Decreased neuroinflammation correlates to higher vagus nerve activity fluctuations in near-term ovine fetuses: a case for the afferent cholinergic anti-inflammatory pathway?

Background

Neuroinflammation in utero may contribute to brain injury resulting in life-long neurological disabilities. The pivotal role of the efferent cholinergic anti-inflammatory pathway (CAP) in controlling inflammation, e.g., by inhibiting the HMGB1 release, via the macrophages’ α7 nicotinic acetylcholine receptor (α7nAChR) has been described in adults, but its importance in the fetus is unknown. Moreover, it is unknown whether CAP may also exert anti-inflammatory effects on the brain via the anatomically predominant afferent component of the vagus nerve.

Methods

We measured microglial activation in the ovine fetal brain near term 24 h after the umbilical cord occlusions mimicking human labor versus controls (no occlusions) by quantifying HMGB1 nucleus-to-cytosol translocation in the Iba1+ and α7nAChR+ microglia. Based on multiple clinical studies in adults and our own work in fetal autonomic nervous system, we gauged the degree of CAP activity in vivo using heart rate variability measure RMSSD that reflects fluctuations in vagus nerve activity.

Results

RMSSD correlated to corresponding plasma IL-1β levels at R = 0.57 (p = 0.02, n = 17) and to white matter microglia cell counts at R = −0.89 (p = 0.03). The insult increased the HMGB1 translocation in α7nAChR+ microglia in a brain region-dependent manner (p < 0.001). In parallel, RMSSD at 1 h post insult correlated with cytosolic HMGB1 of thalamic microglia (R = −0.94, p = 0.005), and RMSSD at pH nadir correlated with microglial α7nAChR in the white matter (R = 0.83, p = 0.04). Overall, higher RMSSD values correlated with lower HMGB1 translocation and higher α7nAChR intensity per area in a brain region-specific manner.

Conclusions

Afferent fetal CAP may translate increased vagal cholinergic signaling into suppression of cerebral inflammation in response to near-term hypoxic acidemia as might occur during labor. Our findings suggest a new control mechanism of fetal neuroinflammation via the vagus nerve, providing novel possibilities for its non-invasive monitoring in utero and for targeted treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0567-x) contains supplementary material, which is available to authorized users.

Neonatal outcomes are associated with latency after preterm premature rupture of membranes

OBJECTIVE

To determine factors associated with latency time to birth after preterm premature rupture of membranes (PPROM) and the impact on neonatal outcomes.

STUDY DESIGN

Data on singleton pregnancies with PPROM (n=1535 infants) were prospectively collected in a computerized perinatal/neonatal database at a tertiary care perinatal center. Latency was characterized as ≤72h versus >72 h after PPROM.

RESULT

The percentage of women with latency to birth >72 h decreased from 67% in very preterm (gestational age (GA) 25 to 28 weeks) to 10% in late preterm women (GA 33 to 36 weeks). PPROM women with latency ≤72 h were more likely to have pregnancy-induced hypertension and birth weight <3%; PPROM women with latency >72 h were more likely to have received steroids and develop clinical chorioamnionitis. PPROM <32 weeks GA with latency ≤72 h was associated with a two-fold higher incidence of severe neonatal morbidity, while PPROM between 29 to 34 weeks GA and latency ≤72 h was associated with a higher incidence of moderate neonatal morbidity.

CONCLUSION

A latency period >72 h was associated with a decreased incidence of adverse neonatal outcomes up to 32 weeks GA for severe and 34 weeks GA for moderate morbidity indices.

Sagittal sinus blood flow in the ovine fetus as a continuous measure of cerebral blood flow: relationship to behavioural state activity

Superior sagittal sinus blood flow (Q(ss)) was studied over a 6-h period in nine chronically catheterized fetal sheep as a continuous measure of cerebral blood flow to determine the change in blood flow values and in measures of blood flow variability in relation to behavioural state activity. Mean Q(ss) was increased during the low voltage (LV)/rapid eye movement (REM) state compared to the high voltage (HV)/NREM state by approximately 25%, and was further increased during periods of LV/REM with fetal breathing movements. The increase in Q(ss) was abrupt and began at the transition to LV/REM, with the rate of change 2-fold greater than that during transition to HV/NREM, where the decrease in Q(ss) was gradual and began prior to the evident state change. Q(ss) showed considerable fluctuation, which tended to be greater during the HV/NREM state compared to the LV/REM state when analyzed using measures of longer term variability. Q(ss) thus provides for a continuous measure of cerebral blood flow in the ovine fetus, with the approximately 25% increase with change from the HV/NREM to LV/REM state similar to that previously reported using radioactive microspheres. The abrupt increase in Q(ss) at the transition to LV/REM versus the gradual decrease in Q(ss) before transition to HV/NREM would suggest that the state-related change in brain blood flow is better linked to the presence of the LV electrocorticogram and favours its active generation.

Adaptation of cardiovascular responses to repetitive umbilical cord occlusion in the late gestation ovine fetus

1. The impact of repeated umbilical cord occlusion on the normal maturation of fetal heart rate (FHR) and mean arterial pressure (MAP) and the cardiovascular responses to successive umbilical cord occlusion was investigated over a 21 day period in the latter part of gestation. 2. Fifteen chronically instrumented sheep (control group n = 6; occlusion group n = 9) were studied for 21 days (113-133 days of gestation, term = 145 days) with umbilical cord occlusions (90 s duration) performed every 30 min for 1-4 h each day. On days 1, 9 and 18, FHR, FHR variation and MAP were monitored continuously and fetal arterial blood gases, pH and metabolites were measured at predetermined intervals. The baroreflex response to 75-100 microg phenylephrine (I.V.) was tested on days 1 and 18. 3. Basal FHR decreased (DeltaFHR: control, 34.6 +/- 3.6 beats x min(-1); occlusion, 36.9 +/- 2.7 beats x min(-1)) and MAP increased (DeltaMAP: control, 3.1 +/- 1.7 mmHg; occlusion, 5.2 +/- 2.1 mmHg) to a similar extent in control and occlusion groups between days 1 and 21 of the study. There was a small decline in FHR variation over the 21 day study in occlusion, but not control, group fetuses. 4. The magnitude of the fall in FHR decreased and the rise in MAP increased, despite similar changes in blood gases in response to umbilical cord occlusion, over the course of the 21 day study. Despite a significant decline in the ratio of DeltaFHR to DeltaMAP on days 9 and 18 compared to day 1, there was no difference between control and occlusion groups in baroreflex sensitivity. However DeltaFHR/DeltaPO2, an index of chemoreceptor sensitivity, had decreased by day 9 and 18 compared to day 1. 5. The cardiovascular responses to umbilical cord occlusion are altered with repetitive occlusions during the latter part of gestation, with a decrease in DeltaFHR/DeltaMAP, which does not involve changes in baroreflex sensitivity, but may involve changes in chemoreceptor sensitivity. However, repeated umbilical cord occlusion appears to have no impact on baseline cardiovascular control since there was no change in the normal maturational decrease in FHR and rise in MAP.

Intermittent umbilical cord occlusion in the ovine fetus: effects on blood glucose, insulin, and glucagon and on pancreatic development

OBJECTIVE

To determine whether repetitive umbilical cord occlusion resulting in fetal hypoxemia but not cumulative acidosis also affects fetal glucose levels and the levels of the regulatory hormones insulin and glucagon, by altering glucose delivery and with repetitive insults by inducing fetal glucose production, thus possibly affecting pancreatic development.

METHODS

Fifteen chronically catheterized fetal sheep were studied over 21 days. Umbilical cord occlusions (UCOs) (duration 90 seconds) were performed every 30 minutes for 3-4 hours each day. Fetal arterial blood was sampled at predetermined times on days 1, 9, and 18 for blood gases, pH, glucose, lactate, insulin, and glucagon. When animals were sacrificed, fetal pancreatic tissues were collected for insulin immunostaining.

