The effect of oxygen content during an initial sustained inflation on heart rate in asphyxiated near-term lambs.

OBJECTIVE: At birth, an initial sustained inflation (SI) uniformly aerates the lungs, increases arterial oxygenation and rapidly improves circulatory recovery in asphyxiated newborns. We hypothesised that lung aeration, in the absence of an increase in arterial oxygenation, can increase heart rate (HR) in asphyxiated near-term lambs. INTERVENTIONS: Lambs were delivered and instrumented at 139±2 days of gestation. Asphyxia was induced by umbilical cord clamping and then delaying the onset of ventilation until mean carotid arterial pressures (CAPs) had decreased <20 mm Hg. Lambs then received a single 30-s SI using nitrogen (N2; n=6), 5% oxygen (O2; n=6), 21% O2 (n=6) or 100% O2 (n=6) followed by ventilation in air for 30 min. MAIN OUTCOME MEASURES: HR, CAP and pulmonary blood flow (PBF) were continuously recorded. RESULTS: HR and PBF increased more quickly in lambs resuscitated with 100% and 21% O2 than with 5% O2 or N2. HR and PBF recovery in the 5% O2 group was delayed relative to all other oxygen SI groups. HR in 5%, 21% and 100% O2 groups reached 100 bpm before the SI was complete. HR and PBF in the N2 group did not increase until 10 s after the SI was completed and ventilation was initiated with air. CAP tended to increase quicker in all O2 groups than in N2 group. CONCLUSIONS: Oxygen content during an SI is important for circulatory recovery in asphyxiated lambs. This increase in HR is likely driven by the increase in PBF and venous return to the heart.

The impact of maternal synthetic glucocorticoid administration in late pregnancy on fetal and early neonatal hypothalamic-pituitary-adrenal axes regulatory genes is dependent upon dose and gestational age at exposure.

In this study, we determined the gene and/or protein expression of hypothalamic-pituitary-adrenal (HPA) axis regulatory molecules following synthetic glucocorticoid exposures. Pregnant sheep received intramuscular saline or betamethasone (BET) injections at 104 (BET-1), 104 and 111(BET-2) or 104, 111 and 118 (BET-3) days of gestation (dG). Samples were collected at numerous time-points between 75 dG and 12 weeks postnatal age. In the BET-3 treatment group, fetal plasma cortisol levels were lower at 145 dG than controls and gestational length was lengthened significantly. The cortisol:adrenocorticotropic hormone (ACTH) ratio in fetal plasma of control and BET-3 fetuses rose significantly between132 and 145 dG, and remained elevated in lambs at 6 and 12 weeks of age; this rise was truncated at day 145 in fetuses of BET-3 treated mothers. After BET treatment, fetal and postnatal pituitary proopiomelanocortin mRNA levels were reduced from 109 dG to 12 weeks postnatal age; pituitary prohormone convertase 1 and 2 mRNA levels were reduced at 145 dG and postnatally; hypothalamic arginine vasopressin mRNA levels were lowered at all time-points, but corticotrophin-releasing hormone mRNA levels were reduced only in postnatal lambs. Maternal BET increased late fetal and/or postnatal adrenal mRNA levels of ACTH receptor and 3ß hydroxysteroid dehydrogenase but decreased steroidogenic acute regulatory protein and P450 17-α hydroxylase. The altered mRNA levels of key HPA axis regulatory proteins after maternal BET injections suggests processes that may subserve long-term changes in HPA activity in later life after prenatal exposure to synthetic glucocorticoids.

Effects of glucocorticoid treatment given in early or late gestation on growth and development in sheep.

Antenatal corticosteroids are used to augment fetal lung maturity in human pregnancy. Dexamethasone (DEX) is also used to treat congenital adrenal hyperplasia of the fetus in early pregnancy. We previously reported effects of synthetic corticosteroids given to sheep in early or late gestation on pregnancy length and fetal cortisol levels and glucocorticoids alter plasma insulin-like growth factor (IGF) and insulin-like growth factor binding protein (IGFBP) concentrations in late pregnancy and reduce fetal weight. The effects of administering DEX in early pregnancy on fetal organ weights and betamethasone (BET) given in late gestation on weights of fetal brain regions or organ development have not been reported. We hypothesized that BET or DEX administration at either stage of pregnancy would have deleterious effects on fetal development and associated hormones. In early pregnancy, DEX was administered as four injections at 12-hourly intervals over 48 h commencing at 40-42 days of gestation (dG). There was no consistent effect on fetal weight, or individual fetal organ weights, except in females at 7 months postnatal age. When BET was administered at 104, 111 and 118 dG, the previously reported reduction in total fetal weight was associated with significant reductions in weights of fetal brain, cerebellum, heart, kidney and liver. Fetal plasma insulin, leptin and triiodothyronine were also reduced at different times in fetal and postnatal life. We conclude that at the amounts given, the sheep fetus is sensitive to maternal administration of synthetic glucocorticoid in late gestation, with effects on growth and metabolic hormones that may persist into postnatal life.

The cerebral critical oxygen threshold of ventilated preterm lambs and the influence of antenatal inflammation.

