NEW YORK (Reuters Health) – Contrary to prior assumptions, prolongation of atrioventricular conduction in fetuses exposed to maternal anti-Ro antibodies does not predict progression to complete atrioventricular block at birth, a Canadian team reports.

Consequently, the rationale for treating early low-grade block with dexamethasone to prevent complete atrioventricular (AV) block is questionable, they point out in the March 29 issue of the Journal of the American College of Cardiology.

Dr. Edgar T. Jaeggi, with the Hospital for Sick Children in Toronto, Ontario, and colleagues conducted a prospective study in 165 fetuses of 142 anti-Ro antibody-positive women, to evaluate anti-Ro antibody-mediated effects on the fetal heart and to develop management recommendations.

Echocardiography was performed weekly between gestational weeks 19 and 24 to measure AV intervals.

During the observation period, AV conduction was consistently normal in 150 of the fetuses. Nonetheless, complete AV block was diagnosed in one of these fetuses at week 28, according to the report.

The other 15 fetuses were seen to have AV prolongation or type 1 second-degree heart block during the observation period. However, none of them developed complete heart block, the investigators found. At birth, three of the 15 infants were diagnosed with first-degree block, but this either resolved spontaneously or did not progress.

“We suggest that transplacental treatment should be restricted to those fetuses with progressive AV block or with additional findings of autoantibody-mediated pathology, such as endocardial fibroelastosis and effusions,” Dr. Jaeggi and colleague advise.

“In contrast,” they add, “in isolated AV prolongation up to 6 z-scores, close monitoring for disease progression without treatment is recommended. The utility of this approach needs confirmation in a large prospective, multicenter study.”

J Am Coll Cardiol 2011;57:1487–1492.