in fetuses with a structurally normal heart or in the setting of congenital heart disease. The most common heart abnormality is left atrial isomerism, which can be found as early as the first trimester or present later in pregnancy.

In 90% of isolated CHB the condition is caused by the presence of the maternal autoantibodies anti-Ro (SS-a) or anti-La (SS-B). Most mothers are asymptomatic. In fetuses with cardiac defects, CHB may present in the first trimester, whereas in cases of isolated, autoantibody-related CHB the condition usually presents in the second trimester.

The four-chamber view confirms cardiomegaly and typical echogenic patches within the myocardium. In some cases there is progression of biventricular dysfunction and cardiomegaly, atrioventricular valve regurgitation resulting in pericardial effusion and fetal hydrops.

Prevalence

Complete heart block is found in about 1 in 20,000 live births.

Other abnormalities

Complete heart block is not associated with extracardiac defects or chromosomal anomalies

Prognosis

The prognosis of complete heart block depends on the presence of cardiac defects, the ventricular rate and the occurrence of hydrops. Fetal hydrops is associated with a poor prognosis. No prenatal intervention, including maternal administration of steroids for those with autoantibodies or invasive fetal cardiac pacing, has been proven to be useful. Postnatal cardiac assessment is essential but only a minority of patients require early implantation of a pacemaker.