Doppler Velocimetry of the Uteroplacental Circulation

Chapter 16 Doppler Velocimetry of the Uteroplacental Circulation Edwin R. Guzman, Eftichia Kontopoulos, Ivica Zalud Introduction blood flow at term was estimated at 500±700 ml/min with a two- to threefold decrease in uteroplacental Since the last edition of this volume, interest has con- perfusion noted in the presence of preeclampsia. tinued in uterine artery Doppler velocimetry (UADV) Uterine artery Doppler velocimetry was first re- as a screening technique to predict adverse pregnancy ported by Campbell and colleagues in 1983 [10]. They outcomes such as preeclampsia, for pregnancy risk showed that, compared to pregnancies with normal scoring, and as an entry criterion for randomized uterine artery waveforms, pregnancies with abnormal trials on medical therapies for the prevention of pre- uterine artery Doppler waveforms were associated with eclampsia and intrauterine growth restriction. These more proteinuric hypertension, required more antihy- areas have been updated and UADV for predicting pertensive therapy, and resulted in lower birth weights pregnancy outcome in medical conditions other than and younger gestational ages at birth. Thus the capabil- preeclampsia has been added. The areas regarding ity for this potentially safe, noninvasive, prospective non-Doppler assessment of uterine artery flow, phys- means of analyzing uterine artery blood flow during iology and development of uterine artery flow, and pregnancy was realized and set off a wave of interest the development of uterine artery Doppler waveform and research that has continued until today. remain essential and have been retained for historical purposes. Anatomy of Uterine Circulation Non-Doppler Methods The uterine artery originates from the internal iliac to Measure Uterine Artery Blood Flow artery and meets the uterus just above the cervix. The main uterine artery branches into the arcuate ar- Methods developed to measure uteroplacental blood teries, which arch anteriorly and posteriorly and ex- flow in humans, such as the nitrous oxide technique tend inward for about one-third of the thickness of based on the Fick principle [1, 2], clearance of radio- the myometrium (Fig. 16.1). They are tortuous and active 24N [3±6], and the flow probe technique [7], vary in thickness and in the area they supply. The are obviously not suitable for human application. arcuate artery network anastomoses near the midline They suffer from various limitations, such as ques- [11]. The radial arteries arise from this network, are tionable accuracy, invasiveness, requirement for ra- directed toward the uterine cavity, and become spiral diation, and being unsuitable for longitudinal obser- arteries when they enter the endometrium. vations. Once it was realized that the placenta must There are a variety of arterial anastomoses of the utilize maternal dehydroisoandrosterone sulfate, testo- human uterine circulation that have been demon- sterone, and androstenedione in the intervillous space strated by anatomic and radiographic studies and for the synthesis of estradiol, a technique that mea- uterine perfusion experiments [11±14]. Ipsilateral sured the rate of estradiol synthesis was developed to connections between uterine and ovarian arteries assess intervillous space perfusion in human preg- have been demonstrated. Also identified are contra- nancies [8, 9]. This technique was tedious, required lateral anastomoses between the right and left uterine sophisticated laboratory equipment (which made it arteries and their branches. The uterine circulation is expensive), made use of isotope-labeled prehormone, also connected to the systemic circulation, for exam- and proved to be of questionable accuracy. Its clinical ple the inferior mesenteric, middle sacral, and inferi- applicability is limited. Nonetheless, when all these or epigastric arteries. During pregnancy these anasto- techniques were applied to human pregnancies they motic connections can increase in size and function had surprisingly similar results. Normal uterine after occlusion of major vessels. 