Reference Curve for the Mean Uterine Artery Pulsatility Index in Singleton Pregnancies

Abstract

Introduction: Doppler sonography of the uterine artery (UA) is done to monitor pregnancies, because the detected flow patterns are useful to draw inferences about possible disorders of trophoblast invasion. Increased resistance in the UA is associated with an increased risk of preeclampsia and/or intrauterine growth restriction (IUGR) and perinatal mortality. In the absence of standardized figures, the normal ranges of the various available reference curves sometimes differ quite substantially from one another. The causes for this are differences in the flow patterns of the UA depending on the position of the pulsed Doppler gates as well as branching of the UA. Because of the discrepancies between the different reference curves and the practical problems this poses for guideline recommendations, we thought it would be useful to create our own reference curves for Doppler measurements of the UA obtained from a singleton cohort under standardized conditions.

Material and methods: This retrospective cohort study was carried out in the Department of Obstetrics of the Charité - Universitätsmedizin Berlin, the Department for Obstetrics and Prenatal Medicine of the University Hospital Halle (Saale) and the Center for Prenatal Diagnostics and Human Genetics Kurfürstendamm 199. Available datasets from the three study locations were identified and reference curves were generated using the LMS method. Measured values were correlated with age of gestation, and a cubic model and Box-Cox power transformation (L), the median (M) and the coefficient of variation (S) were used to smooth the curves.

Results: 103 720 Doppler examinations of the UA carried out in singleton pregnancies from the 11th week of gestation (10 + 1 GW) were analyzed. The mean pulsatility index (Mean PI) showed a continuous decline over the course of pregnancy, dropping to a plateau of around 0.84 between the 23rd and 27th GW, after which it decreased again.

Conclusion: Age of gestation, placental position, position of pulsed Doppler gates and branching of the UA can all change the flow pattern. The mean pulsatility index (Mean PI) showed a continuous decrease over time. There were significant differences between our data and alternative reference curves. A system of classifying Doppler studies and a reference curve adapted to the current technology are urgently required to differentiate better between physiological and pathological findings.

Keywords: Doppler; growth retardation; placenta; preeclampsia; ultrasound; uterine artery.

Fig. 1

Correct positioning of the pulsed Doppler gates on the UA proximal to crossing the external iliac artery.

Fig. 2

Dot plot of measurements used to calculate Mean PI according to the age of gestation.

Fig. 3

5th, 50th and 95th percentiles of the mean pulsatility index of the uterine arteries (Mean PI, mean of the right and left uterine arteries) together with the formula for the linear trend which corresponds to a weekly decrease in the Mean PI of about 0.026.

Fig. 4

Comparisons of the 5th, 50th and 95th percentiles of the reference curve of Gomez et al. with our reference curve over the course of pregnancy. Up until the 24th week of pregnancy, our figures are lower than those of the Gomez curve, even though measurements for the Gomez curve were obtained distal to the crossing of the arteries and values should therefore be lower. After about the 24th GW, the 95th percentile of our data is above the 95th percentile of the Gomez curve, which would appear to indicate a broader distribution.

Fig. 5

Distribution of mean pulsatility indices (Mean PI) between the 24th and 32nd week of gestation. 75% of measured PI values were less than 0.98.

Fig. 6

Distribution of mean pulsatility indices (Mean PI) between the 22nd and the 24th week of gestation. Compared to the Papageorghiou curve (median: 1.04; 95th percentile: 1.63) our figures showed, as expected, a shift to lower mean pulsatility indices between the 22nd and 24th GW (median 0.84; 95th percentile 1.34)

Abb. 1

Korrekte Positionierung des Pulsed-Doppler-Gates auf der AU proximal der Überkreuzung der A. iliaca externa.

Abb. 2

Dotplot der Messungen des Mean PI in Abhängigkeit des Gestationsalters.

Abb. 3

Darstellung der 5., 50. und 95. Perzentile des mittleren Pulsatilitätsindex der Aa. uterinae (Mean PI, Mittel aus rechter und linker A. uterina) sowie Formel der Trendgerade, was einem wöchentlichen Abfall des mittleren PI von ca. 0,026 entspricht.

Abb. 4

Vergleich der 5., 50. und 95. Perzentile der Referenzkurve von Gomez et al. mit der Referenzkurve unserer Daten im Schwangerschaftsverlauf. Unsere Daten liegen bis zur 24. SSW unter denen der Gomez-Kurve, obwohl diese distal der Überkreuzung abgeleitet wurden und daher niedrigere Werte zeigen sollten. Die 95. Perzentile unserer Daten liegt ab ca. der 24. SSW oberhalb der 95. Perzentile der Gomez-Kurve, was für eine breitere Streuung spricht.

Abb. 5

Verteilung der mittleren Pulsatilitätsindizes (Mean PI) zwischen 24 und 32 SSW. 75% der Messungen liegen unter einem PI 0,98.

Abb. 6

Verteilung der mittleren Pulsatilitätsindizes (Mean PI) zwischen 22 und 24 SSW. Im Vergleich zu der Kurve von Papageorghiou (Median: 1,04; 95. Perzentile: 1,63) zeigen unsere Werte erwartungsgemäß eine Verschiebung zu niedrigeren mittleren Pulsatilitätsindizes zwischen der 22. und 24. SSW (Median 0,84; 95. Perzentile 1,34).