Uterine artery Doppler blood flow resistance indices in the second half of pregnancy: measures of central tendency, distribution, inter-ratio dependency, and explanation of variation.

Study objectives: To evaluate the three commonly used indices of Doppler blood flow resistance in clinical practice for the uterine artery. By assessing the measures of central tendency, dispersion, and variation, to determine the most appropriate index for clinical use transabdominally in the second half of pregnancy. Methods: A prospective study of uterine artery Doppler measurement in singleton pregnancies between 18 and 32 weeks gestation. Using previously described methods, two independent measurements were obtained transabdominally from each side in all subjects. The systolic/diastolic ratio (SD ratio), pulsatility index (PI), and resistance index (RI) were recorded from both the right and left uterine artery in turn. All measurements were obtained by a single operator (MOT) using the same machine (Philips SD 800). The results were analyzed statistically for the measures of dispersion, distribution, and the variation explained. Results: There were 1796 observations from 449 subjects in total, with the SD ratio showing the widest dispersion. The PI measurements were intermediate with respect to both, while the RI showed the least dispersion. The RI distribution was the closest to a naturally distributed (Gaussian) pattern, followed by the PI. The SD ratio did not appear to conform to a normal distribution. The interquartile ranges (IRQ) for the SD ratio were 0.71, 0.73 (left- and right-uterine artery, respectively); PI: 0.42, 0.41; and RI: 0.15, 0.17. The mean of three derived measurements of the RI was sufficient to account for over 99% of the observed variation. Conclusions: Differences reported between the different resistance and pulsatility Doppler indices could be due to either random or systematic error, or a combination of both. The observations from this study suggest that when the RI is used, these are kept to a minimum. These findings may be helpful in selecting the appropriate index for uterine artery Doppler screening and clinical applications... [ABSTRACT FROM AUTHOR]