J.W. Knox Ritchie

Effect of placental embolization on the umbilical arterial velocity waveform in fetal sheep.

Absent diastolic velocity and reversed diastolic velocity have been observed in the umbilical artery of growth-retarded human fetuses by means of Doppler ultrasonography. Because the physiologic mechanism is unknown, the purpose of this study was to determine whether such waveform changes could be induced in sheep fetuses by embolization of the resistance vessels of the placenta. Seven chronically catheterized sheep fetuses were instrumented with Doppler crystals mounted on one umbilical artery. The placenta was progressively embolized from the fetal side with plastic microspheres. In all cases the umbilical arterial waveform showed a progression from normal to zero diastolic velocity with embolization. In six of seven animals diastolic velocity eventually reversed in direction before fetal death. We conclude that occlusion of placental arteries and arterioles in fetal sheep recreates the waveform changes observed in severely growth-retarded human fetuses.

Localization of two angiogenic growth factors (PDECGF and VEGF) in human placentae throughout gestation

The spatiotemporal distribution of two angiogenic growth factors, platelet-derived endothelial cell growth factor (PDECGF) and vascular endothelial growth factor (VEGF) were determined using immunohistochemistry on sections of human placentae from each trimester of pregnancy. In the first trimester PDECGF was detected in trophoblast and in a band in the centre of the villous core. During gestation staining spread throughout the stroma but began to weaken in trophoblast until, by term, it was found only in stroma and in some endothelial cells. VEGF was detected exclusively in cytotrophoblast during the first trimester and then in syncytiotrophoblast throughout the remainder of pregnancy. Western blot analysis revealed that PDECGF antisera bound to three bands approximately 27, 47 and 94 kDa. The lowest band was not detected in platelet lysate and may represent an alternatively processed form of this peptide in placenta. VEGF antisera bound strongly to bands approximately 36, 46, 54, 56 and 64 kDa. The intensity of most bands increased between the first and second trimesters, consistent with an increased level of angiogenesis as the placenta develops. The presence of both factors in trophoblast in early pregnancy may be indicative of the trophoblast playing an active role in influencing the development of the villous vascular network.

The effects of maternal aerobic exercise on human placental development: Placental volumetric composition and surface areas

The histomorphometry of term placentae from women who exercised regularly throughout either the first half or all of pregnancy was compared to that of placentae from matched controls to determine if regular exercise during pregnancy produced histomorphometric evidence of altered development and transport capacity. Conventional stereological techniques were used to estimate placental volumetric composition, surface areas, and villous and vascular configurations in the three groups. Exercise confined to early pregnancy increased the parenchymal component of the placenta, total vascular volume and site-specific capillary volume and surface area. Exercise throughout pregnancy increased these and multiple other histomorphometric parameters associated with the rate of placental perfusion and transfer function. However, significant changes were confined to villi > 80 microns in diameter. The localization of both the timing of the stimulus and the anatomical sites affected indicates that regular, sustained exercise modifies placental development primarily in early and mid-pregnancy. We speculate that the lack of significant changes in the structure and configuration of the smaller villi indicates that other adaptive mechanisms, such as increased rates of placental blood flow, must be well developed by the latter portion of the mid-trimester and adequately maintain fetal oxygenation and substrate delivery throughout the third trimester.

Effect of placental resistance, arterial diameter, and blood pressure on the uterine arterial velocity waveform: A computer modeling approach

A computer model was used to simulate velocity waveforms that can be visualized in the human uterine artery using Doppler ultrasound. It was found that increasing uteroplacental vascular resistance from normal caused an increase in the systolic/diastolic velocity ratio (S/D) and pulsatility index (PI) of the waveform. Increasing uteroplacental resistance also caused the appearance of a dicrotic notch. Reducing the uterine artery radius increased the S/D and PI and this effect was accentuated at high placental resistance. In contrast, increasing mean arterial pressure in the uterine artery had little effect on S/D and PI. Results suggest that waveform shape abnormalities observed in obstetric patients with pregnancy-induced hypertension are primarily caused by high uteroplacental vascular resistance and a reduced uterine arterial diameter.

