Cerebral oxygen consumption during asphyxia in fetal sheep

Cerebral blood flow and cerebral arteriovenous oxygen content difference were measured in 17 fetal sheep, and cerebral oxygen uptake was calculated. The measurements were made under control conditions and after profound fetal asphyxia induced of uterine blood flow for up to 90 min. In 14 of the fetal sheep, sequential measurements were made to examine hemodynamic changes and cerebral oxygen consumption at comparable intervals up to 36 min of asphyxia. These fetuses initially had elevated blood pressure and lowered heart rate became hypoxemic, hypercarbic, and acidotic. There was an initial decrease in cerebral oxygen consumption. Sequential measurements, however, showed a relative stability in this decreased oxygenation during 4 to 36 min of asphyxia despite a progressive metabolic acidosis. The cerebral fractional oxygen extraction remained unchanged despite a mean pH of 6.98 at 36 min. The calculated cerebral oxygen uptake during asphyxia in all 17 sheep was grouped according to whether the ascending aortic oxygen content was greater or less than 1.0 mmol/l. In the first group with mean ascending aortic oxygen content of 1.3 mmol/l, blood flow to the brain was increased and cerebral oxygen consumption was 85% of control. In the second group with mean arterial blood oxygen content of 0.8 mmol/l, there was a narrowing of the arteriovenous oxygen content difference, but no further increase in cerebral blood flow. Cerebral oxygen consumption was only 48% of control in this more asphyxiated group. We conclude that the degree of hypoxemia in the second group represents a point where physiologic mechanisms cannot compensate, and may be associated with neuronal damage.     

Renal sympathetic nerve activity during asphyxia in fetal sheep

The sympathetic nervous system (SNS) is an important mediator of fetal adaptation to life-threatening in utero challenges, such as asphyxia. Although the SNS is active well before term, SNS responses mature significantly over the last third of gestation, and its functional contribution to adaptation to asphyxia over this critical period of life remains unclear. Therefore, we examined the hypotheses that increased renal sympathetic nerve activity (RSNA) is the primary mediator of decreased renal vascular conductance (RVC) during complete umbilical cord occlusion in preterm fetal sheep (101 ± 1 days; term 147 days) and that near-term fetuses (119 ± 0 days) would have a more rapid initial vasomotor response, with a greater increase in RSNA. Causality of the relationship of RSNA and RVC was investigated using surgical (preterm) and chemical (near-term) denervation. All fetal sheep showed a significant increase in RSNA with occlusion, which was more sustained but not significantly greater near-term. The initial fall in RVC was more rapid in near-term than preterm fetal sheep and preceded the large increase in RSNA. These data suggest that although RSNA can increase as early as 0.7 gestation, it is not the primary determinant of RVC. This finding was supported by denervation studies. Interestingly, chemical denervation in near-term fetal sheep was associated with an initial fall in blood pressure, suggesting that by 0.8 gestation sympathetic innervation of nonrenal vascular beds is critical to maintain arterial blood pressure during the rapid initial adaptation to asphyxia.     

Pulmonary pressure-flow relationships in the fetal lamb during in utero ventilation

Pressure-flow relationships in the ventilated lung have not been previously determined in undelivered fetal sheep. Therefore we studied 11 late-gestation chronically prepared fetal sheep during positive-pressure ventilation with different gas mixtures to determine the roles of mechanical distension and blood gas tensions on pressure-flow relationships in the lung. Ventilation with 3% O2-7% CO2 produced a substantial fall in pulmonary vascular resistance even though arterial blood gases were not changed. Increases in pulmonary arterial PO2 during ventilation were associated with falls in pulmonary vascular resistance beyond that measured during mechanical distension. Decreases in pulmonary arterial PCO2 and associated increases in pH were also associated with falls in pulmonary vascular resistance. Pulmonary blood flow ceased at a pulmonary arterial pressure that exceeded left atrial pressure, indicating that left atrial pressure does not represent the true downstream component of driving pressure through the pulmonary vascular bed. The slope of the driving pressure-flow relationship in the normal mature fetal lamb was therefore different from the ratio of pulmonary arterial pressure to pulmonary arterial flow. We conclude that mechanical ventilation, increased PO2 and decreased PCO2, and/or increased pH has an important influence on the fall in pulmonary vascular resistance elicited by positive pressure in utero ventilation of the fetal lamb and that the downstream driving pressure for pulmonary blood flow exceeds left atrial pressure.     

