Some effects of maternal hyperoxia and hypoxia on the blood gas tensions and vascular pressures in the foetal sheep

Fetal cardiovascular response to acute hypoxia during maternal anesthesia

Preclinical imaging studies of fetal hemodynamics require anesthesia to immobilize the animal. This may induce cardiovascular depression and confound measures under investigation. We compared the impact of four anesthetic regimes upon maternal and fetal blood gas and hemodynamics during baseline periods of normoxia, and in response to an acute hypoxic challenge in pregnant sheep. Merino ewes were surgically prepared with maternal and fetal vascular catheters and a fetal femoral artery flow probe at 105-109 days gestation. At 110-120 days gestation, ewes were anesthetized with either isoflurane (1.6%), isoflurane (0.8%) plus ketamine (3.6 mg·kg-1 ·h-1 ), ketamine (12.6 mg·kg-1 ·h-1 ) plus midazolam (0.78 mg·kg-1 ·h-1 ), propofol (30 mg·kg-1 ·h-1 ), or remained conscious. Following 60 min of baseline recording, nitrogen was administered directly into the maternal trachea to displace oxygen and induce maternal and thus fetal hypoxemia. During normoxia, maternal PaO2 was ~30 mmHg lower in anesthetized ewes compared to conscious controls, regardless of the type of anesthesia (p < .001). There was no effect of anesthesia on fetal mean arterial blood pressure (MAP; p > .05), but heart rate was 32 ± 8 bpm lower in fetuses from ewes administered isoflurane (p = .044). During maternal hypoxia, fetal MAP increased, and peripheral blood flow decreased in all fetuses except those administered propofol (p < .05). Unexpectedly, hypoxemia also induced fetal tachycardia regardless of the anesthetic regime (p < .05). These results indicate that despite maternal anesthesia, the fetus can mount a cardiovascular response to acute hypoxia by increasing blood pressure and reducing peripheral blood flow, although the heart rate response may differ from when no anesthesia is present.

Current paradigms and new perspectives on fetal hypoxia: implications for fetal brain development in late gestation

The availability of oxygen to the fetus is limited by the route taken by oxygen from the atmosphere to fetal tissues, aided or diminished by pregnancy-associated changes in maternal physiology and, ultimately, a function of atmospheric pressure and composition of the mother's inspired gas. Much of our understanding of the fetal physiological response to hypoxia comes from experiments designed to elucidate the cardiovascular and endocrine responses to transient hypoxia. Complementing this work is equally impactful research into the origins of intrauterine growth restriction in which animal models designed to restrict the transfer of oxygen from the maternal to the fetal circulation were used. A common assumption has been that outcomes measured after a period of hypoxia are related to cellular deprivation of oxygen and reoxygenation: an assumption based on a focus on what we can see "under the streetlights." Recent studies demonstrate that availability of oxygen may not tell the whole story. Transient hypoxia in the fetal sheep stimulates transcriptomics responses that mirror inflammation. This response is accompanied by the appearance of bacteria in the fetal brain and other tissues, likely resulting from a hypoxia-stimulated release of bacteria from the placenta. The appearance of bacteria in the fetus after transient hypoxia complements the recent discovery of bacterial DNA in the normal human placenta and in the tissues of fetal sheep. An understanding of the mechanism of the physiological, cellular, and molecular responses to hypoxia requires an appreciation of stimuli other than cellular oxygen deprivation: stimuli that we would have never known about without looking "between the streetlights," illuminating direct responses to the manipulated variables.

The influence of maternal oxygen inhalation on human placental and umbilical venous blood flow

The effect of maternal short-term inhalation of oxygen (51/min) on intervillous (IVBF) and umbilical vein blood flow (UVBF) was studied in 22 cases during the third trimester of pregnancy. The maternal paO2 levels increased significantly (P less than 0.001) after O2 inhalation. The mean IVBF level was 190 +/- 66 (SD) ml/min per 100 ml of the intervillous space before inhalation and 125 +/- 58 ml afterwards, the decrease being significant (P less than 0.01). UVBF maintained its original level after O2 inhalation. The human fetus seems not to compensate for alterations in oxygen delivery and reduced IVBF after maternal oxygen inhalation, by means of changes in UVBF.