228 E.R. Guzman et al. Fig. 16.1. Anatomy of the arcuate, radial, and spiral arteries during pregnancy. (From [112] with permission) Normal Growth and Development teries. Once again the trophoblasts replace the endo- of Uteroplacental Circulation thelium and establish themselves in the muscular media. The elastic and muscular tissue of the myome- During Pregnancy trial segments of the spiral arteries is gradually lost and replaced with fibrinoid material (Fig. 16.2). This During the first 12 weeks of pregnancy cytotropho- condition, along with the increase in blood flow and blasts invade the spiral arterial walls in the decidua the associated hemodynamic forces, converts the en- and replace the endothelium and muscular media tire length of the spiral arteries from small muscular with a matrix of cytotrophoblasts and fibrinoid and arteries to dilated, tortuous uteroplacental vessels fibrous tissue [15, 16]. The fibrinoid material is a (Fig. 16.3). At term these changes can be seen at the complex of maternal fibrin and other plasma constit- distal portions of the radial arteries. The mean exter- uents plus proteinaceous material derived from the nal diameter of the myometrial segments of the spiral trophoblastic cells. Beginning at about 12 weeks' ge- arteries is approximately 500 lm, an increase from station and continuing throughout the remainder of 200±300 lm in the nonpregnant state. The small mus- the second trimester, the endovascular trophoblasts cular arteries that branch off the radial and spiral ar- move into the myometrial segments of the spiral arteries, the basal arteries, do not undergo these Fig. 16.2. Normal pregnancy. Spiral artery at the myometrial junction shows extensive structural alterations (physiologic changes). (Reprinted from [15] with permission) a Chapter 16 Doppler Velocimetry of the Uteroplacental Circulation 229 Fig. 16.3. Fully developed physiologic changes in the uteoplacental arteries during normal pregnancy. Hatched portions of the wall of these vessels indi- cate the extent of the physiologic changes. I.V.S. intrvillous space. (From [113] with permission) changes. In all, approximately 100±150 converted Abnormal Development spiral arteries supply the placental bed. It is believed of the Uteroplacental Circulation that these uteroplacental vessels have lost their ability to respond to vasoactive substances. in the Presence of Essential The question arises whether this change from Hypertension, Preeclampsia, small muscular arteries to dilated tortuous vessels is and Intrauterine Growth Restriction entirely responsible for the increase in flow from 100 ml/min to 500±800 ml/min. All large vessels of According to Brosens et al. [15], Robertson et al. the pregnant uterus, regardless of whether they sup- [16], and Khong et al. [18], a lack of endovascular in- ply the placental bed, undergo hyperplasia and hy- filtration by trophoblasts into the myometrial portion pertrophy [17]. Thus the increase in cross-sectional of the placental bed spiral arteries is a consistent area leads to a reduction in resistance and further de- finding in the presence of preeclampsia. The physio- velopment of the uterine circulation. This increase in logic changes of the placental bed spiral arteries luminal diameter of the large vessels probably ac- extend only to the deciduomyometrial junction counts for the increase in uteroplacental flow during (Fig. 16.4). With preeclampsia the spiral arteries may the third trimester. In addition, the effects of the hor- remain unconverted throughout their decidual and mones estrogen and progesterone, the increased myometrial length (Fig. 16.5). Thus there is both an blood volume and cardiac output, the diminished incompleteness in the degree of endovascular tropho- blood viscosity, and the lowered peripheral resistance blastic invasion of the spiral arteries, being confined influence uterine flow. to the decidual portion, and a reduction in the num- ber of uteroplacental arteries formed. The diameter of the spiral arteries remains at 200±300 lm. Sheppard and Bonnar [19] reported the picture not to be so clear-cut, as they observed physiologic changes in the myometrial spiral arteries in preeclamptic pregnan- Fig. 16.4. Difference between normal and preeclamptic pregnancies regarding the extent of physiologic changes in the uteroplacental arteries. With preeclampsia these changes do not extent beyond the deciduomyometrial junction. (From [113] with permission) 230 E.R. Guzman et al. Fig. 16.5. Preeclampsia. Myometrial spiral artery is unaffected by physiologic changes and retains a normal internal elastic lamina. (Reprinted from [15] with permission) cies. What has not been studied is whether the large correlate with the severity and duration of the hyper- vessels in the uterine artery circulation in the pres- tension. This lesion is not seen with preeclampsia ence of preeclampsia undergo the same degree of hy- unless there is a history of essential hypertension perplasia as is seen in normotensive pregnancies. (Fig. 16.7). When essential hypertension is compli- Another placental bed lesion seen with preclamp- cated by superimposed preeclampsia, both acute sia is an acute arteriopathy, termed acute atherosis by atherosis and hyperplastic arteriosclerosis are seen Zeek and Assali [20]. Here the wall of the spiral ar- [15, 16]. tery shows fibrinoid necrosis with lipophages, and In normotensive pregnancies resulting in intrauter- there is a mononuclear cellular infiltrate

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