Effect of carotid denervation on plasma vasopressin levels during acute hypoxia in the late-gestation sheep fetus

Dissecting the contributions of the peripheral chemoreflex and myocardial hypoxia to fetal heart rate decelerations in near-term fetal sheep

Brief repeated fetal hypoxaemia during labour can trigger intrapartum decelerations of the fetal heart rate (FHR) via the peripheral chemoreflex or the direct effects of myocardial hypoxia, but the relative contribution of these two mechanisms and how this balance changes with evolving fetal compromise remain unknown. In the present study, chronically instrumented near-term fetal sheep received surgical vagotomy (n = 8) or sham vagotomy (control, n = 11) to disable the peripheral chemoreflex and unmask myocardial hypoxia. One-minute complete umbilical cord occlusions (UCOs) were performed every 2.5 min for 4 h or until arterial pressure fell below 20 mmHg. Hypotension and severe acidaemia developed progressively after 65.7 ± 7.2 UCOs in control fetuses and 49.5 ± 7.8 UCOs after vagotomy. Vagotomy was associated with faster development of metabolic acidaemia and faster impairment of arterial pressure during UCOs without impairing centralization of blood flow or neurophysiological adaptation to UCOs. During the first half of the UCO series, before severe hypotension developed, vagotomy was associated with a marked increase in FHR during UCOs. After the onset of evolving severe hypotension, FHR fell faster in control fetuses during the first 20 s of UCOs, but FHR during the final 40 s of UCOs became progressively more similar between groups, with no difference in the nadir of decelerations. In conclusion, FHR decelerations were initiated and sustained by the peripheral chemoreflex at a time when fetuses were able to maintain arterial pressure. After the onset of evolving hypotension and acidaemia, the peripheral chemoreflex continued to initiate decelerations, but myocardial hypoxia became progressively more important in sustaining and deepening decelerations. KEY POINTS: Brief repeated hypoxaemia during labour can trigger fetal heart rate decelerations by either the peripheral chemoreflex or myocardial hypoxia, but how this balance changes with fetal compromise is unknown. Reflex control of fetal heart rate was disabled by vagotomy to unmask the effects of myocardial hypoxia in chronically instrumented fetal sheep. Fetuses were then subjected to repeated brief hypoxaemia consistent with the rates of uterine contractions during labour. We show that the peripheral chemoreflex controls brief decelerations in their entirety at a time when fetuses were able to maintain normal or increased arterial pressure. The peripheral chemoreflex still initiated decelerations even after the onset of evolving hypotension and acidaemia, but myocardial hypoxia made an increasing contribution to sustain and deepen decelerations.

Physiological control of fetal heart rate variability during labour: Implications and controversies

The interpretation of fetal heart rate (FHR) patterns is the only available method to continuously monitor fetal wellbeing during labour. One of the most important yet contentious aspects of the FHR pattern is changes in FHR variability (FHRV). Some clinical studies suggest that loss of FHRV during labour is a sign of fetal compromise so this is reflected in practice guidelines. Surprisingly, there is little systematic evidence to support this observation. In this review we methodically dissect the potential pathways controlling FHRV during labour-like hypoxaemia. Before labour, FHRV is controlled by the combined activity of the parasympathetic and sympathetic nervous systems, in part regulated by a complex interplay between fetal sleep state and behaviour. By contrast, preclinical studies using multiple autonomic blockades have now shown that sympathetic neural control of FHRV was potently suppressed between periods of labour-like hypoxaemia, and thus, that the parasympathetic system is the sole neural regulator of FHRV once FHR decelerations are present during labour. We further discuss the pattern of changes in FHRV during progressive fetal compromise and highlight potential biochemical, behavioural and clinical factors that may regulate parasympathetic-mediated FHRV during labour. Further studies are needed to investigate the regulators of parasympathetic activity to better understand the dynamic changes in FHRV and their true utility during labour. This article is protected by copyright. All rights reserved.

Breath of Life: Heart Disease Link to Developmental Hypoxia

Heart disease remains one of the greatest killers. In addition to genetics and traditional lifestyle risk factors, we now understand that adverse conditions during pregnancy can also increase susceptibility to cardiovascular disease in the offspring. Therefore, the mechanisms by which this occurs and possible preventative therapies are of significant contemporary interest to the cardiovascular community. A common suboptimal pregnancy condition is a sustained reduction in fetal oxygenation. Chronic fetal hypoxia results from any pregnancy with increased placental vascular resistance, such as in preeclampsia, placental infection, or maternal obesity. Chronic fetal hypoxia may also arise during pregnancy at high altitude or because of maternal respiratory disease. This article reviews the short- and long-term effects of hypoxia on the fetal cardiovascular system, and the importance of chronic fetal hypoxia in triggering a developmental origin of future heart disease in the adult progeny. The work summarizes evidence derived from human studies as well as from rodent, avian, and ovine models. There is a focus on the discovery of the molecular link between prenatal hypoxia, oxidative stress, and increased cardiovascular risk in adult offspring. Discussion of mitochondria-targeted antioxidant therapy offers potential targets for clinical intervention in human pregnancy complicated by chronic fetal hypoxia.

Plasma vasopressin levels are closely associated with fetal hypotension and neuronal injury after hypoxia-ischemia in near-term fetal sheep

BACKGROUND

Sensitive biomarkers are needed to rapidly identify high-risk infants after hypoxia-ischemia for neuroprotective treatment. Hypotension is a key determinant of hypoxic-ischemic neural injury, and a potent stimulus of humoral pressors including angiotensin-II and arginine vasopressin. We therefore aimed to quantify the relationship between vasopressin and angiotensin-II levels in the latent phase after hypoxia-ischemia induced by umbilical cord occlusion (UCO) with both the severity of preceding hypotension and subsequent neuronal injury.