RESULTS

Blood glucose decreased acutely with each UCO but showed a cumulative increase of approximately 30% over the course of each sampling day. Although plasma insulin levels also increased over the course of sampling on days 9 and 18, plasma glucagon levels remained unchanged throughout the study. The percentage of pancreatic islet cells immunopositive for insulin, which averaged 67%, was also unchanged in experimental compared with control animals.

CONCLUSION

Umbilical cord occlusion during the latter part of pregnancy, which caused severe but limited hypoxemia, also resulted in acute decreases in blood glucose levels because of reduced exogenous glucose delivery and a cumulative increase in glucose in response to repetitive insults, possibly by inducing fetal glucose production, enhancing glucose delivery, or both. However, repetitive UCO as studied had minimal effect on plasma insulin levels and no effect on glucagon levels or on pancreatic immunostaining for insulin, and thus had no evident effect on pancreatic development.

The effect of intermittent umbilical cord occlusion on insulin-like growth factors and their binding proteins in preterm and near-term ovine fetuses

Intermittent umbilical cord compression with resultant fetal hypoxia can have a negative impact on fetal growth and development. Insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) are the most important regulators of fetal growth. In preterm (107-108 days of gestation) and near-term (128-131 days of gestation) ovine fetuses, we have determined the effect of intermittent umbilical cord occlusion (UCO) over a period of 4 days on the profile and expression of IGFs and IGFBPs. In experimental group animals (preterm n=7; near term n=7) UCOs were carried out by complete inflation of an occluder cuff (duration 90 s) every 30 min for 3-5 h each day, while control fetuses (preterm n=7; near term n=7) received no UCOs. Ewes were euthanized at the end of day 4, and fetal heart, lung, kidney, liver, skeletal muscle and placenta were collected. During UCOs, PO(2! ) fell (by approximately 13 mmHg), pH fell (by approximately 0.05) and PCO(2) increased (by approximately 7 mmHg), and changed to a similar extent in both preterm and near-term groups. In both preterm and near-term groups, there was no difference in fetal body or organ weight between UCO and control fetuses. No significant changes were observed in plasma IGF-I and -II concentrations or IGFBP-1, -2, -3 or -4 levels throughout the 4-day study at either gestational age. In the preterm group UCO fetuses, IGF-II mRNA (1.2-6.0 kb) levels were lower in fetal lung (33%, P<0.05), heart (54%, P<0.01) and skeletal muscle (29%, P<0.05), but there were no differences in IGF-I mRNA levels (7.3 kb); IGFBP-2 mRNA (1.5 kb) levels were lower in the right lobe of the liver (42%, P<0.05) and kidney (22%, P<0.01), but hig! her in the heart (72%, P<0.01), while IGFBP-4 (2.4 kb) levels were lower in skeletal muscle (21%, P<0.01). In the near-term group UCO fetuses, IGFBP-2 mRNA levels were greater in the placenta (39%, P<0.05). Thus, intermittent UCO as studied has a greater effect on the expression of genes encoding certain peptides of the fetal IGF system in selected tissues in preterm fetuses than that in near-term fetuses. Altered IGFBP-2 mRNA levels with reduced IGF-II mRNA levels in selected tissues may mediate changes in growth and/or differentiation that might become apparent if the length of the UCO study were extended.

Activation of the hypothalamic-pituitary-adrenal axis with repetitive umbilical cord occlusion in the preterm ovine fetus

OBJECTIVE

To determine whether repeated hypoxic insults with umbilical cord occlusion over 4 days will lead to activation of the hypothalamic-pituitary-adrenal (HPA) axis altered adrenocortical responsiveness in the preterm ovine fetus.

METHODS

Umbilical cord occlusions of 90 seconds duration were performed every 30 minutes for 3 to 5 hours each day (experimental group n = 7, control group n = 7; at 112-116 days' gestation, term = 147 days). Arterial blood was sampled at predetermined times for blood gases and pH, plasma ACTH, and cortisol. Pituitary proopiomelanocortin (POMC) and glucocorticoid receptor (GR) mRNA also were localized and quantified by in situ hybridization.

RESULTS

During umbilical cord occlusions fetal arterial oxygen pressure (approximately 17 mmHg) and pH (approximately 0.05) decreased, and carbon dioxide pressure increased (approximately 8 mmHg) as measured on days 1 and 4, but with no cumulative blood gas or pH change over successive occlusions for any of the 4 study days. Plasma ACTH increased, as measured after cord occlusion and over the course of successive cord occlusions on days 1 and 4, and returned to control values by the next day. The cumulative increase in ACTH was much less on day 4 than day 1 (15 +/- 3 compared with 101 +/- 25 pg/mL, P <.05). Plasma cortisol increased, as measured after cord occlusion and over the course of successive cord occlusions on day 4 only (2.7 +/- 0.4 to 4. 7 +/- 0.3 ng/mL, P <.05). POMC mRNA increased 2.5-fold in the pars distalis of the pituitaries from cord occlusion compared to control fetuses, but was unchanged in the pars intermedia. GR mRNA, which was detected in the pars distalis only, was unaltered.

CONCLUSION

Repetitive umbilical cord occlusion in the preterm ovine fetus resulted in the activation of the HPA axis, with increased adrenocortical responsiveness over time, and involved differential regulation of POMC mRNA expression in the pars distalis and pars intermedia of the pituitary, but with no change in GR.

Cardiovascular and metabolic responses to intermittent umbilical cord occlusion in the preterm ovine fetus

OBJECTIVE

To determine the cardiovascular and metabolic responses to umbilical cord occlusion in the preterm ovine fetus and the impact of repetitive intermittent insults over a 4-day period.

METHODS

Repetitive umbilical cord occlusions (experimental group, n = 7; control group, n = 7) were performed daily (112-115 days' gestation, term = 147 days). Mean arterial pressure (MAP), fetal heart rate (FHR), and FHR variation were monitored, and arterial blood was sampled at predetermined intervals.

RESULTS

During umbilical cord occlusions, arterial oxygen pressure (PaO2) (approximately 17 mmHg) and glucose (approximately 0.3) millimoles per liter (mmol/L) fell and arterial carbon dioxide pressure (approximately 8 mmHg) rose (P < .01) to a similar extent on days 1 and 4. Umbilical cord occlusion produced a rise in lactate over the course of successive umbilical cord occlusions each day, the magnitude of which tended to be reduced by day 4 (0.3 +/- 0.1 versus 0.6 +/- 0.1 mmol/L). Control hour FHR and MAP were unaltered over the 4 days, but the delta (delta) FHR to delta PaO2 ratio during umbilical cord occlusions was less on day 4 than on day 1 (6.0 +/- 0.4 versus 10.9 +/- 1.5 beats per minute/mmHg; P < .01). During occlusion hours, high FHR variation episodes, as a measure of fetal activity, were reduced (14.6 +/- 1.5 versus 4.2 +/- 1.3 min/h; P < .01), whereas the reduction in short-term (7.4 +/- 0.7 to 5.8 +/- 0.6 milliseconds; P < .05) and long-term (34.9 +/- 2.7 to 30.0 +/- 0.6 milliseconds; P < .05) FHR variation reached significance only on day 4.

CONCLUSION

The increase in lactate and reduced high-FHR variation episodes over successive umbilical cord occlusions may affect fetal growth and development. Furthermore, repeated umbilical cord occlusions over several days alter the preterm FHR response to subsequent stresses, suggesting an altered chemoreflex response.