Effects of naloxone on breathing movements during hypercapnia in the fetal lamb

To determine whether endogenous opioids influence the fetal breathing response to CO2 we have investigated the effect of the opiate antagonist, naloxone on the incidence, rate, and amplitude of breathing movements during hypercapnia in fetal lambs in utero. In 20 experiments in six pregnant sheep (130-145 days gestation) hypercapnia was induced by giving the ewe 4-6% CO2-18% O2 in N2 to breathe for 60 min. After 30 min of hypercapnia either naloxone (13 experiments) or saline (7 experiments) was infused intravenously for the remaining 30 min. During hypercapnia breath amplitude increased from 5.8 +/- 0.5 to 9.1 +/- 1.2 mmHg (P less than 0.001), and infusion of naloxone was associated with a further significant increase to 15.7 +/- 1.2 mmHg (P less than 0.001). Naloxone had no effect on the incidence or rate of breathing movements during hypercapnia. After hypercapnia there was a significant decrease in the incidence of fetal breathing movements in the naloxone group (14.7 +/- 3.2%). Infusion of saline during hypercapnia had no effect on incidence, rate, or amplitude of fetal breathing movements. These results suggest that endogenous opioids act to suppress or limit breath amplitude during hypercapnia but do not affect rate or incidence of breathing movements.     

Hemodynamic responses to alpha-adrenergic blockade during hypoxemia in the fetal lamb.

The mechanisms responsible for the increase in umbilical venous and hepatic vascular resistance during hypoxemia are poorly understood. To assess the relative importance of alpha-adrenergic receptors, we produced an acute, severe hypoxemia in chronically instrumented fetal sheep. While fetal arterial oxygen saturation was maintained at the same level, we then injected phentolamine, a selective alpha-adrenoreceptor blocker. We found that the hypoxemia-induced vasoconstriction of the umbilical veins and hepatic vasculature was reversed by alpha blockade. Thus, alpha-adrenergic stimulation is necessary to maintain vasoconstriction of the umbilical veins and hepatic vasculature during acute fetal hypoxemia. Furthermore, alpha-adrenergic stimulation is responsible for the hypoxemia-induced vasoconstriction of the gut, spleen, and lower carcass. Thus, the alpha-adrenergic system mediates important fetal hemodynamic adaptations to acute hypoxemia. However, the alpha-adrenergic system is not responsible for the hypoxemia-induced constriction of the renal vasculature.     

Lack of change in the composition of fetal lamb lung surfactant during gestation

Fetal surfactant from lamb lung fluids collected daily from day 114 to day 146 of gestation, was isolated by centrifugation (pellet material) and further purified by sucrose density gradient centrifugation. The concentration of the pellet material from lung fluid (crude surfactant) increased from day 125 till day 135 and fluctuated strongly from that period onwards, whereas lung fluid secretion increased linearly until a few days before parturition. The pellet phospholipid composition changed with gestational age, suggesting biochemical maturation of the surfactant-producing system. The purified surfactant fraction, of which approximately 85% was phosphatidylcholine, did not change however from day 122 onwards except for a small increase in the percentage of phosphatidylglycerol. Alveolar wash surfactant or the lamellar body material, isolated from fetal lungs at different gestational ages had the same composition as surfactant from lung fluids. Only the composition of lamellar bodies of '125 day' lungs differed slightly from that of the lung fluid surfactant. The similar characteristics of all purified surfactant fractions throughout gestation indicate that, in the fetal lamb, lung maturation is associated with an increase in surfactant production no significant changes in phospholipid composition.     

Prostaglandins in the circulation of the fetal lamb

Prostaglandin E and PGF have been measured in the plasma of chronically catheterized fetal lambs throughout the last 20–35 days (0.73 onwards) of gestation. The mean concentration of PGE was higher than that of PGF. There was a significant increase in the concentration of PGE but little change in the concentration of PGF in samples of fetal plasma taken within 24 h of parturition. In contrast, at this time in maternal utero-ovarian venous plasma, there was a large increase in PGF, but relatively little change in PGE. There was a significant decrease in the concentration of PGE and PGF in the plasma of lambs within 12 h after birth compared to the levels found in the same animals as fetuses a few hours previously. The physiological importance of these changes is discussed.     

Venous responses to hypoxemia in the fetal lamb

The factors regulating umbilical venous return and its distribution between the ductus venosus and liver are poorly understood. This study was designed to determine where the major changes in resistance to umbilical venous return occur in response to fetal hypoxemia. In eight chronically-instrumented fetal lambs, during control and hypoxemic periods, we measured pressure in the descending aorta, extra-abdominal umbilical vein, portal sinus, and inferior vena cava; we also measured blood flow using radionuclide-labeled microspheres. During the control period, the umbilical arteries and placental vasculature accounted for 82% of total resistance to umbilical-placental blood flow, the umbilical veins for 11%, and the ductus venosus and liver for 7%. Hypoxemia increased resistance in the umbilical veins more than twofold, but did not affect resistance in the umbilical arteries or placenta. Although combined liver/ductus venosus resistance did not change, hepatic vascular resistance increased, and ductus venosus resistance decreased. We conclude that the major increase in resistance to umbilical venous return in response to hypoxemia resides in the umbilical veins. This increased resistance may improve maternal-fetal blood gas exchange by increasing the fetal surface area in the placenta.     