METHODS

Chronically instrumented near-term fetal sheep underwent sham-UCO or UCO for either 15 min or until mean arterial pressure was <8 mmHg. Neuronal injury was assessed after 72 h recovery.

RESULTS

Umbilical cord occlusion was associated with severe hypotension that recovered after UCO; two fetuses developed profound secondary hypotension within 6 h and died. Vasopressin levels but not angiotensin-II were significantly elevated 1-3 h after UCO and were closely associated with the severity of hypotension during UCO and the subsequent severity of neuronal loss in the parasagittal and lateral cortex, caudate nucleus and putamen. The Youden cut-point for vasopressin at 1 h was 180.0 pmol/L, with sensitivity 100% and specificity 92.3% for severe neuronal injury or death.

CONCLUSION

Vasopressin levels shortly after moderate-severe hypoxia-ischemia may be a useful early biomarker to guide the timely implementation of neuroprotective treatment.

IMPACT

It can be difficuIt to rapidly identify infants who might benefit from therapeutic hypothermia. We investigated whether increases in plasma pressor hormones early after hypoxia-ischemia were biomarkers for neonatal hypoxic-ischemic encephalopathy using near-term fetal sheep. Arginine vasopressin levels were elevated at 1-3 h after hypoxia-ischemia and were predictive of the severity of preceding hypotension and subsequent risk of severe neuronal injury or death after hypoxia-ischemia. Arginine vasopressin may help identify neonates at high risk of hypoxic-ischemic encephalopathy early within the therapeutic window for hypothermia.

Effects of antenatal dexamethasone and hyperglycemia on cardiovascular adaptation to asphyxia in preterm fetal sheep

Antenatal glucocorticoids improve outcomes among premature infants but are associated with hyperglycemia, which can exacerbate hypoxic-ischemic injury. It is still unclear how antenatal glucocorticoids or hyperglycemia modulate fetal cardiovascular adaptations to severe asphyxia. In this study, preterm fetal sheep received either saline or 12 mg im maternal dexamethasone, followed 4 h later by complete umbilical cord occlusion (UCO) for 25 min. An additional cohort of fetuses received titrated glucose infusions followed 4 h later by UCO to control for the possibility that hyperglycemia contributed to the cardiovascular effects of dexamethasone. Fetuses were studied for 7 days after UCO. Maternal dexamethasone was associated with fetal hyperglycemia (P < 0.001), increased arterial pressure (P < 0.001), and reduced femoral (P < 0.005) and carotid (P < 0.05) vascular conductance before UCO. UCO was associated with bradycardia, femoral vasoconstriction, and transient hypertension. For the first 5 min of UCO, fetal blood pressure in the dexamethasone-asphyxia group was greater than saline-asphyxia (P < 0.001). However, the relative increase in arterial pressure was not different from saline-asphyxia. Fetal heart rate and femoral vascular conductance fell to similar nadirs in both saline and dexamethasone-asphyxia groups. Dexamethasone did not affect the progressive decline in femoral vascular tone or arterial pressure during continuing UCO. By contrast, there were no effects of glucose infusions on the response to UCO. In summary, maternal dexamethasone but not fetal hyperglycemia increased fetal arterial pressure before and for the first 5 min of prolonged UCO but did not augment the cardiovascular adaptations to acute asphyxia.

Perinatal cardiopulmonary adaptation to the thin air of the Alto Andino by a native Altiplano dweller, the llama

Most mammals have a poor tolerance to hypoxia, and prolonged O₂ restriction can lead to organ injury, particularly during fetal and early postnatal life. Nevertheless, the llama (Lama Glama) has evolved efficient mechanisms to adapt to acute and chronic perinatal hypoxia. One striking adaptation is the marked peripheral vasoconstriction measured in the llama fetus in response to acute hypoxia, which allows efficient redistribution of cardiac output toward the fetal heart and adrenal glands. This strong peripheral vasoconstrictor tone is triggered by a carotid body reflex and critically depends on α-adrenergic signaling. A second adaptation is the ability of the llama fetus to protect its brain against hypoxic damage. During hypoxia, in the llama fetus there is no significant increase in brain blood flow. Instead, there is a fall in brain O₂ consumption and temperature, together with a decrease of Na⁺-K⁺-ATPase activity and Na⁺ channels expression, protecting against seizures and neuronal death. Finally, the newborn llama does not develop pulmonary hypertension in response to chronic hypoxia. In addition to maintaining basal pulmonary arterial pressure at normal levels the pulmonary arterial pressor response to acute hypoxia is lower in highland than in lowland llamas. The protection against hypoxic pulmonary arterial hypertension and pulmonary contractile hyperreactivity is partly due to increased hemoxygenase-carbon monoxide signaling and decreased Ca²⁺ sensitization in the newborn llama pulmonary vasculature. These three striking physiological adaptations of the llama allow this species to live and thrive under the chronic influence of the hypobaric hypoxia of life at high altitude.

