Validation of the Doppler PR interval in the fetus.

Fetal congenital heart is thought to be an inflammatory disease that is mediated by maternal antiSSA/Ro and anti-SSB/La antibodies, and that is characterized by injury to the atrioventricular node. Several treatment strategies have been proposed for the fetus with congenital heart block, including maternal steroid administration. When therapy is initiated after the onset of third-degree heart block, conduction remains abnormal, but therapy during second-degree heart block has been associated with reversion to first-degree block at birth. Early detection of cardiac abnormalities in a fetus at risk for congenital heart block has been described with the use of echocardiography. In this case, the mother was a 35-year-old gravida 2, para 1 with Sjogren syndrome. She had 1 prior child with congenital heart block and was under intensive echocardiographic surveillance according to a previously published protocol. At 21 weeks gestation, the fetus developed pericardial effusion, decreased left ventricular function, and premature atrial contractions. Dexamethosone therapy was initiated after these findings were obtained, at a dose of 4 mg once daily given orally to the mother. Recently, Glickstein and colleagues described the use of a new echocardiographic measurement, the mechanical PR interval, to assess the fetal conduction system. They recently published normative data for fetuses based on the measurement from the onset of the mitral A wave to the onset of aortic flow as estimated by pulsed wave Doppler in normal fetal echocardiograms. The normal fetal Doppler mechanical PR interval was less than 150 ms. Importantly, this measurement has not been validated in fetuses with a conduction abnormality. In this report, we describe the retrospective application of the mechanical PR interval to the case described previously. All fetal echocardiograms were performed with commercial echocardiography equipment (Acuson Corp, Mountain View, Calif). Doppler tracings were obtained and recorded onto S-VHS tape at a sweep speed of 100 mm/s. Serial PR data are shown in Figure 1. This demonstrates an acute rise in the PR interval from 124 ms at 20 weeks gestation to an abnormally prolonged value of 194 ms at 21 weeks gestation, which was sustained in the following week, and was then followed by a decrease in the PR interval corresponding with the onset of dexamethasone therapy. At 1 week of age, the PR interval determined by electrocardiogram was longer than the mechanical PR interval that had been previously determined at 26 weeks gestation (160 ms vs 139 ms), but was in a similar range. This is the first documentation of the use of the mechanical PR interval in a fetus with confirmed conduction abnormality. The sequential changes we observed validate the basic premise of the mechanical PR interval, even if the actual measurement differed somewhat from the electrocardiogram obtained after delivery (approximately 10 weeks later). There has been considerable discussion of the use of maternal steroids in utero for the prevention/reversal of congenital heart block. However, if therapy is begun after the onset of third-degree heart block, the likelihood is high that conduction abnormalities will persist. Thus, an early marker of cardiac conduction abnormalities is highly desirable to begin early, targeted therapy. The mechanical PR interval may be the appropriate tool to determine this. A prospective, observational, multicenter trial to evaluate the use of the mechanical PR interval in fetuses at risk of congenital heart block is currently ongoing to answer this important question (the PRIDE [PR Interval and Dexamethasone Evaluation] Study of Congenital Heart Block, supported by National Institute of Arthritis and Musculoskeletal and Skin Diseases grant AR46256).