Fetal and maternal hemodynamic responses to exercise in pregnancy assessed by Doppler ultrasonography

It is common for women to undertake vigorous exercise in the late phase of pregnancy. This may have detrimental effects on the blood flow to the uterus and placenta or from the fetus to the placenta. Fifteen pregnant women with no obstetric or medical complications were subjected to a 5-minute exercise period. The maternal heart rate and blood pressure were elevated after exercise. The uteroplacental and umbilical circulations were assessed with Doppler ultrasonography. The ratio of the systolic/diastolic velocity in the uterine artery was elevated, which suggests that uteroplacental vascular resistance increased. The fetal heart rate was elevated after exercise, whereas the systolic/diastolic velocity ratio in the umbilical artery was unaltered. We conclude that moderate maternal exercise causes increased resistance to blood flow in the uterine circulation, whereas the umbilical circulation remains unaltered.

Epidural anaesthesia and blood flow velocity in mother and fetus.

Doppler ultrasound has recently been used to assess changes in blood velocity in the uterine and umbilical arteries. Alterations in the ratio of systolic to diastolic velocity (SID ratio) are believed to reflect changes in placental vascular resistance. We have used this technique to assess potential beneficial or detrimental effects of epidural anaesthesia on blood flow to the placenta. Continuous wave Doppler ultrasound was used to measure the SID ratio in the uterine and umbilical arteries of 12 patients undergoing epidural anaesthesia prior to elective caesarean section. Anaesthesia was achieved using lidocaine andepinephrine. The SID ratio in both the uterine and umbilical arteries remained unaltered either by the fluid preload or by the epidural anaesthesia. It is concluded that epidural anaesthesia using this technique has neither a beneficial nor detrimental effect on uterine or umbilical blood velocity in the uncomplicated pregnancy.RésuméRécemment, l’ultrasonographie (Doppler) a été utilisée pour ivaluer les changements de la vitesse de perfusion sanguine dans les arteres uterines et ombilicales. II a été poslulé que les changements dans le rapport de vitesse systolique vs dyastolique (S/D) sont fonction des changements de résistance vasculaire placentaire. Nous avons utilisé cette technique pour mesurer les effets béneféques ou néfastes de l’anesthésié sous épidurale sur le volume sanguin placentaire. Le rapport S/D des artéres uterines et ombilicales de 12 gestantes ayant un accouchement par césarienne sous anesthésie épidurale fut mesure à l’aide d’ondes continues. Epinéphrine et lidocaine furent utilisées pour l’anesthésie. Ni la perfusion liquidienne interveineuse ni l’anesthésie épidurale ne causérent de perturbations au niveau de lartére utérine ou ombilicale. En conclusion, l’utilisation de cette technique lors l’anesthésie sous épidurale ne nous a pas permis de déceler d’effets bénéfiques ou néfastes sur la vitesse de perfusion sanguine lors de grossesses normales.

Hypoxic acidemia, hyperviscosity, and maternal hypertension do not affect the umbilical arterial velocity waveform in fetal sheep.

The effect of hypoxic acidemia, hyperviscosity, and maternal hypertension on the umbilical arterial velocity waveform was studied in 23 chronically catheterized fetal sheep. Fetal hypoxic acidemia induced by lowering the maternal inspired oxygen concentration (n = 7) caused no change in the ratio of systolic/diastolic blood velocity even when fetal arterial pH was as low as 6.8. Fetal blood hyperviscosity (n = 7) induced by exchange transfusion with packed maternal blood cells increased placental vascular resistance by greater than or equal to 50% but had no significant effect on the systolic/diastolic ratio. Similarly, maternal hypertension induced by intravenous infusion of angiotensin II to the ewe (n = 9) did not affect the systolic/diastolic ratio despite a 50% increase in maternal arterial blood pressure. We conclude that umbilical arterial velocity waveform abnormalities observed in growth-restricted human fetuses are probably not a direct result of fetal hypoxemia or hyperviscosity or maternal hypertension.

The influence of spontaneous accelerations of fetal heart rate on umbilical artery velocity waveforms

For clinical interpretation of Doppler waveforms, it is important to establish the extent to which fetal heart rate changes affect the umbilical artery velocity waveform. Umbilical artery waveforms were measured with continuous wave ultrasonography during spontaneous accelerations of the fetal heart rate in 20 uncomplicated, near-term pregnancies. On average, an acceleration of 20 beats/min of fetal heart rate within an individual was associated with a reduction in the systolic/diastolic velocity ratio of 0.25. There was, however, considerable variability in the response, and in six patients the systolic/diastolic ratio actually increased with heart rate. We conclude that fetal heart accelerations within the normal range cause only small and variable changes in the systolic/diastolic ratio.

Umbilical artery blood flow velocity in pregnancies complicated by systemic lupus erythematosus

We evaluated the role of umbilical artery Doppler velocimetry in the surveillance of pregnancies complicated by systemic lupus erythematosus (SLE).