The physiology of intrapartum fetal compromise at term

Uterine contractions in labor result in a 60% reduction in uteroplacental perfusion, causing transient fetal and placental hypoxia. A healthy term fetus with a normally developed placenta is able to accommodate this transient hypoxia by activation of the peripheral chemoreflex, resulting in a reduction in oxygen consumption and a centralization of oxygenated blood to critical organs, namely the heart, brain, and adrenals. Providing there is adequate time for placental and fetal reperfusion between contractions, these fetuses will be able to withstand prolonged periods of intermittent hypoxia and avoid severe hypoxic injury. However, there exists a cohort of fetuses in whom abnormal placental development in the first half of pregnancy results in failure of endovascular invasion of the spiral arteries by the cytotrophoblastic cells and inadequate placental angiogenesis. This produces a high-resistance, low-flow circulation predisposing to hypoperfusion, hypoxia, reperfusion injury, and oxidative stress within the placenta. Furthermore, this renders the placenta susceptible to fluctuations and reduction in uteroplacental perfusion in response to external compression and stimuli (as occurs in labor), further reducing fetal capillary perfusion, placing the fetus at risk of inadequate gas/nutrient exchange. This placental dysfunction predisposes the fetus to intrapartum fetal compromise. In the absence of a rare catastrophic event, intrapartum fetal compromise occurs as a gradual process when there is an inability of the fetal heart to respond to the peripheral chemoreflex to maintain cardiac output. This may arise as a consequence of placental dysfunction reducing pre-labor myocardial glycogen stores necessary for anaerobic metabolism or due to an inadequate placental perfusion between contractions to restore fetal oxygen and nutrient exchange. If the hypoxic insult is severe enough and long enough, profound multiorgan injury and even death may occur. This review provides a detailed synopsis of the events that can result in placental dysfunction, how this may predispose to intrapartum fetal hypoxia, and what protective mechanisms are in place to avoid hypoxic injury.

Mechanisms of death in structurally normal stillbirths

Objectives To investigate mechanisms of in utero death in normally formed fetuses by measuring amniotic fluid (AF) biomarkers for hypoxia (erythropoietin [EPO]), myocardial damage (cardiac troponin I [cTnI]) and brain injury (glial fibrillary acidic protein [GFAP]), correlated with risk factors for fetal death and placental histopathology. Methods This retrospective, observational cohort study included intrauterine deaths with transabdominal amniocentesis prior to induction of labor. Women with a normal pregnancy and an indicated amniocentesis at term were randomly selected as controls. AF was assayed for EPO, cTnI and GFAP using commercial immunoassays. Placental histopathology was reviewed, and CD15-immunohistochemistry was used. Analyte concentrations >90th centile for controls were considered "raised". Raised AF EPO, AF cTnI and AF GFAP concentrations were considered evidence of hypoxia, myocardial and brain injury, respectively. Results There were 60 cases and 60 controls. Hypoxia was present in 88% (53/60), myocardial damage in 70% (42/60) and brain injury in 45% (27/60) of fetal deaths. Hypoxic fetuses had evidence of myocardial injury, brain injury or both in 77% (41/53), 49% (26/53) and 13% (7/53) of cases, respectively. Histopathological evidence for placental dysfunction was found in 74% (43/58) of these cases. Conclusion Hypoxia, secondary to placental dysfunction, was found to be the mechanism of death in the majority of fetal deaths among structurally normal fetuses. Ninety-one percent of hypoxic fetal deaths sustained brain, myocardial or both brain and myocardial injuries in utero. Hypoxic myocardial injury was an attributable mechanism of death in 70% of the cases. Non-hypoxic cases may be caused by cardiac arrhythmia secondary to a cardiac conduction defect.

The peripheral chemoreflex: indefatigable guardian of fetal physiological adaptation to labour

The fetus is consistently exposed to repeated periods of impaired oxygen (hypoxaemia) and nutrient supply in labour. This is balanced by the healthy fetus's remarkable anaerobic tolerance and impressive ability to mount protective adaptations to hypoxaemia. The most important mediator of fetal adaptations to brief repeated hypoxaemia is the peripheral chemoreflex, a rapid reflex response to acute falls in arterial oxygen tension. The overwhelming majority of fetuses are able to respond to repeated uterine contractions without developing hypotension or hypoxic-ischaemic injury. In contrast, fetuses who are either exposed to severe hypoxaemia, for example during uterine hyperstimulation, or enter labour with reduced anaerobic reserve (e.g. as shown by severe fetal growth restriction) are at increased risk of developing intermittent hypotension and cerebral hypoperfusion. It is remarkable to note that when fetuses develop hypotension during such repeated severe hypoxaemia, it is not mediated by impaired reflex adaptation, but by failure to maintain combined ventricular output, likely due to a combination of exhaustion of myocardial glycogen and evolving myocardial injury. The chemoreflex is suppressed by relatively long periods of severe hypoxaemia of 1.5-2 min, longer than the typical contraction. Even in this setting, the peripheral chemoreflex is consistently reactivated between contractions. These findings demonstrate that the peripheral chemoreflex is an indefatigable guardian of fetal adaptation to labour.

The fetal brain sparing response to hypoxia: physiological mechanisms

How the fetus withstands an environment of reduced oxygenation during life in the womb has been a vibrant area of research since this field was introduced by Joseph Barcroft, a century ago. Studies spanning five decades have since used the chronically instrumented fetal sheep preparation to investigate the fetal compensatory responses to hypoxia. This defence is contingent on the fetal cardiovascular system, which in late gestation adopts strategies to decrease oxygen consumption and redistribute the cardiac output away from peripheral vascular beds and towards essential circulations, such as those perfusing the brain. The introduction of simultaneous measurement of blood flow in the fetal carotid and femoral circulations by ultrasonic transducers has permitted investigation of the dynamics of the fetal brain sparing response for the first time. Now we know that major components of fetal brain sparing during acute hypoxia are triggered exclusively by a carotid chemoreflex and that they are modified by endocrine agents and the recently discovered vascular oxidant tone. The latter is determined by the interaction between nitric oxide and reactive oxygen species. The fetal brain sparing response matures as the fetus approaches term, in association with the prepartum increase in fetal plasma cortisol, and treatment of the preterm fetus with clinically relevant doses of synthetic steroids mimics this maturation. Despite intense interest into how the fetal brain sparing response may be affected by adverse intrauterine conditions, this area of research has been comparatively scant, but it is likely to take centre stage in the near future.

