Umbilical artery flow velocity waveforms during acute hypoxemia and the relationship with hemodynamic changes in the fetal lamb

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The effect of hypoxemia on the pulsatility index (PI) of the umbilical artery flow velocity waveform and placental vascular resistance was studied. Fetal hypoxemia was induced by maternal breathing of a low-oxygen gas mixture. Umbilical venous blood flow was measured with an electromagnetic flowmeter. Placental vascular resistance (PVR) was defined as the ratio perfusion pressure (mean arterial pressure minus umbilical venous pressure) and umbilical blood flow. Umbilical artery velocity waveforms were obtained by a 5-MHz pulsed Doppler device around one umbilical artery in 4 lambs and by a transcutaneous 4-MHz continuous wave Doppler transducer in 3 lambs. Fetal arterial oxygen content was lowered from 2.28 ± 0.18 to 0.93 ± 0.15 mM(p < 0.05), while pCO₂ and pH remained unchanged. Control values of the hemodynamic variables were compared with values during deepest hypoxemia. Fetal heart rate, mean arterial and umbilical venous pressure, PVR and the umbilical artery PI did not significantly change, whereas umbilical blood flow increased from 436 ± 64.7 to 491 ± 65.9ml/min (p < 0.05) during deepest hypoxemia. Individual regression analysis, however, showed a significant inverse correlation of umbilical venous pressure whereas PVR had a positive correlation with actual oxygen content. It is concluded that acute fetal hypoxemia slightly decreases PVR, but does not affect the umbilical artery PI in sheep. Decreasing fetal oxygenation is associated with an increase in pressure in the umbilical vein.

Section: Discussionmentioning

confidence: 99%

Section: Surgerymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Umbilical Artery Pulsatility Index and Placental Vascular Resistance during Acute Hypoxemia in Fetal Lambs

Huisseling¹,

Hasaart²,

Muijsers³

et al. 1991

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“…The abdomen was opened through a paramedian incision. An inflatable balloon occluder was placed around the maternal common iliac artery' [13]. The fetal lambs (mean weight 2,780 g. range 1.600-4,000 g) were approached by hysterotomy in the uterine wall lying over the fetal pelvis.…”

Section: Surgerymentioning

confidence: 99%

Transcutaneous and Arterial Carbon Dioxide Tension at Various Conditions of Fetal Stress in Lambs

Bergmans

Stevens²,

Keunen³

et al. 1997

In an acute fetal lamb model the relation between arterial PCO₂ (PaCO₂), transcutaneous PCO₂ (tcPCO₂) and pH was studied at different conditions of stress. Occlusion of the maternal common iliac artery for 8 min, umbilical cord obstruction for 5 min and placental embolization were performed subsequently, every time with an interval of 1 h for fetal recuperation. During the first 2 experiments arterial values changed rapidly after occlusion and returned nearly to normal within 30-60 min after the end of occlusion. TcPCO₂ started to increase several minutes after occlusion and reached its maximum about 5 min after the end of occlusion. Afterwards a gradual decrease towards the onset value was observed. During placental embolization tcPCO₂ did not increase until 15 min before fetal death. It is concluded that tcPCO₂ can follow a gradual trend, but not quick changes in PaCO₂.

“…The pulsatility index (PI), as determined by Doppler ultrasound, is assumed to reflect impedance at the site of measurement [6], In a previous study it has been shown that the PI of the femoral artery blood velocity waveform (femoral PI) increased in each of 4 fetal lambs, whereas the PI of the umbilical artery blood velocity waveform (umbilical PI) in a group of 6 fetal lambs did not change in response to progressively reduced uterine blood flow [7], As a consequence, the femoral PI might be used as a tool for noninvasive estimation of fetal oxygenation. Unfortunately however, the value of a wave form index also depends on variables such as heart rate [8,9], In order to dispose of a parameter possibly less sensi tive to variations in heart rate and more sensitive to varia tions in fetal blood flow distribution, we also determined the umbilical PI. This study was designed to examine the hypothesis that, in normally grown fetal sheep, the ratio femoral Pl/umbilical PI correlates with fetal oxygen con tent to such a degree that this ratio may be used for nonin vasive estimation of fetal oxygenation.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Maternal Hypoxemia on the Umbilical and Femoral Artery Blood Flow Velocity Waveforms and the Relationship with Mean Arterial Pressure in Fetal Sheep

Muijsers

Huisseling

Hasaart

1993

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The relationship between the ratio femoral pulsatility index (PΙ)/umbilical PI and arterial oxygen content was examined in 8 fetal sheep at various stages of hypoxemia. This relationship was characterized by an inconsistent increase of the ratio femoral PI/umbilical PI at oxygen content levels beneath 1.0 mM. The correlation coefficient of the linear regression between both variables was only ––0.54, indicating that prediction of fetal oxygenation on the basis of Doppler waveform measurements in peripheral fetal arteries might be invalid. The average correlation coefficient of the linear regressions between the femoral PI and fetal mean arterial pressure was 0.87 ± 0.15 (SD), whereas the average correlation between the umbilical PI and fetal mean arterial pressure was 0.04 ± 0.61. These data suggest that the degree of change in fetal mean arterial pressure considerably depends on the degree of change in vascular resistance in the fetal carcass.