Institute of Medical Science, University of Toronto, Toronto, Ontario, M5S 1A8, Canada.
Division of Cardiology, The Hospital for Sick Children, Toronto, Ontario, M5G 1X8, Canada.
J Physiol. 2020 Aug;598(15):3259-3281. doi: 10.1113/JP279725. Epub 2020 May 29.
Human fetal Doppler ultrasound and invasive blood gas measurements obtained by cordocentesis or at the time of delivery reveal similarities with sheep (an extensively used model for human fetal cardiovascular physiology). Oxygen saturation (SO ) measurements in human fetuses have been limited to the umbilical and scalp vessels, providing little information about normal regional SO differences in the fetus. Blood T2 MRI relaxometry presents a non-invasive measure of SO in the major fetal vessels. This study presents the first in vivo validation of fetal vessel T2 oximetry against the in vitro T2-SO relationship using catheterized sheep fetuses and compares the normal SO in the major vessels between the human and sheep fetal circulations. Human fetal vessel SO by T2 MRI confirms many similarities with the sheep fetal circulation and is able to demonstrate regional differences in SO ; in particular the significantly higher SO in the left versus right heart.
Blood T2 magnetic resonance imaging (MRI) relaxometry non-invasively measures oxygen saturation (SO ) in major vessels but has not been validated in fetuses in vivo. We compared the blood T2-SO relationship in vitro (tubes) and in vivo (vessels) in sheep, and measured SO across the normal human and sheep fetal circulations by T2. Singleton pregnant ewes underwent surgery to implant vascular catheters. In vitro and in vivo sheep blood T2 measurements were related to corresponding SO measured using a blood gas analyser, as well as relating T2 and SO of human fetal blood in vitro. MRI oximetry was performed in the major vessels of 30 human fetuses at 36 weeks (term, 40 weeks) and 10 fetal sheep (125 days; term, 150 days). The fidelity of in vivo fetal T2 oximetry was confirmed through comparison of in vitro and in vivo sheep blood T2-SO relationships (P = 0.1). SO was similar between human and sheep fetuses, as was the fetal oxygen extraction fraction (human, 33 ± 11%; sheep, 34 ± 7%; P = 0.798). The presence of streaming in the human fetal circulation was demonstrated by the SO gradient between the ascending aorta (68 ± 10%) and the main pulmonary artery (49 ± 9%; P < 0.001). Human and sheep fetal vessel MRI oximetry based on T2 is a validated approach that confirms the presence of streaming of umbilical venous blood towards the heart and brain. Streaming is important in ensuring oxygen delivery to these organs and its disruption may have important implications for organ development, especially in conditions such as congenital heart disease and fetal growth restriction.
人类胎儿多普勒超声和通过脐带穿刺或分娩时获得的侵入性血气测量结果与绵羊（广泛用于人类胎儿心血管生理学的模型）相似。人类胎儿的氧饱和度 (SO ) 测量仅限于脐部和头皮血管，对胎儿正常区域 SO 差异的信息提供很少。血液 T2 MRI 弛豫测量法提供了一种非侵入性的胎儿主要血管 SO 测量方法。本研究首次使用导管化绵羊胎儿对胎儿血管 T2 血氧计进行了体内验证，并比较了人类和绵羊胎儿循环中主要血管的正常 SO 。胎儿 T2 MRI 血管 SO 证实与绵羊胎儿循环有许多相似之处，并且能够显示 SO 的区域差异；特别是左心与右心之间的 SO 显著升高。
血液 T2 磁共振成像 (MRI) 弛豫测量法可无创测量主要血管中的氧饱和度 (SO )，但尚未在体内胎儿中得到验证。我们比较了绵羊的离体（管）和体内（血管）血 T2-SO 关系，并通过 T2 测量正常的人类和绵羊胎儿循环中的 SO 。妊娠单胎母羊接受了植入血管导管的手术。离体和体内绵羊血 T2 测量与使用血气分析仪测量的相应 SO 相关，以及与离体人胎儿血的 T2 和 SO 相关。对 30 名 36 周（足月，40 周）和 10 名胎儿绵羊（125 天；足月，150 天）的主要血管进行了 MRI 血氧计检查。通过比较离体和体内绵羊血 T2-SO 关系，证实了体内胎儿 T2 血氧计的准确性（P=0.1）。人类和绵羊胎儿的 SO 相似，胎儿氧摄取分数也相似（人类，33±11%；绵羊，34±7%；P=0.798）。通过升主动脉（68±10%）和主肺动脉（49±9%；P<0.001）之间的 SO 梯度，证明了人类胎儿循环中的血流存在。基于 T2 的人类和绵羊胎儿血管 MRI 血氧计是一种经过验证的方法，证实了脐静脉血流向心脏和大脑的流动存在。这种流动对于确保向这些器官输送氧气很重要，其中断可能对器官发育有重要影响，特别是在先天性心脏病和胎儿生长受限等情况下。