Cardiovascular function in term fetal sheep conceived, gestated and studied in the hypobaric hypoxia of the Andean altiplano

High-altitude hypoxia causes intrauterine growth restriction and cardiovascular programming. However, adult humans and animals that have evolved at altitude show certain protection against the effects of chronic hypoxia. Whether the highland fetus shows similar protection against high altitude gestation is unclear. We tested the hypothesis that high-altitude fetal sheep have evolved cardiovascular compensatory mechanisms to withstand chronic hypoxia that are different from lowland sheep. We studied seven high-altitude (HA; 3600 m) and eight low-altitude (LA; 520 m) pregnant sheep at ∼90% gestation. Pregnant ewes and fetuses were instrumented for cardiovascular investigation. A three-period experimental protocol was performed in vivo: 30 min of basal, 1 h of acute superimposed hypoxia (∼10% O2) and 30 min of recovery. Further, we determined ex vivo fetal cerebral and femoral arterial function. HA pregnancy led to chronic fetal hypoxia, growth restriction and altered cardiovascular function. During acute superimposed hypoxia, LA fetuses redistributed blood flow favouring the brain, heart and adrenals, whereas HA fetuses showed a blunted cardiovascular response. Importantly, HA fetuses have a marked reduction in umbilical blood flow versus LA. Isolated cerebral arteries from HA fetuses showed a higher contractile capacity but a diminished response to catecholamines. In contrast, femoral arteries from HA fetuses showed decreased contractile capacity and increased adrenergic contractility. The blunting of the cardiovascular responses to hypoxia in fetuses raised in the Alto Andino may indicate a change in control strategy triggered by chronic hypoxia, switching towards compensatory mechanisms that are more cost-effective in terms of oxygen uptake.

The effect of late gestation foetal hypoglycaemia on cardiovascular and endocrine function in sheep

An appropriate foetal cardiovascular (CV) response to reduced substrate supply (e.g. oxygen or other nutrients) is vital for growth and development, and may impact on CV control. The prevailing nutritional environment and associated CV changes may influence subsequent CV responses to challenges during late gestation, for example, umbilical cord occlusion (UCO). We investigated the effect of low-circulating glucose on foetal CV control mechanisms and response to UCO. Under general anaesthesia, late gestation foetal sheep (n = 7, 119 days gestational age (dGA), term ∼147 days) were implanted with vascular catheters, a bladder catheter, electrocardiogram electrodes and an umbilical cord occluder. Mean arterial pressure (MAP), heart rate (HR) and kidney function were monitored during maternal saline (MSAL, 125dGA) and insulin (MINS, 126dGA) infusion, and foetal CV responses were assessed during incremental doses of angiotensin II, a 90-s total UCO, and administration of phenylephrine to assess baroreflex function. During MINS infusion, the decrease in maternal and foetal blood glucose was associated with a small but significant decrease in foetal HR and reduced foetal baroreflex sensitivity (P < 0.05). The increase in foetal MAP during a 90-s UCO was greater during hypoglycaemia (P < 0.05). The MAP response to angiotensin II was not affected by hypoglycaemia. Decreased foetal HR and baroreflex sensitivity and increased CV responsiveness to UCO during hypoglycaemia indicates altered CV homoestatic mechanisms. The combination of altered nutrition and a CV challenge, such as UCO, during late gestation may have a cumulative effect on foetal CV function.

Chemoreflex activity increases prostaglandin endoperoxide synthase mRNA expression in the late-gestation fetal sheep brain

Fetal sheep defend blood pressure, blood volume, and blood gases using baro- and chemoreflexes that influence autonomic and neuroendocrine responses. The local generation of prostanoids within the fetal brain is also an important component in activating hormone responses to these stimuli, but the relationship between the reflexes and prostanoid biosynthesis is unclear. The present study was performed to test the hypothesis that the abundances of prostaglandin biosynthetic enzymes in the fetal brain are dependent upon the activity of the baro- and chemoreflex pathways. We subjected chronically catheterized fetal sheep in late gestation to a 10-minute period of brachiocephalic occlusion (BCO), a stimulus that provokes brisk cardiovascular and neuroendocrine responses. We compared the central nervous system abundance of prostaglandin endoperoxide synthases 1 and 2 (PGHS-1 and PGHS-2) after BCO to (1) fetal sheep that had been subjected to BCO after chronic sinoaortic denervation plus bilateral vagotomy and (2) fetal sheep in which the N-methyl d-aspartate (NMDA) receptor antagonist, ketamine, had been administered prior to BCO. Abundances of messenger RNA (mRNA) for PGHS-1 and of mRNA and protein for PGHS-2 in fetal hippocampus were reduced significantly by either prior denervation or ketamine administration. Prostaglandin endoperoxide synthases 1 and 2 mRNA in pituitary were decreased and increased, respectively, by ketamine pretreatment. The results of this study are consistent with the conclusion that the expression of PGHS-1 and -2 in fetal hippocampus and pituitary are influenced by the baro- and/or chemoreflex pathways within the fetal brain in late gestation.

