P05.29: Evaluation of PR interval in fetuses of Anti Ro positive pregnancies

Immune mediated complete heart block (CHB) occurs in 3% of fetuses of mothers with Anti Ro or La antibodies (Ro/La). The disease is progressive and early detection of PR interval prolongation could rationalise treatment with steroids to ameliorate damage. We report mechanical and electrical PR intervals of Ro/La fetuses. Retrospective audit January 1997 to December 2004 of all Ro/La mothers. Mechanical PR interval was measured from the onset of the mitral A wave Doppler to the onset of aortic ejection at the level of the left ventricular inflow and outflow. Twenty-three fetuses had non-invasive fetal ECG (fECG) and mechanical and electrical fetal PR intervals were compared with normal reference ranges. Ninety-one mechanical PR intervals were measured in 42 Ro/La women. Twenty-three also had 35 fECG. Six had mechanical PR interval >95th centile for gestation, three of whom developed CHB. The sensitivity of mechanical PR interval to detect heart block was 75% and the specificity was 92%, with a false positive rate of 8%. One fetus showed normal mechanical PR interval at 22 weeks but initial fECG PR interval was >95th centile and progressed to CHB in spite of immediate steroid therapy. Measurement of mechanical PR interval is moderately valuable in assessment of this high-risk population but fetal ECG has unmasked considerable variability in PR interval in those studied serially and is likely to be the more useful tool to permit a therapeutic trial of steroid efficacy in preventing progression to CHB.