Abstract

Objective: Approximately 2.8% of pregnancies are Ro/La antibody positive. 3–15% of fetuses develop complete heart block (CHB). First-degree atrioventricular heart block (1° AVB) is reported in a third of Ro/La fetuses but as most have a normal postnatal ECG this may reflect inadequacies of Doppler measurement techniques.

Methods: Comparison was made between mechanical (mPR) and electrical (ePR) intervals obtained prospectively using Doppler and non-invasive fetal ECG (fECG) in 52 consecutive Ro/La pregnancies in 46 women carrying 54 fetuses in an observational study at a fetal medicine unit.

121 mPR and 37 ePR intervals were recorded in 49 Ro/La fetuses. Five were referred with CHB and excluded. ePR was measured successfully in 35/37 (94%) and mPR was measured in all cases. 1° AVB was defined as PR >95% CI. Logistic regression predicted abnormal final fetal rhythm from first mPR or ePR.

Results: The ePR model gave 66.7% sensitivity (6 of 8 final abnormal fetal rhythm cases were predicted correctly in fetuses >20 weeks) and 96.2% specificity. mPR gave 44.4% sensitivity (4 of 9 cases) and 88.5% specificity. Z scores for ePR (zPR) were calculated from 199 normal fetuses. The area under the receiver operator characteristic (ROC) curve was 0.88 (95% CI, 0.754 to 1.007). A cut-off of 1.65 gave a sensitivity of 87.5% and specificity of 95% for those with prolonged and normal ePR intervals, respectively.

Conclusion: zPR is better than mPR at differentiating between normal and prolonged PR intervals, suggesting that fECG is the diagnostic tool of choice to investigate the natural history and therapy of conduction abnormalities in Ro/La pregnancies.