Interaction of PGHS-2 and glutamatergic mechanisms controlling the ovine fetal hypothalamus-pituitary-adrenal axis

Prostaglandins, generated within the fetal brain, are integral components of the mechanism controlling the fetal hypothalamus-pituitary-adrenal (HPA) axis. Previous studies in this laboratory demonstrated that prostaglandin G/H synthase isozyme 2 (PGHS-2) inhibition reduces the fetal HPA axis response to cerebral hypoperfusion, blocks the preparturient rise in fetal plasma ACTH concentration, and delays parturition. We also discovered that blockade of N-methyl-d-aspartate (NMDA) receptors reduces the fetal ACTH response to cerebral hypoperfusion. The present study was designed to test the hypothesis that PGHS-2 action and the downstream effect of HPA axis stimulation are stimulated by NMDA-mediated glutamatergic neurotransmission. Chronically catheterized late-gestation fetal sheep (n = 8) were injected with NMDA (1 mg iv). All responded with increases in fetal plasma ACTH and cortisol concentrations. Pretreatment with resveratrol (100 mg iv, n = 5), a specific inhibitor of PGHS-1, did not alter the magnitude of the HPA axis response to NMDA. Pretreatment with nimesulide (10 mg iv, n = 6), a specific inhibitor of PGHS-2, significantly reduced the HPA axis response to NMDA. To further explore this interaction, we injected NMDA in six chronically catheterized fetal sheep that were chronically infused with nimesulide (n = 6) at a rate of 1 mg/day into the lateral cerebral ventricle for 5-7 days. In this group, there was no significant ACTH response to NMDA. Finally, we tested whether the HPA axis response to prostaglandin E(2) (PGE(2)) is mediated by NMDA receptors. Seven chronically catheterized late-gestation fetal sheep were injected with 100 ng of PGE(2), which significantly increased fetal plasma ACTH and cortisol concentrations. Pretreatment with ketamine (10 mg iv), an NMDA antagonist, did not alter the ACTH or cortisol response to PGE(2). We conclude that generation of prostanoids via the action of PGHS-2 in the fetal brain augments the fetal HPA axis response to NMDA-mediated glutamatergic stimulation.

Normal fetal physiology and behavior, and adaptive responses with hypoxemia

The principal objective of antenatal testing is to detect fetal hypoxia before the fetus has suffered lasting harm. This article summarizes some of the mechanisms by which fetal oxygen consumption is maintained under normal conditions, and adaptations that occur in response to hypoxemia. Alterations in the fetal heart rate are prominent in the fetal response to hypoxemia and are the basis of some methods of antenatal testing. The principal mechanisms underlying these fetal heart rate changes are described. Adaptations in the distribution of blood flow that permit the fetus to deal with reduced oxygen levels are summarized. Developmental trends in fetal motility and behavior are outlined, and also how these are modified by fetal growth retardation and maternal diabetes. Fetal movements are suppressed during acute hypoxemia, but with gradually developing hypoxemia, normal movement patterns may continue until the fetus becomes acidemic. This may limit the use of these biophysical variables in antenatal testing.

Ketamine inhibits fetal ACTH responses to cerebral hypoperfusion

The present study tested the effect of ketamine on the fetal reflex responses of late-gestation sheep to brachiocephalic occlusion (BCO), a stimulus that mimics the reduction in cerebral blood flow that results from severe fetal hypotension. Ketamine, a dissociative anesthetic and known noncompetitive antagonist of N-methyl D-aspartate (NMDA) receptors, has previously been shown to impair chemoreceptor responsiveness. Studies from this laboratory suggest that fetal reflex ACTH responses to hypotension are largely mediated by chemoreceptors; therefore, we hypothesized that ketamine would inhibit the reflex hormonal response to BCO. Chronically catheterized fetal sheep were subjected to acute cerebral hypoperfusion through occlusion of the brachiocephalic artery. Fetal blood pressure and heart rate were continuously recorded, and fetal blood samples drawn during the experiment were analyzed with specific hormone assays. Our results demonstrate that ketamine attenuates hemodynamic responses to cerebral hypoperfusion and is a potent inhibitor of ACTH and proopiomelanocortin (POMC)/pro-ACTH release. These data support the hypothesis that fetal reflex responses hypotension are chemoreceptor mediated. Given the potency with which ketamine inhibits ACTH response to fetal hypotension, we suggest that the use of ketamine or other anesthetic or analgesic drugs that block or otherwise interact with the NMDA-glutamate pathways, in late pregnancy or in preterm newborns be reconsidered.

Cortisol and ACTH responses to severe asphyxia in preterm fetal sheep

It has been hypothesized that the hypothalamic-pituitary-adrenal (HPA) axis is immature in the preterm fetus and that this compromises their ability to adapt to hypoxic stress; however, there are few direct data. We therefore examined the effects of asphyxia on HPA responses in chronically instrumented preterm fetal sheep (104 days of gestation; term is 147 days), allocated to a sham control group (n = 7) or 25 min of complete umbilical cord occlusion (n = 8), followed by recovery for 72 h. During umbilical cord occlusion there was a rapid rise in ACTH levels (230.4 +/- 63.5 versus 14.1 +/- 1.8 ng ml(-1) in sham controls, 16-fold) and cortisol levels (7.4 +/- 4.9 versus 0.2 +/- 0.1 ng ml(-1), 31-fold), with further increases after release of cord occlusion. ACTH levels were normalized by 24 h, while plasma cortisol levels returned to sham control values 72 h after asphyxia. Fetal arterial blood pressure was elevated in the first 36 h, with a marked increase in femoral vascular resistance, and correlated positively with cortisol levels after asphyxia (P = 0.05). In conclusion, the preterm fetus shows a brisk, substantial HPA response to severe hypoxia.

