Fetal and maternal Doppler flow velocity waveforms in normal sheep pregnancy

Accuracy of ultrasonography in early pregnancy diagnosis in the ewe

Nonbred and pregnant ewes were examined ultrasonographically at intervals of 4 to 6 days on Days 17 to 34 after estrus. Each ewe was diagnosed as pregnant or nonpregnant, and a score for degree of certainty in the diagnosis was recorded. The goal of the study was to define criteria that could be used for identification and accuracy of diagnosis of an early conceptus and to ascertain the confidence which the operator had in makeing the diagnosis. Pregnancy was retrospectively confirmed by ultrasonographic detection of an embryo proper and by embryonic heartbeat on Days 21 to 34, and later judged against the number of lambs born to each ewe. The percentage of ewes accurately diagnosed pregnant by ultrasonography was not significantly higher than that by guessing (50%) before Day 24, but reached 85% on Days 32 and 34. However, the ability to detect nonpregnant ewes by ultrasonography was higher (P<0.01), with a greater specificity starting on Days 21 to 23 (80%) and reaching 98% by Days 32 to 34. Before Day 24, the diagnosis of pregnancy in many cases was based primarily upon the ultrasonographic appearance of the uterine lumen and location of the uterus in relation to the bladder rather than upon detection of the conceptus. For the certainty score there was a main effect of day (P<0.01) but not for the reproductive status (pregnant vs nonpregnant). The certainty score increased in all ewes among days, and was highest on Days 32 to 34. It was concluded that real time transrectal ultrasonographic scanning of sheep between Days 24 and 34 of gestation offers a safe, accurate and practical means for diagnosing pregnancy.

Fetoplacental biometry and umbilical artery Doppler velocimetry in the overnourished adolescent model of fetal growth restriction

OBJECTIVE

The purpose of this study was to evaluate ultrasonographically fetal growth trajectories, placental biometry, and umbilical artery (UA) Doppler indices in growth-restricted pregnancies of overnourished adolescent ewes and normally developing pregnancies of control-fed ewes.

STUDY DESIGN

Singleton pregnancies were established using embryo transfer in 42 adolescent ewes that were overnourished (n = 27) or control-fed (n = 15) and were scanned at weekly intervals from 83-126 days' gestation and necropsied at 131 days' gestation (term = 145 days).

RESULTS

Ultrasonographic placental measurements were reduced and UA Doppler indices were increased from 83 days' gestation; measurements of fetal abdominal circumference and femur length, renal volume and tibia length, and biparietal diameter were reduced from 98, 105, and 112 days' gestation, respectively, in overnourished vs control-intake pregnancies.

CONCLUSION

Overnourishment of adolescent sheep dams produced late-onset asymmetric fetal growth restriction that was commensurate with brain sparing. Ultrasonographic placental biometry was already reduced and UA Doppler indices increased by mid gestation in overnourished pregnancies, preceding reduced fetal growth velocity and indicating an early nutritionally mediated insult on placental development.

Mild maternal undernutrition in the first half of ovine pregnancy influences placental morphology but not fetal Doppler flow velocity waveforms and fetal heart size

AIMS

We wanted to investigate whether experimental dietary manipulations during early pregnancy influence placental growth and subsequently the cardiovascular system as assessed non-invasively by ultrasonography in the sheep fetus.

METHODS

21 ewes bearing singletons of uniform age were randomly assigned for the first half of pregnancy to one of the following study groups: fed 100% of their nutritional requirements (i.e. controls), global reduction in total intake by 30% (i.e. 70% global) and reduction in protein intake by 30% (i.e. 70% protein).

RESULTS

Placentas from the 70% protein group had significantly more small placentomes, but significantly fewer large placentomes compared to the 70% global group. However, there were no significant differences between the three dietary groups for either the aortic or the umbilical Doppler velocimetry parameters and the fetal heart rate or heart size.

CONCLUSIONS

The present study shows that early mild maternal undernutrition produces subtle changes in cotelydonary weight. However, Doppler and echocardiographic parameters were not affected by these changes.

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.

Does the umbilical artery systolic/diastolic ratio reflect flow or acidosis? An umbilical artery Doppler study of fetal sheep

We studied four fetal sheep at 114 to 139 days' gestation to evaluate the effect of acute placental embolization on placental perfusion, fetal oxygenation and acid-base status, and the umbilical artery systolic/diastolic ratio. Studies were performed after maternal anesthesia was induced in an acute model, with serial umbilical artery embolizations performed at 20 to 30 minute intervals. Fetal umbilical flow, systolic/diastolic ratio, and arterial blood gas levels were followed up. One animal maintained a normal systolic/diastolic ratio despite a 50% drop in flow after the first injection and pH of 6.91, and died after the second injection. The remaining three animals showed significantly decreased flow after the third injection (p less than 0.02), with no change in the systolic/diastolic ratio. The systolic/diastolic ratio increased significantly after the fourth injection (p less than 0.05) when flow had dropped even further. Corresponding decreases in PO2 and pH did not achieve statistical significance. The magnitude of all changes increased with subsequent embolizations. We conclude that in this model significant reductions in flow result in small increases in the systolic/diastolic ratio and that greater magnitude changes in the systolic/diastolic ratio appear only with severely diminished flow and PO2, when acidosis develops.

Pulsed wave Doppler velocimetry in the descending aorta of chronically instrumented fetal sheep

Pulsed wave Doppler sonography was used to determine peak systolic velocity (Vpeak), mean maximum velocity (Vmean) and Pulsatility Index (PI) in the descending aorta of chronically instrumented, unanesthetized fetal sheep (days 120-130 of pregnancy). Measurements were done 1-4 days after surgery using a duplex mechanical sector scanner (4.5 MHz) and offset attached pulsed wave Doppler probe (2.25 MHz) during periods of control, intrauterine cooling and rewarming. Mean values derived from the envelope curve of maximum shift frequencies were (controls): thoracic: Vmean 60.9 +/- 9.1 cm/sec, Vpeak 118.9 +/- 17.8 cm/sec, PI 1.57 +/- 0.24; abdominal: Vmean 49.6 +/- 10.9 cm/sec, Vpeak 98.9 +/- 20.4 cm/sec, PI 1.59 +/- 0.41. Vpeak was positively correlated to arterial pressure (r = 0.23) and PI showed an inverse correlation to heart rate (r = -0.22). During cold stress (reduction of fetal core temperature by 2.7 degrees C) and consecutive rewarming Vpeak and Vmean were positively correlated to fetal core temperature and arterial pressure (r = 0.2-0.26). PI revealed a weak negative dependency from core temperature and heart rate (r = -0.19 and -0.18 respectively).

The effect of heart rate on Doppler flow velocity systolic-diastolic ratios in umbilical and uteroplacental arterial waveforms

In an analysis of the influence of heart rate on Doppler flow velocity waveform-systolic/diastolic ratios from 1555 umbilical artery waveform studies and from 1497 uteroplacental waveform studies, a negative correlation was observed between the ratio and the corresponding heart rate. This relationship was not influenced by gestational age and was similar in pregnancies with normal outcomes when compared with all pregnancies. The magnitude of this inverse relationship is such that the effect of heart rate on systolic/diastolic ratios must be considered when Doppler flow velocity waveform analyses are used as experimental tools to assess placental vascular resistance but it does not appear to be an important factor in general clinical practice when measurements are made during fetal inactivity and when heart rates are within normal ranges.