Perinatal inflammation is associated with adverse neurodevelopmental outcomes, which may be partly due to changes in the cerebral oxygen delivery/consumption relationship. We aimed to determine the critical oxygen delivery threshold of the brain of preterm, ventilated lambs and to determine whether the critical threshold is affected by exposure to inflammation in utero. Pregnant ewes received intra-amniotic injection of lipopolysaccharide or saline at 125 or 127 days of gestation. Pulmonary and systemic flow probes and catheters were surgically positioned in the fetus immediately before delivery at 129 days of gestation. After delivery, lambs were ventilated for 90 min using a positive end-expiratory pressure recruitment strategy. Cardio-respiratory variables and blood gases were measured regularly. Systemic and cerebral oxygen delivery, consumption (Fick), and extraction were calculated, and the relationship between cerebral delivery and consumption analyzed. Linear regression was used to define the transition or "critical" oxygen threshold as the point at which the slope of the oxygen delivery/consumption curve changed to be > 10°. Four subgroups were defined according to the calculated critical threshold. A total of 150 measurements were recorded in 18 lambs. Fetal cerebral oxygen consumption was increased by antenatal lipopolysaccharide (P < 0.05). The postnatal critical oxygen threshold was 3.6 ml·kg⁻¹·min⁻¹, corresponding to cerebral oxygen consumption of 0.73 ml·kg⁻¹·min⁻¹. High oxygen delivery and consumption were associated with increased pulmonary and carotid blood flow and systemic extraction compared with low oxygen delivery and consumption. No postnatal effect of antenatal inflammation was observed. Inflammation in utero increases fetal, but not postnatal, cerebral oxygen consumption. Adverse alterations to pulmonary blood flow can result in reduced cerebral blood flow, oxygen delivery, and consumption. Regardless of exposure to inflammation, there is a consistent postnatal relationship between cerebral oxygen delivery and consumption.

Differential appearance of placentomes and expression of prostaglandin H synthase type 2 in placentome subtypes after betamethasone treatment of sheep late in gestation.

UNLABELLED: Inappropriate fetal exposure to maternal glucocorticoid (GC) has been proposed as a mechanism for fetal programming where the effects of GC may be mediated by the placenta. However, the consequences of maternal GC on placental morphology and enzyme expression are unclear. OBJECTIVES: We used betamethasone (BET) to determine effects on placentome subtype distribution and expression of prostaglandin H synthase type 2 (PGHS-2) enzyme. METHODS: Pregnant sheep carrying male fetuses were randomized to receive injections of saline (n = 30) or one (104 days of gestation, (dG); n = 6), two (104, 111 dG; n = 6) or three (104, 111, 118 dG; n = 11) doses of BET (0.5 mg/kg). Placental tissue was collected prior to (75, 84, 101 dG), during (109, 116 dG) and after BET (122, 132, 146 dG). RESULTS: Total number of placentomes was not different between gestational ages. A- and B-subtypes were most affected by prenatal BET exposure; numbers of A-subtypes were increased and numbers of B-subtypes were decreased compared to controls at 116 dG. At term numbers of A-subtypes were lower after BET, but the weight range distribution was similar to controls. In controls, placental PGHS-2 protein levels increased with gestational age and PGHS-2 localized primarily to uninuclear trophoblast cells. After BET, PGHS-2 protein in C-subtypes at term was significantly increased compared to A-subtypes. CONCLUSIONS: Maternal BET treatment in late gestation affects the proportions of placentome subtypes and their differential expression of PGHS-2. Our data do not support previous hypotheses that A-subtypes develop into B-, C- and D-subtypes over the course of gestation.

All-trans retinoic acid and intra-amniotic endotoxin-mediated effects on fetal sheep lung.

All-trans retinoic acid (RA) is a potent modulator of lung development. Chorioamnionitis, which is frequently associated with preterm birth, causes fetal lung inflammation and improves lung function but also results in alveolar simplification and microvascular injury. Endotoxin-mediated chorioamnionitis reduces RA concentration in the fetal lung to 16% of control values. We hypothesized that administration of RA to the fetus before induction of chorioamnionitis would preserve septation of the distal airspaces. Time-mated ewes with singletons were assigned to receive a fetal intramuscular treatment with 20,000 IU of RA in olive oil (or olive oil only) 3 hr prior to intra-amniotic injection of endotoxin (20 mg, E. coli 055:B5) or saline, at 124-day gestational age and 7 days after the fetal treatment. The right cranial lung lobe was processed for morphometric analysis. RA treatment did not affect chorioamnionitis-induced fetal and systemic inflammation or interleukin-8 concentrations in lung tissue. RA administration alone did not alter lung structure. Relative to control lungs (5 +/- 3 mL/kg), lung volume increased similarly with endotoxin (22 +/- 4 mL/kg) or RA plus endotoxin (20 +/- 3 mL/kg; P < 0.05). Alveolar wall thickness was 4.2 +/- 0.3 mum after endotoxin-induced chorioamnionitis, 6.0 +/- 0.4 mum in controls (P < 0.05 versus endotoxin) and 5.5 +/- 0.2 mum after RA and endotoxin (P < 0.05 versus control, n.s. versus endotoxin). The ratio of airspace versus tissue was 4.6 +/- 0.3 in endotoxin-induced chorioamnionitis, 2.1 +/- 0.3 in controls and 4.1 +/- 0.5 after RA and endotoxin. We conclude that fetal treatment with RA did not prevent inflammation-induced alveolar simplification.

