Evolution of the Fetal Atrioventricular Interval from 6 to 40 Weeks of Gestation

Doppler-based methods of estimating the atrioventricular interval are commonly used as a surrogate for the electrical PR in fetuses at risk of conduction abnormalities; however, to date, normal values for the fetal atrioventricular interval and an understanding of the evolution of its components in the late first trimester are lacking. We sought to investigate changes in the fetal atrioventricular interval from the first trimester to 40 weeks gestational age, and to explore functional and electrophysiological events that potentially impact its evolution. We prospectively examined healthy pregnancies by fetal echocardiography from 6 to 40 weeks' gestational age. The atrioventricular interval, heart rate, isovolumic contraction time, and A-wave duration were measured from simultaneous ventricular inflow-outflow Doppler tracings. Regression analysis was used to examine relations with gestational age, and linear relations with heart rate were assessed by Pearson's correlation coefficient. Data were collected in 305 fetuses from 279 pregnancies. Atrioventricular interval demonstrated an inverse relation with heart rate (r = -0.45, p <0.0001), dramatically decreasing before 10 weeks and slowly increasing thereafter. Between 6 and 9 weeks, isovolumic contraction time acutely decreased approaching 0, thereafter minimally increasing to term. In contrast, from 6 weeks, the A-wave duration linearly increased through gestation, and negatively correlated with heart rate (r = -0.62, p <0.0001). In conclusion, we have established normal measures of the atrioventricular interval from 6 to 40 weeks' gestational age. Before 10 weeks, a prolonged atrioventricular interval in healthy fetuses largely reflects the lengthened isovolumic contraction time which is likely influenced by the evolution of ventricular function and afterload.