Effects of sustained hypoxaemia with 72 hours recovery on 11beta-hydroxysteroid dehydrogenase types 1 and 2 gene expression in near-term fetal sheep

The study examined the effects of 8 h sustained hypoxaemia, with 72 h recovery, on the expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) types 1 and 2 in near-term fetal sheep. Placental tissue and fetal liver and kidney were collected at Days 135-138 gestation 72 h after 8 h sustained hypoxaemia induced by lowering maternal inspired oxygen with (n = 9) and without (n = 6) metabolic acidosis or after 8 h normoxia (n = 6). In hypoxic fetuses with metabolic acidosis, a significant increase in the level of 11beta-HSD2 mRNA in the kidney compared with controls was correlated significantly with degree of associated fetal acidaemia, but there were no corresponding increases in the tissue level of 11beta-HSD2 activity. Hence, a time lag may exist between the mRNA and activity. Alternatively, the translation of 11beta-HSD2 mRNA may be inhibited. In contrast, levels of 11beta-HSD1 mRNA in the placenta and fetal liver were unchanged 72 h after sustained hypoxaemia. These results indicate that sustained fetal hypoxaemia with metabolic acidosis selectively up-regulates 11beta-HSD2 mRNA expression in the near-term fetal sheep kidney. This may be a re-bound effect at 72 h following an initial down-regulation as observed in a previous study.

Metabolic and circulatory adaptations to chronic hypoxia in the fetus

When oxygenation is compromised the fetus is capable of a number of adaptive responses, both protective and potentially pathologic, which can be categorized as those affecting fetal metabolism and those affecting fetal oxygen transport. However, both the extent and the duration of the impairment in oxygenation will bear on these adaptive responses. While fetal O2 extraction is increased when oxygenation is acutely compromised thus maintaining O2 consumption, with chronic hypoxemia there is a decrease in O2 consumption paralleling that in O2 delivery and contributed to by the resultant fall-off in growth and alterations in behavioural activity. While a redistribution of blood flow to vital organs continues to be evident, this will be less pronounced than that seen with acute hypoxemia reflecting diminished hormonal changes, underlying metabolic alterations, and the extent to which fetal blood gases are normalized. Much of this information is based on experimental data using unanesthetized fetal sheep with chronic catheterization; however, clinical outcome data and the use of investigative techniques including ultrasound scanning and cordocentesis have supported the relevance of this experimental data to the human situation.

The effects of 'sleep promoting agents' on behavioural state in the ovine fetus

Fetal behavioural states, with similarities to adult sleep states, exist in both the human and ovine fetus near term. The purpose of the present study was to determine the effects of intracerebral administration of pharmacologic agents, known to affect sleep states in the adult, on fetal behavioural states and physiologic correlates using the chronically catheterized ovine fetus near term. Each drug was infused into either the cisterna magna or lateral ventricle for 90 min in one of two doses. Carbachol (1.35 x 10(-5) and 4.25 x 10(-6) M) led to an increase in low-voltage ECOG, eye movement and FBM activities, while scopolamine (4.68 x 10(-4) and 1.56 x 10(-4) M) led to a decrease in low-voltage ECOG and eye movement activity with an increase in high-voltage ECOG activity. L-5-Hydroxytryptophan (5-HTP) (2.04 x 10(-3) and 6.81 x 10(-4) M) infusion led to an increase in FBM, while VIP (3.00 x 10(-7) and 1.00 x 10(-7) M) infusion had no effect on fetal behavioural state parameters. Study results indicate that fetal behavioural states can be altered pharmacologically and in a manner similar to that seen in the adult but with notable differences that may relate to species, developmental or dose-response issues.

Nuchal cord evident at birth impacts on fetal size relative to that of the placenta

We sought to determine whether umbilical cord complications, as evidenced by a nuchal cord at the time of birth, affects birth weight, placental weight and, therefore, the birth to placental weight ratio as a mechanism whereby fetal and placental growth may be differentially affected. The computerized perinatal database of St. Joseph's Health Centre, London, Ontario, was used to obtain the birth weight, placental weight, umbilical cord gases, and nuchal cord status, for all term singleton liveborn infants between January, 1991 and December, 1994. The effect of no nuchal cord versus nuchal cord on birth weight, placental weight, and the birth to placental weight ratio was determined, along with the interactive effects of gestational age and umbilical cord gases, and the 'dose response' effect of the number of cord encirclements. Infants with the cord around the neck were smaller (P < 0.001), their placentas larger (P = 0.001), and their birth to placenta weight ratio also smaller (P < 0.001), with a 'dose response' relationship apparent as these effects were greater in those infants with more than one cord encirclement. Although gestational age and umbilical cord veno-arterial PO2 and PCO2 differences were also found to impact on birth weight, placental weight, and the birth to placental weight ratio, these effects were independent of the effect of nuchal cord complications. Umbilical cord complications is evidenced by a nuchal cord at the time of birth, are associated with a decrease in fetal size relative to that of the placenta, although this is likely to be of little biological significance for most nuchal cord infants.

Prediction of umbilical artery base excess by intrapartum fetal oxygen saturation monitoring

OBJECTIVE

Our purpose was to evaluate the predictive value of intrapartum fetal oxygen saturation as monitored by reflectance pulse oximetry (SpO2) for metabolic acidosis at birth.

STUDY DESIGN

An observational study was carried out on intrapartum patients at > or = 35 weeks' gestation having either a nonreassuring fetal heart rate pattern, intrauterine growth restriction, or thick meconium. Fetal oxygen saturation monitoring was performed with use of the Nellcor N-400 monitor and the FS-14 fetal oxygen sensor. Mean values of SpO2 from the last 30 minutes of monitoring were correlated with umbilical artery base excess and pH at birth, with use of regression analysis, whereas the prediction of acidosis by SpO2 at different thresholds was tested with use of receiver-operator characteristic curve calculations.

RESULTS

Fifty-four patients met the criteria for data analysis, with a mean SpO2 monitoring time of 150 +/- 124 minutes (SD) and a mean signal loss of 30% +/- 20%. Mean fetal SpO2 for the last 30 minutes of monitoring averaged 42.1% +/- 9.9% and, for individual patient studies, correlated significantly with calculated oxygen saturation in the umbilical vein (r = 0.52, p < 0.001) and in the umbilical artery (r = 0.34, p = 0.02) as measured at birth. However, the correlation with umbilical artery base excess values at birth was somewhat weaker (r = 0.30, p < 0.05), as was the correlation with umbilical artery pH values (r = 0.26, p = 0.05). Receiver-operator characteristic curve calculations were all nonsignificant when SpO2 from the last 30 minutes of monitoring was used as a diagnostic test for predicting acidosis at birth.

CONCLUSIONS

Intrapartum fetal SpO2 as monitored in the current study was of limited use as a diagnostic test for predicting acidosis at birth, regardless of the SpO2 cutoff value used.

Tissue DNA synthesis in the preterm ovine fetus following 8 hours of sustained hypoxemia

OBJECTIVE

Protein synthesis is significantly decreased in the near-term ovine fetus in response to induced hypoxemia of several hours' duration. We therefore sought to determine the extent to which DNA synthesis rates as an index of tissue mitotic activity are also affected by similarly induced compromises in fetal oxygenation.

METHODS

Fetal sheep were studied at 0.75 of gestation during a normoxic control period and an 8-hour experimental period of either sustained hypoxemia induced by lowering maternal inspired oxygen concentration of 11-8% (hypoxia group, n = 7) or continued exposure to room air (control group, n = 5). To estimate DNA synthesis rate, [3H]-thymidine (1 mCi/kg) was injected intravenously into each fetus at the beginning of the experimental period.

RESULTS

Sustained hypoxemia with a reduction in fetal arterial O2 content from (mean +/- standard error of the mean) 4.3 +/- 0.1 to 1.5 +/- 0.1 mmol/L by the end of study resulted in a variable degree of fetal acidemia, 7.26 +/- 0.03 (range from 7.41 to 7.10), which was entirely metabolic in nature.