Calcitonin gene-related peptide contributes to the umbilical haemodynamic defence response to acute hypoxaemia

Despite clinical advances in obstetric practice, undiagnosed fetal hypoxaemia still contributes to a high incidence of perinatal morbidity. The fetal defence to hypoxaemia involves a redistribution of blood flow away from peripheral circulations towards essential vascular beds, such as the umbilical, cerebral, myocardial and adrenal circulations. In marked contrast to other essential vascular beds, the mechanisms mediating maintained perfusion of the umbilical circulation during hypoxaemia remain unknown. This study determined the role of calcitonin gene-related peptide (CGRP) in the maintenance of umbilical blood flow during basal and hypoxaemic conditions. Under anaesthesia, five sheep fetuses were instrumented with catheters and a Transonic probe around an umbilical artery, inside the fetal abdomen, at 0.8 of gestation. Five days later, fetuses were subjected to 0.5 h hypoxaemia during either i.v. saline or a selective CGRP antagonist in randomised order. Treatment started 30 min before hypoxaemia and ran continuously until the end of the challenge. The CGRP antagonist did not alter basal blood gas or cardiovascular status in the fetus. A similar fall in Pa,O2 occurred in fetuses during either saline (21 +/- 0.8 to 9 +/- 0.9 mmHg) or antagonist treatment (20 +/- 0.9 to 9 +/- 1.2 mmHg). Hypoxaemia during saline led to significant increases in arterial blood pressure, umbilical blood flow and umbilical vascular conductance. In marked contrast, hypoxaemia during CGRP antagonist treatment led to pronounced falls in both umbilical blood flow and umbilical vascular conductance without affecting the magnitude of the hypertensive response. In conclusion, CGRP plays an important role in the umbilical haemodynamic defence response to hypoxaemia in the late gestation fetus.

Long-term hypoxia alters endocrine and physiologic responses to umbilical cord occlusion in the ovine fetus

OBJECTIVE

This study was designed to determine the effect of umbilical cord occlusion (UCO) on fetal endocrine responses in the long-term hypoxemic (LTH) ovine fetus.

METHODS

Pregnant ewes were maintained at high altitude (3820 m) from day 30 of gestation. Normoxic control and LTH fetuses were catheterized, and an inflatable occluder was placed on the umbilical cord at day 132 of gestation. In the LTH group, maternal oxygen tension was maintained at approximately 60 mmHg by nitrogen infusion through a maternal tracheal catheter. On day 137, two 5-minute UCOs were performed. On day 139, the study was repeated with a 10-minute UCO.

RESULTS

Basal adrenocorticotropic hormone (ACTH) levels and peak responses to the first 5-minute UCO were not different between control and LTH fetuses (17.6 +/- 4.0 to 418.8 +/- 41.3 in controls, 25.7 +/- 4.0 to 530.0 +/- 93.0 pg/mL in LTH fetuses). A similar pattern was observed during the second UCO. Basal cortisol levels were similar in both groups. In response to UCO, a significant increase in cortisol was observed in both groups, but peak concentrations in the LTH group were significantly higher than those in the control group (23.9 +/- 4.8 versus 14.8 +/- 2.9 ng/mL, respectively, P <.05). The second occlusion also increased cortisol concentrations, but no differences were observed between groups. After the 10-minute UCO, the ACTH and cortisol responses were similar to the first 5-minute occlusion, with higher cortisol levels in the LTH fetuses.

CONCLUSION

Despite similar ACTH responses to UCO, the cortisol response was greater in the LTH fetuses than in normoxic controls. LTH appears to result in enhanced adrenal sensitivity to a secondary stressor or altered cortisol metabolism.

Adrenocortical responsiveness is blunted in twin relative to singleton ovine fetuses

Twin fetuses experience much higher rates of perinatal mortality/morbidity than age- and weight-matched singletons. Across species, the prepartum increase in fetal plasma cortisol is responsible for maturing a number of systems in preparation for birth and the immediate postnatal period. In sheep, it is known that basal adrenocortical function is delayed in twins relative to singletons. Thus, it could be argued that relative immaturity in twins may explain their increased susceptibility to stress in the perinatal period and their relatively poor perinatal outcome. However, whether adrenocortical responsiveness to stress is also diminished in the twin fetus and whether the fetal cardiovascular, metabolic and endocrine defences to acute stress are comparatively weak in the twin fetus is unknown. This study investigated the effect of twinning on adrenocortical responsiveness to either the physiological stress of acute hypoxaemia or to an exogenous ACTH test, and on the fetal cardiovascular, metabolic and endocrine responses to acute hypoxaemic stress. Twenty Welsh Mountain sheep fetuses were chronically instrumented (1-2% halothane) at 121 +/- 3 days of gestation (term is ca 145 days) with amniotic and vascular catheters and with a transit-time flow probe around a femoral artery. The animals were divided into two groups based upon fetal number (singletons, n= 10; twins, n= 10), as determined at surgery. At 130 +/- 2 days, a 1 h episode of acute, isocapnic hypoxaemia (to reduce carotid P(O(2)) to 12 +/- 1 mmHg) was induced in all fetuses by reducing the maternal inspired O(2) fraction (F(IO(2)); 9% O(2) in N(2)). Fetal cardiovascular variables were recorded at 1 s intervals throughout the experimental protocol and arterial blood samples taken at appropriate intervals for biophysical (blood gases, glucose, lactate) and endocrine (catecholamines, vasopressin, cortisol, ACTH) measures. At 133 +/- 2 days a 2.5 microg bolus dose of synthetic ACTH (Synacthen; Ciba Pharmaceuticals, UK) was injected i.v. into eight of the singleton and six of the twin fetuses to determine adrenocortical steroidogenic sensitivity to exogenous ACTH. Under basal conditions, twins had lower plasma cortisol concentration, arterial blood pressure and femoral blood flow relative to singleton fetuses. Twins responded to acute hypoxaemia with similar pressor and vasopressor responses compared to singleton fetuses. However, the rate pressure product, an index of myocardial work, tended to decrease during hypoxaemia in twins, in contrast to the increase observed in singletons. Similar increases in the fetal plasma concentrations of ACTH, AVP, noradrenaline and adrenaline were observed during hypoxaemia in both groups; however, both the increments in fetal plasma concentration of cortisol in response to acute hypoxaemia and to exogenous ACTH were blunted in twins relative to singletons. This study shows that basal adrenocortical function as well as adrenocortical responsiveness is blunted in the twin relative to the singleton fetus. Further, the mechanism for adrenocortical blunting resides at the level of the adrenal cortex rather than higher up the axis. Relative adrenocortical immaturity in the twin fetus may reflect a specific endocrine adaptation to prolong gestation in multiple ovine pregnancies; however, such an adaptation does not affect the cardiovascular, metabolic or endocrine defence responses to acute hypoxaemia in the twin fetus.

