Gestational hypertension (GHTN) and PE are placental-derived diseases[3]. When a pregnant woman suffers from HDP, placental function decreases[9].The placenta plays a very important role during pregnancy. In the past, we evaluated placental function by observing the maturity of the placenta and measuring various blood flow parameters of the fetus[10].SWE is a new non-invasive imaging technique that can assess the stiffness of soft tissues.In recent years, scholars have begun to study elastography of the placenta. Cimsit[11]performs SWE on the placenta of singleton pregnancy from 20 to 23 weeks of gestation. They found that the placental elasticity value of the early-onset preeclampsia group was significantly higher than that of the normal pregnancy group, and they believed that SWE could be used to evaluate the function of the placenta.
Our research shows that the placenta Young's modulus of pregnant women with HDP(gestational hypertension and PE) is higher than that of normal pregnant women, which is consistent with the results of Karaman[12].We speculate that this may be related to the pathophysiology and pathological changes of the placenta[13].The inadequate invasion of trophoblast leads to the lack of vascular remodeling of maternal spiral artery, which leads to high-pressure and low flow placental unit, while the placental blood supply of normal pregnancy should be the opposite[14].The pathology of HDP placenta showed that syncytial cell nodules, trophoblast proliferation, fibrin like deposition and infarction were higher than those of normal pregnancy placenta, and there were changes such as inflammatory infiltration, arteriosclerosis and villous thrombosis. These are consistent with placental hypoperfusion and ischemia and appear to be related to the severity of HDP[15–16]. Sugitani[17] et al. showed that when there is extensive infarction and inflammation in the placenta, the Young's modulus value increases. Therefore, we believe that the placental pathology and pathophysiological changes of the placenta of hypertensive disorder of pregnancy lead to an increase in placental hardness and an increase in its Young's modulus.
Reduced placental blood perfusion and placental infarction in patients with HDP will reduce the effective exchange area of the placenta, Reduced placental blood perfusion and placental infarction in patients with hypertension during pregnancy will reduce the effective exchange area of the placenta, and its placental function will inevitably be affected. The aging of placental function is pathologically manifested as increased placental stiffness, syncytiotrophoblast number, fibrinoid areas[18]. We therefore speculate that the increased hardness of the placenta is related to decreased placental function. Therefore, we believe that ultrasonic shear wave elastography can be used as a way to reflect the function of the placenta.
Preeclampsia is more severe than pregnancy-induced hypertension In this study, the placental Young's modulus of pregnant women with preeclampsia was higher than that of pregnant women with pregnancy-induced hypertension, and our results showed that the placental Young's modulus had a good correlation with the severity of the disease. We believe that the use of shear wave elastography to detect the hardness of the placenta can evaluate the severity of hypertension in pregnancy. The reason may be due to the decrease of placental blood perfusion in preeclampsia, placental infarction, fibrin deposition and other changes are more significant than pregnancy-induced hypertension. Alan[19] et al. measured the shear wave value of the placenta of mild preeclampsia and severe preeclampsia by acoustic radiation pulse elastography. They found that there is a significant difference between the shear wave value of the placenta and the severity of preeclampsia. Relevance, and think that elastography can be used as a supplementary method to assess the severity of preeclampsia. The results of Alan's research are similar to ours.
In this study, the umbilical artery blood flow parameters of most fetuses with hypertension in pregnancy remained within the normal range, so there is no significant correlation between the placental Young's modulus and the fetal umbilical artery blood flow parameters. The reason may be that there is compensation between the mother and the fetus. When the condition is mild, the blood flow of the umbilical artery is not obvious. And because the pregnant women with HDP in this study have been actively treated, the perfusion of the placenta has been significantly improved.
The TI and MI of the device program mode used in this study are within the safety range of the obstetric ultrasound output level recommended by the American Ultrasonic Medical Association (Ti ≤ 0.7, MI ≤ 1.9)[20] .Kılıç[21] also showed that the thermal index and mechanical index of swe were within the safety limit of fetal evaluation. The irradiation dose of ultrasound should consider not only the sound intensity, but also the influence of examination time. Swe ensures the time interval between two measurements by emitting low-density sound radiation force intermittently, avoiding the risk of long-term irradiation to the placenta. In this study, the TI for SWE is only 0.2.Therefore, it is safe to detect placenta by SWE.
Due to the limited depth of SWE, the posterior placenta and lateral placenta cannot be completely detected to know the average SWE value of placenta. Therefore, only the anterior placenta was included in this study.