Influence of oxygen at atmospheric and high pressure on the developing tooth germ in rat embryos

Repeated exposure of pregnant rats to oxygen at high pressure (65 psi) twice a day for seven days increased the height of the tooth germ and ameloblastic layer of the embryo, but the width and odontoblastic layer were not changed. Exposure of pregnant rats to oxygen at atmospheric pressure stimulated the body growth of the embryo in utero.

Foetal placental blood flow in the lamb

1. Fifteen sheep foetuses of 1.5-5.2 kg body weight were prepared with indwelling arterial and venous catheters for experimentation one to six days later.2. Unanaesthetized foetuses were found to have mean arterial and central venous blood pressures of 40 +/- 1.5 (S.E. of mean) and 2.0 +/- 0.3 (S.E. of mean) mm Hg respectively, compared to intra-uterine pressure. Intra-uterine pressure was 16 +/- 0.8 (S.E. of mean) mm Hg with respect to atmospheric pressure at mid-uterine level.3. Mean placental blood flow of the foetuses was 199 +/- 20 (S.E. of mean) ml./(min.kg body wt.). Mean cardiac output in eleven of the foetuses was 658 +/- 102 (S.E. of mean) ml./(min.kg).4. Mean foetal and maternal colloid osmotic pressures were 17.5 +/- 0.7 (S.E. of mean) and 20.5 +/- 0.6 (S.E. of mean) mm Hg respectively at 38 degrees C.5. Intravenous infusions into six ewes of 1.8 mole of mannitol and 0.4 mole of NaCl resulted in significant increases in foetal plasma osmolarity, sodium, potassium, and haemoglobin concentrations, without detectable transfer of mannitol to the foetal circulation.6. In the sheep placenta there is osmotic and hydrostatic equilibration of water. As a consequence, there should be an interaction between foetal placental blood flow and foetal water exchange with the maternal circulation. It was concluded that this interaction tends to stabilize foetal placental blood flow.

Maternal hyperventilation and foetal hypocapnia in sheep

1. In anaesthetized foetal lambs near term, hypocapnia induced by maternal hyperventilation abolished the rise of arterial pressure and femoral vasoconstriction caused by hypoxaemia. This is consistent with interaction of P(CO2) and P(O2) on the foetal aortic bodies.2. In immature lambs (0.6-0.77 of term) maternal hyperventilation caused a fall in foetal carotid P(CO2) commensurate with that in the maternal blood. In mature lambs (at 0.9 or more of term) the fall in foetal carotid P(CO2) was less than that in maternal blood, whether the foetus was exteriorized or in utero.3. The mean transplacental gradient for P(CO2) (maternal arterial-umbilical vascular), when the foetus was replaced with a mechanical pump recirculating foetal blood, was 6.3 mm Hg. This is attributed to placental CO(2) production, and is nearly half the mean P(CO2) gradient (maternal artery-foetal carotid) of about 14 mm Hg during normal maternal ventilation.4. The mean maternal-umbilical transcotyledonary venous gradients (avoiding vascular shunts through the myometrium and intercotyledonary chorion) were for P(CO2) 1.7 mm Hg and for P(O2) 13.4 mm Hg.5. Maternal hyperventilation (P(a, CO2) approximately 20 mm Hg) caused a small fall in mean foetal carotid P(O2) (5 mm Hg), which was readily reversible with no evidence of progressive acidaemia.

PO2, PCO2 and pH levels in the umbilical and uterine blood of the mare and ewe

1. Foetal and maternal blood gas tensions and pH levels have been investigated in the mare and the ewe during late pregnancy under a number of experimental conditions.2. Observations were made on anaesthetized animals with the foetus in utero. Simultaneous blood samples were withdrawn from a maternal artery and uterine vein, and from the two umbilical vessels, through catheters inserted at the beginning of each experiment.3. At normal maternal arterial P(O2) (80-110 mm Hg) the umbilical venous P(O2) of the foal was very high (49 mm Hg) and the P(O2) in the umbilical artery (33 mm Hg) was similar to that in the umbilical vein of the lamb (34 mm Hg).4. The P(O2) difference between the umbilical and uterine venous blood was 17 mm Hg in the ewe but only 4 mm Hg in the mare. The corresponding P(CO2) gradients were about one quarter of those for oxygen in both species.5. When the maternal arterial P(O2) was raised above 100 mm Hg both uterine venous and umbilical P(O2) increased in the mare, and the gradient between the uterine vein and umbilical vein was reversed, whereas little change occurred in the corresponding vessels of the ewe.6. Alterations in maternal arterial P(CO2) were associated with concomitant changes in the blood in the other three vessels but in both species the P(CO2) difference between uterine and umbilical venous blood appeared to remain constant.7. Foetal blood pH levels followed those of the mother if the changes in the maternal blood were of respiratory origin. The effects of prolonged changes in maternal pH on foetal levels were investigated in the ewe. Foetal pH remained unchanged during maternal alkalaemia (pH 7.7) induced by Na(2)CO(3) infusions, but increased during a similar rise in maternal pH induced by hyperventilation.8. The present observations have been compared with findings on the conscious animal and possible explanations for the differences between the two species are discussed.