CONCLUSION

The DNA synthesis rates of most tissues were not significantly changed by the 8 hours of sustained hypoxemia, suggesting that restrictions in protein synthesis in response to fetal hypoxia are initially due to a differential effect on nonmitotic synthetic processes at this stage of development. However, selective decreases in the DNA synthesis rates of the hippocampus (approximately 50%, P < .01), adrenals (approximately 48%, P < .05), and left and right myocardial ventricles (approximately 42% and 27%, respectively, P = .08) were evident which may reflect altered mitotic activity in response to tissue related changes in energy expenditure.

Failure of magnesium sulfate infusion to inhibit uterine activity in pregnant sheep

OBJECTIVES

Our purpose was to determine the effect of magnesium sulfate infusion on nonlabor uterine contractures and corticotropin-induced preterm uterine contractions in pregnant sheep.

STUDY DESIGN

Fetal and maternal vascular catheters and uterine electromyographic electrodes were surgically placed in 15 pregnant sheep between 118 and 125 days' gestation. After 3 to 5 days of recovery, magnesium sulfate was infused into 7 ewes with a 0.11 gm/kg bolus over 20 minutes, followed by 0.08 gm/kg/hr. In 8 animals labor was induced with use of an intrafetal corticotropin infusion, after which 4 ewes received magnesium sulfate and 4 received saline solution. Continuous recordings of uterine electromyographic activity, amniotic pressure, fetal heart rate, blood pressure, and tracheal pressure were made. Maternal and fetal magnesium, calcium, albumin concentrations, and blood gases were determined before and during the infusion.

RESULTS

Maternal magnesium concentrations increased from an average of 0.94 +/- 0.03 mmol/L to 2.73 +/- 0.1 mmol/L at the end of the bolus, remaining elevated (2.44 +/- 0.17 mmol/L) for 8 hours. Fetal magnesium concentrations (0.89 +/- 0.03 mmol/L before the bolus) did not change with the maternal infusion. In ewes not in labor, uterine contractures occurred 3.7 +/- 0.7 times per 2 hours before and did not change significantly with the infusion of magnesium sulfate. During corticotropin-induced preterm labor uterine contractions were present 13 +/- 3.2 times per hour before infusions and were unchanged by infusion of magnesium sulfate to the ewes.

CONCLUSIONS

Magnesium sulfate infusion in pregnant sheep has no effect on either nonlabor uterine contractures or on corticotropin-induced preterm uterine contractions.

Effects of hypoxemia on 11 beta-hydroxysteroid dehydrogenase types 1 and 2 gene expression in preterm fetal sheep

OBJECTIVE

To examine the effect of sustained hypoxia on the expression of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) type 1 and 2 genes in preterm fetal sheep.

METHODS

Fetal liver and kidney as well as placental tissues were collected at days 111-113 of gestation (term = 145 days) after 8 hours of sustained hypoxemia induced by lowering the maternal inspired oxygen (n = 7) or after 8 hours of normoxia to serve as controls (n = 5). Changes in the levels of 11 beta-HSD1 and 11 beta-HSD2 mRNA were determined by Northern blot analysis using ovine 11 beta-HSD types 1 and 2 cDNAs as probes. Levels of 11 beta-HSD2 activity were determined by a standard radiometric conversion assay.

RESULTS

In hypoxic fetuses, there was a tendency for a decrease (P = .08) in levels of 11 beta-HSD2 mRNA in the kidney. This decrease was correlated significantly with the degree of associated fetal acidemia (P < .01). However, there were no corresponding changes in the level of renal 11 beta-HSD2 enzyme activity, indicating that changes in 11 beta-HSD2 mRNA were unlikely carried through to 11 beta-HSD2 protein. In contrast levels of 11 beta-HSD1 mRNA in the placenta and fetal liver were unchanged after sustained hypoxia.

CONCLUSION

These results demonstrate that fetal hypoxemia-induced acidosis selectively down-regulates 11 beta-HSD2 mRNA expression in the preterm fetal sheep kidney. This may provide a further mechanism whereby fetal acidosis alters developmental processes by regulating the bioavailability of glucocorticoids in specific fetal organs through altered local expression of 11 beta-HSD enzymes.

Fetal sheep endocrine responses to sustained hypoxemic stress after chronic fetal placental embolization

The purpose of this study was to determine the endocrine and circulatory responses of the ovine fetus, near term, to sustained hypoxemic stress superimposed on chronic hypoxemia. Fetal sheep were chronically embolized (n = 7) for 10 days between 0.84 and 0.91 of gestation via the descending aorta until arterial oxygen content was decreased by approximately 30%. Control animals (n = 8) received saline only. On experimental day 10, both groups were embolized over a 6-h period until fetal arterial pH decreased to approximately 7.00. Regional distribution of lower body blood flows was measured on day 10, before and at the end of acute embolization. On day 10, the chronically embolized group had lower arterial oxygen content (P < 0.05), Po2 (P < 0.01), and placental blood flow (P < 0.05) than controls and higher prostaglandin E2 (PGE2) and norepinephrine plasma concentrations (both P < 0.05). In response to a superimposed sustained hypoxemic stress, there was a twofold greater increase in PGE2 in the chronically embolized group than in the control group (P < 0.05). However, the increase in fetal plasma cortisol in response to superimposed hypoxemic stress was similar in both groups, despite significantly lower adrenocorticotropic hormone and adrenal cortex blood flow responses in the chronically hypoxemic group (both P < 0.05). We conclude that PGE2 response to a sustained superimposed reduction in placental blood flow, leading to metabolic acidosis, is enhanced under conditions of chronic hypoxemia and may play an important role for the maintenance of the fetal cortisol response to an episode of superimposed acute stress.

Fetal cerebral, circulatory, and metabolic responses during heart rate decelerations with umbilical cord compression

OBJECTIVE

The purpose of this study was to determine the cerebral, circulatory, and metabolic responses of the ovine fetus near term to umbilical cord compression with variable-type fetal heart rate decelerations.

STUDY DESIGN

Nine fetal sheep, at 0.9 of gestation, were studied before, during, and after umbilical cord occlusion for 1-minute and again after repetitive 1-minute cord occlusions every 5 minutes for 1 hour, with resultant fetal heart rate decelerations of approximately 90 beats/min. Brachiocephalic arterial and sagittal venous blood was analyzed for oxygen content, blood gases and pH, glucose, and lactate. Cerebral and upper body blood flow was measured with the microsphere technique.

RESULTS

Umbilical cord occlusion with moderate to severe variable-type fetal heart rate deceleration resulted in an immediate drop in arterial PO2 by approximately 7 torr, an increase in PCO2 by approximately 9 torr, and a small but significant increase in lactate levels. Cerebral oxidative metabolism was well maintained but required an increase in fractional oxygen extraction because the variable change in cerebral blood flow was insufficient to maintain oxygen delivery. A redistribution of upper body blood flow was evident, with that to the bran and heart variably maintained or increased whereas that to muscle tissue was markedly decreased. Repetitive umbilical cord occlusion over 1 hour resulted in a significant drop in fetal arterial pH, with the acidemia mixed as PCO2 increased approximately 6 torr, whereas lactate levels increased almost fourfold.

CONCLUSION

Although cerebral oxidative metabolism appears to be well maintained during moderate to severe variable-type fetal heart rate decelerations with umbilical cord occlusion, the need to increase fractional oxygen extraction and the redistribution of blood flow from carcass tissues may contribute to an accumulation of lactic acid both within the brain and systemically when such an insult occurs repeatedly.

