Resistance index ri normal value

The resistance index ri is an examination item that can often be heard during prenatal check-ups. Most female friends should do this examination as soon as they confirm that they are pregnant, so that they can know whether the development of the fetus in their abdomen is normal. Generally, if the resistance index ri is not in the normal value, it may be because the development of the fetus has become slow, or it may be caused by other reasons.

The purpose of checking the umbilical cord blood flow is to determine the development of the fetus in the uterus, such as whether there is intrauterine growth retardation, whether there is a tendency to develop pregnancy-induced hypertension syndrome, and whether there is intrauterine fetal hypoxia. Umbilical cord blood flow may sometimes show abnormalities due to fetal chromosomal abnormalities, congenital malformations, etc. Abnormal umbilical cord blood flow may also be related to developmental defects and histological abnormalities of the placenta. The normal value is related to the number of weeks of pregnancy. There are three main items, with the S/D value as the main indicator. The normal value is usually an S/D value of less than or equal to 3.0 in the late pregnancy.

Umbilical cord blood flow is closely related to gestational age. Generally, as gestational age increases, umbilical cord blood flow shows a downward trend. The above table shows the umbilical cord blood flow parameters of full-term pregnant women awaiting delivery, which are divided into normal, loosely coiled and tightly coiled types. For the tight-wrap type, cesarean section is recommended to avoid suffocation during delivery.

The normal value of umbilical cord blood flow at 24 weeks is an average of 3.5, with an upper limit of 4.25. Values ​​exceeding this value are abnormal. The pulsatility index (PI) is 1.12±0.17, and the resistance index (RI) is 0.66±0.07. After 30 weeks of pregnancy, when the SV/DV of the umbilical artery is >3.0, PI>1.7, and RI>0.7, fetal distress often occurs.

The peak value of arterial blood flow during systole (A or S) and the trough value during end-diastole (B or D) are used to calculate the S/D or A/B ratio. The resistance index (RI=AB/A) and pulsatility index (PI=AB/mean A, B) can also be calculated. In normal pregnancy, these values ​​decrease with gestational age. Usually, A/B>4 before 24 weeks, A/B less than 3.0 after 30 weeks, and RI less than 0.68

The ratio of the maximum systolic blood flow velocity (S) to the end diastolic blood flow velocity (D) of the umbilical artery blood flow (S/D, A/B) can be used to judge the development of the placenta by observing the dynamic changes of the S/D (A/B) ratio. Under normal circumstances, the placenta gradually increases in size as the fetus develops. Vascular resistance gradually decreases, and the S/D (A/B) ratio gradually decreases. From 25 to 41 weeks of pregnancy, the S/D ratio decreased almost linearly from 2.8 to 2.2. The fetus has intrauterine growth retardation, the placenta and fetus are poorly developed, the placenta does not enlarge, and the vascular resistance does not decrease. Therefore, the S/D ratio does not decrease. Clinically, if S/D does not decrease regularly or even increases, it indicates that the fetus is maldeveloped.

In normal pregnancy, the S/a ratio of the ductus venosus decreases with gestational age, from about 3 at 14 weeks of gestation to about 2 at 42 weeks of gestation.

The S/a ratio of the ductus venosus is associated with fetal hypoxemia.

Umbilical artery values ​​(see figure Umbilical artery impedance) reflect oxygen delivery to the placenta. If this value rises abnormally, the main consequence is fetal intrauterine hypoxia.