Development of a Z-score for the ductal flow pulsatility index in normal fetuses using two-dimensional Doppler echocardiography

OBJECTIVES: To determine gestational age-specific reference ranges and develop a Z-score equation for the fetal ductal pulsatility index (PI), between 24 and 33 + 6 weeks of gestation. METHODS: A prospective cross-sectional study conducted from 2023 to 2024, 226 singleton fetuses between 24 and 33 + 6 weeks were enrolled. The ductus arteriosus (DA) was assessed in the longitudinal plane of the fetus using color Doppler to confirm its course and flow direction, and pulsed Doppler sampling was placed at the proximal DA with an insonation angle <15°. Three consecutive measurements of PI were recorded and averaged. Z-scores were derived from predicted means and standard deviations and reference percentiles (5th, 50th, and 95th) were computed for gestational age groups using quantile regression. Intraobserver reproducibility was evaluated in 30 randomly selected cases with an interval ≥2 weeks using the intraclass correlation coefficient (ICC). RESULTS: The mean gestational age was 28.2 ± 3.1 weeks. The ductal PI showed a mean of 2.4 ± 0.26 with normal distribution (p=0.2525). The Z-score equation for PI was successfully derived Z-score = (Measured PI - 2.4)/0.26. Reference ranges for PI were: 24-27 weeks (5th=2.0; 50th=2.4; 95th=2.7), 28-31 weeks (5th=2.1; 50th=2.5; 95th=2.9), and 32-34 weeks (5th=2.2; 50th=2.4; 95th=2.5). Intraobserver reproducibility for PI was excellent (ICC >0.90). CONCLUSIONS: We established gestational age-specific reference ranges and a Z-score equation for the ductal PI in normal fetuses between 24 and 33 + 6 weeks. These findings provide a standardized tool for fetal cardiologists to identify subtle ductal flow abnormalities and support perinatal risk stratification.