Inhibition of ACTH secretion blocks hypoxia-induced increase of adrenal cortical blood flow in fetal sheep

To examine the role of endogenous adrenocorticotropic hormone (ACTH) in adrenal blood flow responses to hypoxia, we studied unanesthetized ovine fetuses during an intravenous infusion of cortisol or vehicle. Fetal hypoxia was induced after 5 h of cortisol or vehicle infusion. Control fetuses were not made hypoxic. Blood flows were determined before and at three time points during the infusions. At 2 and 6 h of hypoxia, in vehicle-infused fetuses, fetal plasma concentrations of immunoreactive ACTH (irACTH) had risen from 9 +/- 3 (SE) pg/ml to 68 +/- 25 and 127 +/- 37 pg/ml, respectively. No significant change in fetal plasma irACTH occurred in the other groups. Adrenal cortical blood flow rose three- to fourfold during hypoxia in vehicle-infused fetuses but did not change from prehypoxia levels in cortisol-infused fetuses (P < 0.005). Medullary flow rose with hypoxemia, and this was not affected by concurrent cortisol infusion. Adrenal blood flows did not change in the control groups. Thus prior infusion of cortisol suppressed the rise in fetal plasma ACTH during hypoxia and selectively blocked the increase in adrenal cortical blood flow.

A flat decelerative fetal heart rate tracing with normal fetal heart rate variability

We report two cases for which computer interpretation of nonstress test indicated a flat decelerative trace in spite of normal fetal heart rate variability. Fetal behavioral state in the first case and signal loss in the second case were possibly responsible for this computerized interpretation of the tracings in the absence of fetal distress.

Regional brain blood flow in the ovine fetus during transition to the low-voltage electrocortical state

Sequential changes in cerebral blood flow and regional distribution were studied in nine chronically catheterized fetal sheep during the transition to the low-voltage ECOG (REM) state to determine the time course for blood flow change within the brain and whether executive centres for REM state generation might thus be identified. Blood flows were measured during the first, second and third minutes after the transition to the low-voltage ECOG state and during the third minute of the subsequent high-voltage ECOG (NREM) state using the radioactive labelled microsphere technique. Blood flow to the brain was increased during the low-voltage REM state when compared to that of the high-voltage NREM state, with the increase evident when measured during the first minute after the state transition and with no sequential change thereafter. Regional blood flow increases during the low-voltage state were greatest to those areas variously associated with the generation of REM state activity, but were again remarkably stable through the first 3 minutes after the state transition. This rapid increase in blood flow within the brain and the regional hierarchy for such, supports the participation of multiple anatomical areas which are highly integrated and act in concert to give rise to what is known as the REM state.

The fetal brain: metabolic and circulatory responses to asphyxia

The fetal environment is well suited for normal brain growth and development with oxygen availability well in excess of oxidative needs. With impairments in blood gas exchange, cerebral oxidative metabolism is initially maintained by an increase in cerebral blood flow, thus protecting the 'oxygen margin of safety', and, when this response becomes limited, by an increase in the brain's fractional extraction of oxygen. Additional adaptive mechanisms involve substrate alterations and a decrease in energy consuming processes, including growth restriction and behavioural state alterations. Although protective insofar as essential metabolic functions are maintained, pathologic change may become evident as the 'oxygen margin of safety' becomes limited or energy conserving measures give rise to abnormal growth and development.

Cerebral oxidative metabolism in fetal sheep with prolonged and graded hypoxemia

Cerebral oxidative metabolism and associated circulatory responses were determined in 14 unanesthetized fetal sheep near term, during a normoxic control period and subsequently, during four days of prolonged and graded hypoxemia induced by progressively lowering maternal inspired oxygen concentration with 1-2% CO2 added; first day 18%, second day 16%, third day 12-14%, fourth day 10-12%. Preductal arterial and sagittal vein blood samples were analyzed for oxygen content, blood gas tensions and pH. Regional blood flow was measured with a microsphere technique. Cerebral blood flow increased in a stepwise manner with the graded reduction in fetal arterial O2 saturation and continued to be well predicted by blood gas and metabolic alterations, with no adaptive change evident. Cerebral oxidative metabolism remained little changed with chronically induced hypoxemia until arterial O2 saturation was less than 30% and with fetal acidemia evident when decreased to 70% of normoxic control values. Whether the decrease in oxidative metabolism by the brain at this time represents an adaptive response whereby growth and functional alterations lead to a decrease in nonessential energy utilization or rather a pathological change, remains to be determined.

Regional adrenal blood flow responses to adrenocorticotropic hormone in fetal sheep

To determine whether adrenocorticotropic hormone (ACTH) at plasma concentrations measured during mild hypoxemia and at term affects adrenal blood flow, we measured regional blood flows in five unanesthetized normoxemic fetuses (125-130 days gestation) during a 24-h intravenous infusion of ACTH-(1-24) in isotonic saline solution. Another five fetuses received an infusion of vehicle. Blood flows were determined before the infusion, at 2 and 24 h from its onset, and 24 h afterward using radionuclide-labeled microspheres. Blood flow to the adrenal medulla was fivefold greater than that to the adrenal cortex. Adrenal blood flow rose 99% at 24 h of the ACTH infusion. There was a large increase in adrenal cortical blood flow of 272% at this time but medullary blood flow did not change significantly during ACTH infusion. The rise in cortical blood flow was attributable to decreased vascular resistance. No significant alterations occurred in fetal arterial blood pressure and heart rate, or in blood flow to other lower body organs of the fetus or to the placental cotyledons. These findings are consistent with the hypothesis that the increase in adrenal blood flow observed during fetal hypoxia is associated with changes in plasma ACTH concentration. They are also indicative of selective regulation of cortical and medullary blood flows in the sheep fetus at this stage of gestation.

Electrocortical activity, electroocular activity, and breathing movements in fetal sheep with prolonged and graded hypoxemia

OBJECTIVE

Our objective was to determine the effect of a prolonged and graded reduction in fetal arterial oxygen saturation on electrocortical activity and associated biophysical variables.

STUDY DESIGN

Fourteen unanesthetized fetal sheep were studied between 126 and 135 days' gestation with continuous monitoring of electrocortical and electroocular activity and breathing movements, during a 24-hour control period, and subsequently during 4 days of prolonged and graded hypoxemia induced by progressively lowering the maternal inspired oxygen concentration.

RESULTS

Graded reduction in fetal arterial oxygen saturation resulted in little change in arterial pH until close to 30% when metabolic acidemia was apparent. The incidence of low-voltage electrocortical activity, electroocular activity, and breathing movements were marginally decreased with hypoxemia alone; however, a significant decrease was not apparent until associated with the onset of fetal acidemia.

CONCLUSION

Hypoxemia of a chronic nature must approach the level at which acidemia becomes apparent before a marked change in fetal behavioral activity is noted.

The effect of behavioral state on fetal metabolism and blood flow circulation

As in postnatal life, behavioral states when evident in utero are also seen to influence the metabolic activity and/or circulation of several organ systems if not the global metabolic and circulatory activity of the fetus. In some instances this effect is organ specific as in the increased metabolic activity of the brain during the fetal REM state, which may account for the prominence of this state through the perinatal period and contribute to the brain's growth and development at this time. Generalized metabolic and circulatory effects may also be seen as associated with state-related changes in fetal biophysical activity or circulating vasoactive substances. Although the exact functions of sleep and sleep states are not yet known, their influence on the metabolic and circulatory activity of organ systems is seen to onset in early life, suggesting that a need for such may be just as important during early development as in later life.