Effect of nutritional restriction in early pregnancy on isolated femoral artery function in mid-gestation fetal sheep

Unbalanced maternal nutrition affects fetal endocrine and cardiovascular systems, sometimes accompanied by changes in growth, although this is usually in late gestation. We determined the effect of moderate restriction for the first half of gestation of maternal dietary protein, or of total calorific intake on isolated resistance artery function of mid-gestation fetal sheep. Welsh Mountain ewes were nutritionally restricted by 30 % of the recommended nutrient intake (globally restricted) or 30 % of the recommended protein intake (protein-restricted), compared to control ewes fed 100 % of recommended nutrient intake, for ~12 days prior to conception and for the subsequent 70 days of gestation. At mid-gestation, fetal and placental weights were similar in all dietary groups. In isolated femoral arteries, the response curve to noradrenaline was reduced in protein-restricted group fetuses (P < 0.05). Maximal relaxation (P < 0.01) and sensitivity (P < 0.05) to acetylcholine were markedly reduced in protein-restricted group fetuses, and to a smaller extent in globally restricted group fetuses (response curve, P < 0.05). The dilator response (P < 0.05) and sensitivity (P < 0.05) to the alpha2 agonist UK14304 was lower in protein-, but not in globally restricted group fetuses. The response (P < 0.05) and sensitivity (P < 0.05) to the nitric oxide donor sodium nitroprusside were reduced in protein-restricted group fetuses compared to controls. Our data show that dietary imbalance, in particular restricted protein, of the ewe can produce blunting of endothelial-dependent and -independent relaxation in systemic arteries from the mid-gestation fetus. These changes may precede perturbed late-gestation fetal and postnatal cardiovascular control.

Polyuria and impaired renal blood flow after asphyxia in preterm fetal sheep

Renal impairment is common in preterm infants, often after exposure to hypoxia/asphyxia or other circulatory disturbances. We examined the hypothesis that this association is mediated by reduced renal blood flow (RBF), using a model of asphyxia induced by complete umbilical cord occlusion for 25 min (n = 13) or sham occlusion (n = 6) in chronically instrumented preterm fetal sheep (104 days, term is 147 days). During asphyxia there was a significant fall in RBF and urine output (UO). After asphyxia, RBF transiently recovered, followed within 30 min by a secondary period of hypoperfusion (P < 0.05). This was mediated by increased renal vascular resistance (RVR, P < 0.05); arterial blood pressure was mildly increased in the first 24 h (P < 0.05). RBF relatively normalized between 3 and 24 h, but hypoperfusion developed again from 24 to 60 h (P < 0.05, analysis of covariance). UO significantly increased to a peak of 249% of baseline between 3 and 12 h (P < 0.05), with increased fractional excretion of sodium, peak 10.5 +/- 1.4 vs. 2.6 +/- 0.6% (P < 0.001). Creatinine clearance returned to normal after 2 h; there was a transient reduction at 48 h to 0.32 +/- 0.02 ml.min(-1).g(-1) (vs. 0.45 +/- 0.04, P < 0.05) corresponding with the time of maximal depression of RBF. No renal injury was seen on histological examination at 72 h. In conclusion, severe asphyxia in the preterm fetus was associated with evolving renal tubular dysfunction, as shown by transient polyuria and natriuresis. Despite a prolonged increase in RVR, there was only a modest effect on glomerular function.

