Reference ranges for Doppler parameters of the fetal aortic isthmus during the second half of pregnancy

To establish reference ranges for blood flow velocity waveforms (FVW) of the fetal aortic isthmus (AoI) during the second and third trimesters of pregnancy.

This was a prospective cross-sectional observational study involving 458 uncomplicated singleton pregnancies between 19 and 37 weeks of gestation. Fetal AoI Doppler parameters were assessed in either the longitudinal aortic arch view or the three vessels and trachea view. Regression analysis was used to determine gestational-age-specific reference ranges and to construct nomograms for the following Doppler parameters: pulsatility index (PI), resistance index (RI) and peak systolic (PSV), end-diastolic (EDV) and time-averaged maximum (TAMXV) velocities. Intra- and interobserver reproducibility were evaluated by calculating intraclass correlation coefficients (ICCs) and limits of agreement.

Reliable FVW in the AoI were obtained in all cases. Acceptable intra- and interobserver reproducibility was obtained. With advancing gestation, there was a significant increase in PSV, TAMXV and PI, whereas RI and EDV remained constant during the second half of pregnancy. No cases of absent or reversed flow during diastole were detected.

Normal data of the fetal AoI blood FVW throughout the second and third trimesters of pregnancy are provided. The reported Doppler profiles may be of clinical use in the assessment of hemodynamically compromised growth-restricted fetuses.