The Fetal Doppler Mechanical PR Interval: A Validation Study

Abstract

Objective: To evaluate the accuracy of pulsed Doppler-derived fetal PR interval measurements obtained by physicians participating in a multicenter prospective fetal echocardiographic study. Methods: Echocardiograms on healthy fetuses were performed and evaluated by 15 pediatric cardiologists/perinatologists across the United States who are participating in a larger clinical trial involving fetuses at risk for autoantibody-associated congenital heart block. Prior to enrolling women in the main trial, each physician was provided with a teaching tape to demonstrate how the pulsed Doppler-derived PR interval is measured. The procedure involves placing a gated pulsed Doppler sample volume in the left ventricle at the junction of the anterior leaflet of the mitral valve and the left ventricular outflow tract in an apical 5-chamber view, and simultaneously obtaining left ventricular filling and emptying. Time intervals are measured from the onset of the mitral A wave (atrial systole) to the onset of the aortic pulsed Doppler tracing (ventricular systole). This represents the mechanical PR interval. Each physician measured the pulsed Doppler-derived fetal PR interval on 5 different subjects recruited from the physician’s specific site. To validate each physician’s technique, the tapes were sent to a central facility and the same intervals were remeasured by an experienced central reader (D.M.F.). A physician was determined to have adequate ability to measure the fetal PR interval if all 5 measurements were within ± 30 ms of the central reader’s measurements, where 30 ms corresponds to 25% of the mean observed in normative PR interval data. This difference was deemed to be the minimum clinically important difference in Doppler PR interval. Results: Fourteen of the 15 physicians were considered to have adequate ability to measure the fetal PR interval according to our established criterion. The overall mean difference between the physicians and the central reader’s measurements was –0.26 ± 11.04 ms (p = 0.84). In addition, 95% of the observed differences were included in the interval (–22.23 to 21.81), which is well within our clinically acceptable range of ± 30 ms. Conclusions: The pulsed Doppler assessment of the mechanical PR interval in the fetus can be accurately performed after minimal training. This technique may be a valuable tool for identification of early and potentially reversible conduction abnormalities in fetuses at risk for more advanced and permanent forms of heart block associated with maternal antibodies to SSA/Ro-SSB/La.