Oxygen consumption is maintained in fetal sheep during prolonged hypoxaemia

Experiments were conducted in 12 chronically-catheterized pregnant sheep to examine the effect of prolonged hypoxaemia secondary to the restriction of uterine blood flow on fetal oxygen consumption. Surgery was performed at 115 days gestation to place a teflon vascular occluder around the maternal common internal iliac artery and for insertion of vascular catheters. Following a 5-day recovery period, uterine blood flow was reduced in 6 animals for 24 hours and in 6 animals, the occluder was not adjusted. Fetal arterial PO2 decreased from 19.9 +/- 2.0 mmHg to 12.8 +/- 2.0 mmHg and 11.0 +/- 2.0 mmHg at 1 and 24 hours respectively in the experimental group and did not change the control group. Fetal pH decreased from 7.34 +/- 0.01 to 7.25 +/- 0.03 and 7.29 +/- 0.02 at 1 and 24 hours of hypoxaemia respectively. Fetal arterial lactate concentrations remained elevated throughout the experimental period with maximum concentrations of 6.6 +/- 2.1 mmol/l being present at 4 hours compared to 1.3 +/- 0.2 mmol/l during the control period. Umbilical blood flow increased from 186 +/- 19 ml/min/kg to 251 +/- 39 ml/min/kg at 1 h of hypoxaemia and returned to 191 +/- 21 ml/min/kg at 24 h. In association with the progressive fall in oxygen delivery to the fetus, oxygen extraction increased from 0.33 +/- 0.04 to 0.43 +/- 0.04 and 0.54 +/- 0.05 at 1 and 24 hours, respectively. Overall oxygen consumption by the fetus remained unchanged from control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen consumption in the fetal lamb during sustained hypoxemia with progressive acidemia

To investigate the fetal ability to compensate for a sustained reduction in O2 delivery (DO2; umbilical blood flow X umbilical venous O2 content), studies were carried out on eight chronically instrumented fetal lambs made hypoxemic for 7.8 +/- 0.8 (+/- SE) h by lowering maternal inspired O2 concentration to 9-10%. After 1.7 h of hypoxemia, fetal arterial PO2 had fallen from 18.4 +/- 1.2 to 10.4 +/- 0.5 mmHg. Umbilical venous O2 content fell initially by 41.1 +/- 1.8%, but the fall in DO2 was only 23.7 +/- 5.6%, caused by a 29.3 +/- 7.9% rise in umbilical blood flow. Fetal O2 consumption (VO2) was increased significantly by 29.5 +/- 15.2%. However, fetal vascular pH (7.332-7.281) and base excess (0.5 to -4.3 meq/l) were decreased while blood lactate levels were increased (1.55-7.22 mM). With continued hypoxemia, the metabolic acidemia worsened and led to progressive declines in umbilical venous O2 content and DO2. However, VO2 was maintained at the control level until delivery had fallen by 72.5% and arterial pH was 6.843, at which time VO2 decreased by 37.5 +/- 10.7%. It is concluded that the ability of the fetus to compensate for sustained hypoxemia is limited by the progressive metabolic acidemia.

Blood flow and oxygen delivery to fetal organs and tissues during sustained hypoxemia

To examine the fetal cardiovascular responses to a sustained reduction in O2 delivery (DO2), studies were conducted on 13 chronically instrumented fetal lambs (128-138 days gestation) made hypoxemic for 7.9 +/- 0.5 h by lowering maternal inspired O2 concentration to 9-10%. Fetal descending aortic PO2 fell initially from 18.0 +/- 1.0 to 10.7 +/- 0.6 mmHg, whereas pH decreased progressively from 7.326 +/- 0.006 to 6.843 +/- 0.023. Blood flow to the cerebral hemispheres, myocardium, and adrenal glands rose maximally by 110.2 +/- 22.5, 253.7 +/- 41.1, and 338.7 +/- 55.0%. Cerebral hemispheric DO2 fell progressively, whereas DO2 to the myocardium and adrenal was maintained until 7.9 h, when it fell significantly. There was also a rise in blood flow to brown adipose tissue. Blood flow to the gut and skeletal muscle was maintained, whereas flow to the spleen and kidney fell. DO2 to all these tissues fell markedly because of the progressive decline in blood O2 content. It is concluded that fetal cardiovascular function was well maintained in the face of severe hypoxemia and marked acidemia.

Cerebral oxidative metabolism in lambs during perinatal period: relationship to electrocortical state

Cerebral oxidative metabolism and regional blood flow were studied, with changes in behavioral state over the immediate perinatal period, to determine the change in cerebral metabolism at birth and the relationship to behavioral activity. Nine unanesthetized fetal sheep (139-142 days gestation) were each studied during a period of high-voltage and low-voltage electrocorticogram (ECOG) activity and then again after cesarean delivery at 2 h of age while awake and at 24 h of age while awake and during a period of high-voltage ECOG sleep. Preductal arterial and sagittal vein blood samples were analyzed for O2 content, blood gases, and pH. Blood flow was measured with a radioactive microsphere technique. Cerebral oxidative metabolism showed no significant perinatal change, although a coupling to behavioral state was evident with significant increases during both the perinatal low-voltage ECOG state and the awake state at 24 h. Blood flow to the brain showed a significant perinatal decrease after birth and state-related increases during both the prenatal low-voltage ECOG state and the awake state at 24 h consistent with the tight coupling of flow and/or O2 delivery to metabolic needs reported for normal brain tissue. The cerebral metabolic rate of the prenatal low-voltage ECOG state suggested a level of functional excitation comparable with that of the awake state after birth, supporting the importance of the rapid-eye-movement (REM) state mechanism as a source of endogenous stimulation during the perinatal period of brain growth and development.

Fetal adaptive responses to asphyxia

The fetal environment is thus well suited for normal growth and development with oxygen availability exceeding oxidative needs. With impairments in blood gas exchange this excess oxygen acts as a "margin of safety," providing for the maintenance of oxidative metabolism through increases in fractional O2 extraction, although with resultant fetal hypoxemia. Increases in blood O2 capacity and redistribution of cardiac output in response to this hypoxemia further protect fetal oxygenation. Additional adaptive mechanisms involve a decrease in energy-consuming processes, including growth restriction, decreasing fetal movements, and behavioral state alterations. Although protective in so far as essential metabolic functions are maintained, pathologic change may occur as the "oxygen margin of safety" becomes limited or energy-conserving measures give rise to abnormal growth and development.

Regional blood flow change in the lamb during the perinatal period

Regional blood flow distribution was studied with a radioactive-labeled microsphere technique in 11 unanesthetized fetal sheep (139 to 143 days' gestation). After cesarean delivery each neonatal lamb was studied at 2 and 24 hours of age. Blood flow to the cerebral hemispheres and adrenal glands decreased progressively after birth and correlated inversely with the postnatal rise in arterial oxygen content (r = -0.77, p less than 0.001 and r = -0.52, p less than 0.01, respectively). Blood flow to the gastrointestinal tract and kidneys changed little despite the known increase in blood flow from the late fetal to the early neonatal period. Skeletal muscle and brown fat blood flow were both increased when measured at 2 hours after birth, with an even greater increase in their respective oxygen deliveries presumably reflecting their increased metabolic activity with thermogenesis at this time. Variable blood flow changes are thus evident over the immediate perinatal period. In some cases these changes reflect birth-related changes in either blood gases or functional activity whereas in others the changes simply reflect a continuum from the late fetal to the early neonatal period.

Adrenocorticotropic hormone, cortisol, and progesterone changes in the lamb during the perinatal period

Although the changes in circulating concentrations of adrenocorticotropic hormone, cortisol, and progesterone are well established for the fetal and neonatal lamb, there is little information on these hormones in the immediate perinatal period. We have examined the relationship between these hormones and systemic blood gas tensions and substrate concentrations in the perinatal period. Measurements were made in arterial blood of seven unanesthetized fetal sheep at 138 to 141 days' gestation during low- and high-voltage electrocortical activity. After cesarean delivery each newborn lamb was studied again at 2, 5, and 10 minutes and at 2 and 24 hours while awake. There was no correlation between fetal or maternal adrenocorticotropic hormone, cortisol, or progesterone and fetal electrocortical activity. Within 2 to 5 minutes of delivery there was a dramatic increase in neonatal immunoreactive adrenocorticotropic hormone concentrations associated with an increase in plasma cortisol. Neonatal Pao2 rose progressively, but a mixed respiratory and metabolic acidosis was evident during the first 10 minutes after delivery. This was partially corrected by 2 hours of neonatal life and was fully corrected by 24 hours of life. We conclude that rapid responsiveness of the fetal pituitary-adrenal axis occurs in response to birth in the absence of active labor, and we suggest that this may have survival value for the preterm fetus.