The carotid bodies influence growth responses to moderate maternal undernutrition in late-gestation fetal sheep.

OBJECTIVE: To determine the role of carotid sinus innervation on differential fetal organ growth during maternal nutrient restriction in late pregnancy. DESIGN: Randomised controlled study. SETTING: University research facility. SAMPLE: Thirty-nine Merino ewes. METHODS: At 113 days gestational age (dGA), fetuses were bilaterally carotid sinus denervated or sham denervated. From 118 dGA, the surgery groups were subdivided into two dietary groups, and their ewes were fed 100% of nutrient requirements or 50% until tissue collection at 140 dGA. This provided four groups (sham/control diet, sham/restricted diet, denervated/control diet and denervated/restricted diet). MAIN OUTCOME MEASURES: Fetal organ weights and hormone levels and maternal weight change during the dietary restriction. RESULTS: Adrenal glands were larger in sham/restricted diet fetuses than in sham/control diet or denervated/restricted diet fetuses (P < 0.05). Fetal adrenal weight and brain-to-liver weight ratio were positively related to maternal weight change during the nutritional challenge in sham fetuses only (P < 0.05). Fetal liver weight was negatively related to maternal weight change during nutritional challenge in sham fetuses only (P < 0.05). CONCLUSIONS: We have shown a reduction in liver growth but sparing of adrenal growth in response to moderate maternal undernutrition, which is dependent on intact carotid body innervation. This suggests a new role for the carotid bodies in the control of differential organ growth during such undernutrition.

Synthetic glucocorticoids: antenatal administration and long-term implications.

A clear relationship between intrauterine development and later life predisposition to long-term disease is well established. Weight at birth provides a surrogate measure for fetal development and low birth weight predicts changes in most endocrine axes in adulthood. The exposure of the fetus to elevated levels of either endogenous or synthetic glucocorticoids, pre and periconceptional nutritional status and immediate postnatal development including catch-up growth all contribute substantially to the development of adult onset disease. Fetal exposure to high levels of glucocorticoids has direct clinical relevance. Synthetic glucocorticoids (betamethasone/ dexamethasone) are administered to women at risk of preterm delivery to advance fetal maturation and reduce neonatal morbidity and mortality. However, in human pregnancy, evidence suggests that fetal exposure to synthetic glucocorticoids has detrimental effects on birth outcome, childhood cognition and long-term behavior. Studies in animals have established a link between prenatal exposure to synthetic glucocorticoids and alterations in fetal development as well as changes in placental function. These developmental alterations appear to be permanent. Whether this is the case in humans awaits long-term follow-up of children enrolled in randomized controlled trials of prenatal glucocorticoid therapy. The research challenges in this field are now centered on uncovering the mechanisms by which glucocorticoids are involved in programming the fetus for its future life, and discovering ways in which the effectiveness and safety of antenatal glucocorticoids can be enhanced. The purpose of this mini-review is to provide a background into the use of antenatal synthetic corticosteroids and to highlight and summarize recently published clinical and animal-based studies.

The effects of overcoming experimental bladder outflow obstruction in fetal sheep.

OBJECTIVE: To develop an ovine model of fetal bladder outflow obstruction and to investigate the effect on the kidney of surgical relief of the obstruction in the prenatal period. METHODS: Ultrasound examination and amniocentesis were performed on 68 date-bred pregnant ewes at day 57 of pregnancy (term = 150 days). Fetal gender was determined using a molecular technique to identify single male fetuses. The urethra and urachus were ligated at hysterotomy on 20 of these fetuses at 75 days' gestation. Comparisons were made with six controls that did not undergo operation. Changes that occurred in fetal urinary tract appearance were detected using serial ultrasound examinations. Seven obstructed cases chosen at random had further prenatal surgery on day 94 to decompress the obstructed urinary tract by vesicostomy. The animals were killed at 110 days' gestation and post-mortem studies were performed. RESULTS: Fourteen days after surgical obstruction, there were increases in the summed renal lengths (33 mm vs. 28 mm, p = 0.003) and renal pelvis anteroposterior (A-P) diameters (8 mm vs. 5.5 mm, p = 0.02). In the group allocated to receive surgical decompression, 9 days' relief of obstruction resulted in significant reductions in summed renal lengths (30 mm vs. 41 mm, p = 0.024; controls 31 mm) and renal pelvis A-P diameters (5.8 mm vs. 8.9 mm, p = 0.012; controls < 2 mm). Postmortem histological examination in the surgical decompression group revealed an estimated number of glomeruli similar to controls and greater than in the obstructed cases. CONCLUSION: Surgical relief of fetal bladder outflow obstruction in ovine mid-pregnancy results in improved renal appearance on ultrasonic and histopathological examinations.