Observations on the control of cerebral blood flow in the sheep fetus and newborn lamb

Cerebral blood flow was measured with xenon 133 in 21 fetal sheep and 14 newborn lambs under 48 hours old. When the ewe breathed air, the average gray matter blood flow in the fetuses was 70.6 ml/100 g/min, the average white matter blood flow 15.4 ml/100 g/min and average gray matter oxygen consumption 2.06 ml/100 g/min. The corresponding values in the newborn lambs were 87.2, 17.8, and 2.98 ml/100 g/min. A reduction of fetal Pao 2 by 7 to 11 mm Hg at a constant Paco 2 or an increase of Paco 2 at constant Pao 2 caused a rise in blood flow and arterial pressure and a fall in vascular resistance. Intravenous catecholamines caused variable changes in blood flow but no significant change in gray matter oxygen consumption. Occlusion of the umbilical cord caused an average increase in gray matter blood flow of 50% of control, in white matter blood flow of 30%, and in gray matter oxygen consumption of 90%. By the end of the first hour, these values had fallen and were then similar to those seen in the newborn lamb. These changes were not seen in fetuses in which both vagi had been cut nor in two fetuses with a failing circulation.

Application of the therory of heat exchangers to the transfer of inert materials in placentas

Calculations were made on a variety of models to determine to what extent placental structure (vascular geometry and permeability) and placental function (rate and evenness of distribution of fetal and maternal blood flows, and presence of nonexchanging shunts) affect the rate of diffusional exchange across the placental barrier. The results of the calculations are given in the form of a graphical relationship between three dimensionless variables defined in the text. It is shown that experimentally measured transfers can also be represented in this form and thus compared to transfers that are theoretically predicted. The results indicate that biologically probable degrees of regional variation in permeability are harmless, but that this is not true of uneven distributions of maternal-fetal blood flow ratios. They further show that the relative magnitudes of blood flows through nonexchanging shunts can be found by extrapolation of experimental data obtained with substances whose transfers are entirely flow limited, but that the use of such substances cannot help to separate the effects on placental exchange due to vascular geometry from those due to uneven distribution of flow ratios. Methods are indicated with which the value of the placental permeability can be assessed experimentally under various conditions.

Effects of hyperbaric oxygen on uteroplacental and fetal circulation

The effects of hyperbaric oxygen (at 3 atmospheres absolute) on uteroplacental and fetal circulations were studied in pregnant ewes near term. The ewe was given spinal anesthesia, the fetus was marsupialized to the abdominal walls to protect the umbilical circulation, and the fetal head was covered with a saline-filled glove to prevent breathing. During hyperbaric oxygenation, maternal arterial blood P o 2 rose to 1,300 mm Hg while umbilical vein blood P o 2 rose to 300 mm Hg; umbilical arterial Po 2 rose to only 50 mm Hg. Maternal and fetal arterial pressures did not change significantly, but uteroplacental and umbilical flows decreased slightly. Ductus arteriosus blood flows decreased strikingly when the oxygen tension of the pulmonary blood rose; net pulmonary blood flow increased markedly because of a decrease in pulmonary vascular resistance produced by oxygen. Ascending aortic flow increased, but effective fetal cardiac output (aortic plus ductus arteriosus flows) decreased. These studies indicate that the fetal pulmonary vascular bed is sensitive to oxygen in that it undergoes vasodilatation when the oxygen tension of the blood passing through it rises; the ductus arteriosus responds to the same stimulus by constricting. Hyperbaric oxygenation seems to establish a circulatory pattern in the fetus similar to that of the early neonatal period.