Cerebral oxidative metabolism during sustained hypoxaemia in fetal sheep

Cerebral oxidative metabolism was determined in 9 unanaesthetized fetal sheep near term, during a normoxic control period and during sustained hypoxaemia induced by lowering maternal inspired O2 concentration to 11-8% with 3% CO2 added. Preductal arterial and sagittal vein blood samples were analyzed for oxygen content, blood gas tensions and pH. Cerebral blood flow was measured with a radioactively-labelled microsphere technique. Induced fetal hypoxaemia resulted in a metabolic acidaemia which was progressive over several h. Cerebral oxygen consumption was initially marginally decreased in response to induced hypoxaemia with cerebral blood flow increased thus maintaining O2 delivery coupled to cerebral oxygen consumption. With a worsening metabolic acidemia, pHa below 7.15, cerebral blood flow fell as mean arterial pressure fell, but cerebral oxygen consumption was little changed as fractional O2 extraction now increased. With sustained hypoxaemia and profound metabolic acidaemia, pHa below 7.00, fractional O2 extraction also fell resulting in a terminal fall in cerebral oxygen consumption to less than 50% of control values. Although the initial marginal decrease in cerebral oxygen consumption in response to induced hypoxia may represent a protective mechanism whereby the fetal brain decreases nonessential functions thus lowering oxidative needs, the terminal fall in cerebral oxygen consumption suggests pathological alterations within the brain at this time.

Circulatory responses to prolonged hypoxemia in fetal sheep

Experiments were conducted in 11 chronically catheterized pregnant sheep to determine the distribution of blood flow within the fetus during prolonged (48 hours) hypoxemia secondary to the restriction of uterine blood flow. Uterine blood flow was mechanically restricted with a polytetrafluoroethylene vascular clamp placed around the maternal common internal iliac artery such that mean (+/- SEM) fetal arterial oxygen tension decreased from 23.4 +/- 1.9 to 17.3 +/- 0.8 mm Hg at 1 hour of hypoxemia and remained low for 48 hours. There was an initial increase in fetal arterial carbon dioxide pressure from 48.5 +/- 0.9 mm Hg during the control period to 56.2 +/- 2.3 mm Hg at 1 hour; this parameter subsequently returned to control values, whereas base excess showed a transient decrease. Fetal cerebral, myocardial, and adrenal blood flows were significantly increased at 1, 24, and 48 hours of hypoxemia. In contrast, there was no change in nuchal muscle or renal blood flows with hypoxemia of this magnitude. Cotyledonary blood flow increased transiently by 38% at 1 hour of hypoxemia, but was not changed from control at 24 and 48 hours. These experiments demonstrate that the sheep fetus is able to maintain the normal protective circulatory adjustments seen with acute hypoxemia for up to 48 hours in the absence of progressive metabolic acidemia.

Initiation of pulmonary gas exchange by fetal sheep in utero

The role of umbilical cord occlusion in the initiation of breathing at birth was investigated using unanesthetized fetal sheep that were provided with access to a tracheal supply of hyperoxic air. Near-term fetuses were studied in utero to eliminate extraneous sensory stimuli. Gasping movements began 1.4 +/- 0.1 min after cord occlusion. Breathing was irregular for several minutes before continuous breathing (greater than or equal to 40 min-1) began 6 +/- 1 min after cord occlusion (n = 10). Arterial PO2 rose significantly from 18 +/- 2 mmHg before occlusion and was 115 +/- 15 mmHg immediately before cord release at 15 or 30 min. Breathing continued even during high-voltage electrocortical activity. Cord release caused the breathing rate to decrease from 77 +/- 13 min-1 during the last 5 min of cord occlusion to 5 +/- 3 min-1 10 min after cord release (P less than 0.002; n = 7). Results indicate the change from placental to lung gas exchange can occur in the absence of sensory and thermal changes normally present at birth and that the transition is reversible.

Plasma adrenocorticotropic hormone and cortisol and adrenal blood flow during sustained hypoxemia in fetal sheep

We examined the effect of sustained hypoxemia with progressive acidemia on pituitary-adrenal endocrine function (adrenocorticotropic hormone, cortisol) and on adrenal blood flow in fetal sheep. Hypoxemia was induced by the maternal sheep breathing a gas mixture containing 9% oxygen, with 3% carbon dioxide added. Induced hypoxemia resulted in a progressive fetal metabolic acidosis but with little change in maternal pH. During induced hypoxemia there was little change in maternal plasma adrenocorticotropic hormone or cortisol level. Fetal adrenocorticotropic hormone and cortisol increased to peak values within 2.8 hours of induced hypoxia but by 7.2 hours had begun to fall to values that were not significantly different from those at 1.4 hours. Fetal adrenal blood flow (microsphere technique) also increased significantly and remained elevated throughout the duration (7.2 hours) of hypoxemia. The maximum fetal adrenal blood flow achieved during hypoxemia was significantly correlated with the basal (prehypoxemia) flow to the adrenals. We conclude that the changes in fetal adrenocorticotropic hormone, cortisol, and adrenal blood flow seen in short-term hypoxemia are reproduced during sustained hypoxemia with acidemia. Furthermore, the noted rise in the fetal adrenocorticotropic hormone level may be an important factor contributing to the increase in adrenal blood flow during hypoxemia.

Effect of acute hypoglycaemia on cerebral metabolic rate in fetal sheep

Studies were carried out in 11 fetal sheep four days after surgery for insertion of catheters and electrocortical leads. After a 3 h control period an insulin infusion was given to the ewe and maintained for the next 4 h. Fetal arterial glucose fell from 0.85 +/- 0.10 to 0.57 +/- 0.06 mM (SEM) while oxygen content was unchanged (3.80 +/- 0.24 to 3.75 +/- 0.21 mM). Cerebral uptake of oxygen and glucose were determined from samples drawn simultaneously from the axillary artery and sagittal vein and cerebral blood flow (microsphere technique). There was no significant change in uptake of either oxygen or glucose by the fetal brain. We conclude that a rapid fall in fetal glucose levels with no change in oxygen content does not result in decrease in cerebral metabolism measured over a short term.

Cerebral metabolism in fetal lamb after maternal infusion of ethanol

Cerebral uptake of glucose and O2 (VCO2) was measured in 10 chronically catheterized fetal lambs during a control period, after a 1-h maternal infusion of ethanol (1.0 g X kg-1 X h-1) and 1-h postethanol infusion, to determine if alterations in cerebral metabolism might occur. Brachiocephalic artery and sagittal vein blood samples were analyzed for glucose, O2 content, blood gases, pH, and ethanol. Cerebral blood flow (Qc) was measured with a radioactive microsphere technique. VCO2 decreased significantly, from 140 +/- 13 mumol X 100 g-1 X min-1 during the control period to 91 +/- 8 (P less than 0.05) with the ethanol infusion, with a related fall in Qc, 177 +/- 12 to 104 +/- 9 ml X 100 g-1 X min-1 (P less than 0.001), the cerebral arteriovenous O2 difference being little changed. Cerebral glucose uptake, although decreased, was not significantly changed with the ethanol infusion, because the cerebral arteriovenous difference for glucose increased with the related fall in Qc. We conclude that maternal infusion of ethanol results in a decreased fetal VCO2 with a related fall in Qc. If prolonged, this decrease in cerebral oxidative metabolism might well affect cerebral development and provide a mechanism whereby chronic alcohol intake contributes to central nervous system growth anomalies and dysfunction of infants exposed to alcohol in utero.