Lung liquid secretion, flow and volume in response to moderate asphyxia in fetal sheep

The effects of moderate fetal asphyxia, induced by constriction of the maternal common internal iliac artery, on lung liquid secretion, tracheal fluid efflux and lung liquid volume have been investigated in unanaesthetized fetal sheep (111-142 days) in utero. During periods of fetal asphyxia the percent oxygen saturation, PO2, pH, and PCO2 of fetal carotid arterial blood changed from 57.2 +/- 1.3% (mean +/- SEM), 22.9 +/- 0.6 mmHg, 7.35 +/- 0.01 and 45.6 +/- 1.0 mmHg to 26.3 +/- 0.5% (P less than 0.001), 14.7 +/- 0.2 mmHg (P less than 0.001), 7.28 +/- 0.02, (P less than 0.001) and 47.8 +/- 0.4 mmHg (P less than 0.02), respectively. Fetal asphyxia, over 6 h, decreased the efflux of tracheal fluid from 7.07 +/- 0.47 ml/h to 3.97 +/- 0.36 ml/h (P less than 0.01) and, over 4 h, decreased the rate of lung liquid secretion from 9.42 +/- 1.76 ml/h to 4.91 +/- 1.54 ml/h (P less than 0.005), whereas it had no significant effect on lung liquid volume. The incidence of fetal breathing movements decreased from 52.9 +/- 2.5% to 22.6 +/- 3.5% during 6-h periods of fetal asphyxia. Thus, although fetal asphyxia decreased the net production of lung liquid, lung liquid volume was maintained probably, because the net efflux of fluid from the lungs via the trachea decreased to a similar extent.     

The metabolism of palmitic acid in the fetal lamb

[1-14C]Palmitic and [9,10-3H]palmitic acids were injected into the femoral artery of fetal sheep in utero about one month preparturition. The experiment was terminated after 5, 15 or 30 min when the main tissues were removed for analysis of the lipid components. 5 min after injection of the label, most was recovered in the plasma but increasing amounts were recovered later in the liver and heart. Selective loss of 14C-label occurred such that in the plasma, 30 min after injection, the ratio of 3H:14C had changed from 1:1 to 8.4:1. Increasing amounts of the labelled lipid were recovered in esterified form with time after injection, and the 3H:14C ratio differed markedly in specific lipids and tissues. Most of the label was recovered in palmitic acid, but some was also present in myristic and octadecenoic acids. Some evidence was obtained that the latter may have been the delta 11-isomer, which was found in much greater amounts in fetal than maternal tissues. It appears that partial-oxidation and resynthesis of fatty acids occurs in a concerted manner at a rapid rate in fetal sheep. The phenomenon has important implications for the interpretation of the results of much previous work with fetal animals in vivo.     

Glucose and lactate oxidation rates in the fetal lamb

Both glucose and lactate are nutrients of the ovine fetus. Each may be used by the fetus as a fuel for oxidation or as a source of carbon for energy storage and net tissue accretion. The present report describes the oxidation rates of glucose and lactate in vivo for the fetal lamb over a relatively short time period. The fraction of fetal glucose or lactate oxidized was defined as the ratio of 14CO2 excretion across the umbilical circulation to the net entry of [14C]glucose or [14C]lactate into fetal tissues. The fraction of glucose oxidized over a 3-hr study averaged 61.2%, accounting for 2.55 mg X min-1 X kg-1 of glucose oxidized and for 28% of the simultaneous net oxygen uptake. The fraction of lactate oxidized averaged 71.5%, accounting for 4.12 mg X min-1 X kg-1 of lactate oxidized. Oxidation fractions and rates for both glucose and lactate increased with their concentrations in fetal blood suggesting sparing of other fuels for oxidation at higher glucose and lactate concentrations.     

Essential fatty acids in the fetal and newborn lamb

The concentrations of linoleic and linolenic acids and their metabolites in the liver, kidney, brain, erythrocytes and plasma of fetal lambs at various stages of gestation, and of newborn and 2-week-old suckled lambs was determined. Throughout gestation the fetal tissues, erythrocytes and plasma all contained low levels of linoleic and linolenic acids together with consistently high levels of their long-chain polyunsaturated metabolites. The triene: tetraene (eicosa-5,8,11-trienoic acid/arachidonic acid) ratio was always 0.4 or less except at birth when it reached 0.6 in liver and 0.9 in plasma. Milk intake significantly increased the linoleic and linolenic acid levels in the lamb by 2 weeks after birth. These results show that the developing fetal lamb should not be regarded as being deficient in essential fatty acids, as suggested by previous investigators. It is proposed that the total metabolites of linoleic and linolenic acids are the most appropriate measure of the essential fatty acid status of the fetal lamb.