Cerebral oxidative metabolism in the fetal lamb: relationship to electrocortical state

Cerebral oxidative metabolism and blood flow were measured in 14 chronically prepared fetal lambs with changes in fetal electrocortical activity. Myocardial blood flow was also measured with changes in fetal state in order to determine whether metabolic changes observed were organ specific. Samples of preductal arterial and sagittal vein blood were analyzed for oxygen content, blood gases, and pH. Blood flow was measured with a radioactive microsphere technique. Cerebral oxidative metabolism increased significantly from 126 +/- 7 mumol/100 gm/min during the high-voltage electrocortical state to 152 +/- 7 mumol/100 gm-1/min-1 (p less than 0.05) during the low-voltage electrocortical state. The increase in cerebral oxidative metabolism was sustained by an increase in blood flow, 148 +/- 7 ml/100 gm/min to 173 +/- 10 ml/100 gm/min (p less than 0.01), whereas the arterial venous oxygen difference remained unchanged. Changes in myocardial blood flow were in the opposite direction, with a decrease noted during the low-voltage electrocortical state, and were correlated with changes in fetal heart rate. We conclude that cerebral oxidative metabolism is increased during the fetal low-voltage electrocortical state and suggests an important role for the increased incidence of this state during the accelerated growth and development of the brain during the perinatal period.

Insulin hypoglycemia, cerebral metabolism, and neural function in fetal lambs

The effect of insulin-induced hypoglycemia on cerebral oxidative metabolism (CMRO2) was studied in nine late gestational fetal lambs using the radiolabeled microsphere technique for cerebral blood flow and brachiocephalic to sagittal sinus blood O2 content differences. After 4 h insulin infusion to the fetus, arterial glucose fell from control levels of 0.96 +/- 0.11 (SE) to 0.69 +/- 0.09 mmol X l-1. CMRO2 was reduced from 199 +/- 23 to 155 +/- 22 mumol X 100 g-1 X min-1 (P less than 0.05), and cerebral glucose uptake fell from 31 +/- 4 to 25 +/- 4 mumol X 100 g-1 X min-1 (P less than 0.02). During both euglycemia and hypoglycemia, 6 mumol glucose were taken up for each micromole of O2, indicating that glucose was the sole metabolic substrate for oxidative metabolism. Although there was no change in fetal electrocortical activity during the hypoglycemia, fetal breathing movements were present only 19.4 +/- 3.4% of the hypoglycemic hours compared with 36.8 +/- 2.6% of the control period (P less than or equal to 0.01). These results suggest that during rapidly induced fetal hypoglycemia, blood-brain barrier transport of glucose can limit cerebral glucose and O2 uptake, and this decrease in cerebral metabolism is associated with a lowered incidence of fetal breathing movements.

Fetal breathing movements and the response to carbon dioxide in patients on methadone maintenance

Fetal breathing movements were monitored on six methadone maintenance patients and ten healthy control patients, studied while breathing room air and while breathing a prepared gas mixture with 5% carbon dioxide, both before and 2 hours after either the usual daily dose of methadone or a diet drink. There was no difference before and after the diet drink in control patients, who showed a significant increase in the incidence of fetal breathing movements from 37.1% +/- 5.9% (SEM) on room air to 69.4% +/- 2.8% while breathing 5% carbon dioxide (p less than 0.01). In the patients studied on room air before receiving methadone, fetal breathing movements were significantly decreased from those of the control group, 4.7% +/- 1.2% (p less than 0.01), with a further decrease when studied after receiving methadone, 1.3% +/- 0.7%. Fetal breathing movements did increase significantly in response to 5% carbon dioxide both before and after receiving methadone; however, in both instances the incidence was significantly less that that of the control group. The findings of the present study on methadone subjects demonstrate that abnormal function of the respiratory control network is evident in utero, which may be predictive of subsequent neonatal respiratory development.

Cerebral metabolism in hypoglycemic and hyperglycemic fetal lambs

To investigate the mechanism whereby glucose affects fetal breathing movements (FBM), cerebral metabolism was studied in 12 unanesthetized fetal lambs, during fasting-induced hypoglycemia and after a subsequent fetal infusion of glucose. Preductal arterial and sagittal sinus blood samples were analyzed for glucose and oxygen concentrations and for blood gases and pH. Measurements of regional brain blood flow were made with radioactive microspheres. Maternal fasting of 24- to 36-h duration resulted in a decrease in fetal blood glucose from 1.061 +/- 0.085 mmol X l-1 to 0.664 +/- 0.053 (P less than 0.001). Although cerebral glucose and O2 uptake remained unchanged, sagittal sinus CO2 partial pressure (Pco2) and [H+] were significantly decreased and may have contributed to the observed decrease in FBM. A 2-h infusion of glucose to the fetuses of fasted animals resulted in an increase in blood glucose to 2.452 +/- 0.173 mmol X l-1. Cerebral O2 consumption was again unchanged; however, cerebral glucose consumption was significantly increased as were sagittal sinus Pco2 and [H+], which may have contributed to the observed increase in FBM. These findings are consistent with the hypothesis that glucose affects the incidence of FBM in part by altering the environment of central chemoreceptors.

Central chemical regulation of breathing movements in fetal lambs

In 12 chronically prepared fetal lambs between 126 and 136 days of gestation, 17 ventriculocisternal perfusions (123 microliter/min) were performed. The concentration of bicarbonate ([HCO3-]) in the mock cerebrospinal fluid (CSF) perfusate was altered to manipulate the [HCO3-] in the cerebral ventricles. These perfusions did not systematically alter fetal arterial PCO2, PO2, pH, heart rate or mean blood pressure. Fetal breathing movements (FBM) were noted as rhythmic negative intrathoracic pressures with reference to amniotic fluid pressure. The incidence of FBM was determined and expressed as the percent of the 3 h observation period during which breathing movements were present. Perfusions with mock CSF having an approximately normal [HCO3-], resulted in cisternal [HCO3-] of 23.9 +/- 0.8 (SEM) meq/L and an incidence of fetal breathing movements (FBM) of 22.1 +/- 10.0%. Perfusions with an elevated [HCO3-] in the mock CSF increased the cisternal [HCO3-] to 30.7 +/- 0.4 meq/L and lowered the incidence of FBM to 0.6 +/- 0.2%. Perfusions with a lowered [HCO3-] mock CSF decreased the cisternal [HCO3-] to 17.3 +/- 0.8 meq/L and increased the incidence of FBM to 45.7 +/- 6.8%. These perfusions did not alter the relative incidence of low voltage (LV) versus high voltage (HV) electrocortical (ECOG) state. In 4 animals, low [HCO3-] perfusions induced FBM in the normally apneic HV ECOG state. We conclude that in the near-term fetal lamb, central chemoreceptors can modulate the incidence of rhythmic respiratory efforts, that tonic afferent stimuli arising from these receptors are critical for the generation of spontaneous FBMs and that central acidosis is capable of evoking FBMs in the normally apneic HV ECOG state.

Ventriculocisternal cerebrospinal perfusion in unanesthetized fetal lambs

Methods have been developed for the placement of cannulas in the lateral cerebral ventricle and cisternum magnum of the fetal lamb. Three to seven days after surgery, perfusion of the cerebral ventricular system was carried out in unanesthetized lambs in utero. Artificial cerebrospinal fluid (CSF) was perfused at 123 microliter/min for 4-5 h with no change in fetal arterial O2 pressure, CO2 pressure, pH, mean arterial blood pressure, or heart rate. The preparation is suitable for measurements of net formation of fetal central CSF and also for producing changes in the ionic composition of the fetal CSF.