Cardiovascular and endocrine responses to acute hypoxaemia during and following dexamethasone infusion in the ovine fetus

This study investigated the effects of fetal treatment with dexamethasone on ovine fetal cardiovascular defence responses to acute hypoxaemia, occurring either during or 48 h following the period of glucocorticoid exposure. To address the mechanisms underlying these responses, chemoreflex function and plasma concentrations of catecholamines, neuropeptide Y (NPY) and vasopressin were measured. Under general halothane anaesthesia, 26 Welsh Mountain sheep fetuses were surgically prepared for long-term recording at between 117 and 120 days of gestation (dGA; term is approximately 145 days) with vascular catheters and a Transonic flow probe around a femoral artery. Following at least 5 days of recovery, fetuses were randomly assigned to one of two experimental groups. After 48 h of baseline recording, at 125 +/- 1 dGA, half of the fetuses (n = 13) were continuously infused I.V. with dexamethasone for 48 h at a rate of 2.06 +/- 0.13 microg kg-1 h-1. The remaining 13 fetuses were infused with heparinized saline at the same rate (controls). At 127 +/- 1 dGA, 2 days from the onset of infusions, seven fetuses from each group were subjected to 1 h of acute hypoxaemia. At 129 +/- 1 dGA, 2 days after the end of infusions, six fetuses from each group were subjected to 1 h of acute hypoxaemia. Similar reductions in fetal partial pressure of arterial oxygen occurred in control and dexamethasone-treated fetuses during the acute hypoxaemia protocols. In control fetuses, acute hypoxaemia led to transient bradycardia, femoral vasoconstriction and significant increases in plasma concentrations of catecholamines, vasopressin and NPY. In fetuses subjected to acute hypoxaemia during dexamethasone treatment, the increase in plasma NPY was enhanced, the bradycardic response was prolonged, and the plasma catecholamine and vasopressin responses were diminished. In fetuses subjected to acute hypoxaemia 48 h following dexamethasone treatment, femoral vasoconstriction and plasma catecholamine and vasopressin responses were enhanced, whilst the prolonged bradycardia and augmented plasma NPY responses persisted. These data show that fetal treatment with dexamethasone modifies the pattern and magnitude of fetal cardiovascular responses to acute oxygen deprivation. Modifications to different mechanisms mediating the fetal defence responses to acute hypoxaemia that occur during dexamethasone treatment may reverse, persist or even become enhanced by 48 h following the treatment period.

Enhanced nitric oxide activity offsets peripheral vasoconstriction during acute hypoxaemia via chemoreflex and adrenomedullary actions in the sheep fetus

We tested the hypothesis that enhanced nitric oxide (NO) opposes fetal peripheral vasoconstrictor responses to acute hypoxaemia via actions involving the carotid chemoreflex and the adrenal medulla. The hypothesis was tested in the late gestation ovine fetus using a novel NO clamp technique, which involves fetal combined treatment with the NO synthase inhibitor, L-NAME, and the NO donor, sodium nitroprusside. In contrast to treatment with L-NAME alone, combined fetal treatment with L-NAME and nitroprusside prevents generalized vasoconstriction and pronounced hypertension, not only maintaining basal cardiovascular function, but also permitting blockade of the de novo synthesis of NO during hypoxaemia while compensating for the tonic production of the gas. Under general anaesthesia, seven sheep fetuses were surgically prepared with catheters and a femoral Transonic flow probe. Five days after surgery, fetuses were subjected to a 3 h protocol: 1 h normoxia, 1 h hypoxaemia and 1 h recovery. Fetal hypoxaemia was induced during either fetal infusion with saline or treatment with the NO clamp. During saline infusion, fetuses responded to hypoxaemia with transient bradycardia, femoral vasoconstriction and increases in plasma noradrenaline and adrenaline. During fetal treatment with the NO clamp, bradycardia persisted and there were greater peripheral vasoconstrictor and catecholaminergic responses to hypoxaemia. Further analysis showed that NO clamp treatment enhanced the chemoreflex component of the fetal cardiovascular defence to acute hypoxaemia. These data support the hypothesis that enhanced NO synthesis during acute hypoxaemia offsets fetal peripheral vasoconstrictor responses to hypoxaemia via chemoreflex and adrenomedullary actions.

The role of neuropeptide Y in the ovine fetal cardiovascular response to reduced oxygenation

This study investigated the role of neuropeptide Y (NPY) in mediating cardiovascular responses to reduced oxygenation in the late gestation ovine fetus by: (1) comparing the effects on the cardiovascular system of an exogenous infusion of NPY with those elicited by moderate or severe reductions in fetal oxygenation; and (2) determining the effect of fetal I.V. treatment with a selective NPY-Y(1) receptor antagonist on the fetal cardiovascular responses to acute moderate hypoxaemia. Under general anaesthesia, 14 sheep fetuses (0.8-0.9 of gestation) were surgically prepared with vascular and amniotic catheters. In 5 of these fetuses, a Transonic flow probe was also implanted around a femoral artery. Following at least 5 days of recovery, one group of fetuses (n = 9) was subjected to a 30 min treatment period with exogenous NPY (17 microg kg(-1) bolus plus 0.85 microg kg(-1) min(-1) infusion). In this group, fetal blood pressure and heart rate were monitored continuously and the distribution of the fetal combined ventricular output was assessed via injection of radiolabelled microspheres before and during treatment. The second group of fetuses instrumented with the femoral flow probe (n = 5) were subjected to a 3 h experiment consisting of 1 h of normoxia, 1 h of hypoxaemia, and 1 h of recovery during a slow I.V. infusion of vehicle. One or two days later, the acute hypoxaemia protocol was repeated during fetal I.V. treatment with a selective NPY-Y(1) receptor antagonist (50 microg kg(-1) bolus + 1.5 microg kg(-1) min(-1) infusion). In these fetuses, fetal arterial blood pressure, heart rate and femoral vascular resistance were recorded continuously. The results show that fetal treatment with exogenous NPY mimics the fetal cardiovascular responses to asphyxia, and that treatment of the sheep fetus with a selective NPY-Y(1) receptor antagonist does not affect the fetal cardiovascular response to acute moderate hypoxaemia. These results support a greater role for NPY in mediating the fetal cardiovascular responses to acute asphyxia than to acute moderate hypoxaemia.

Effects of prevailing hypoxaemia, acidaemia or hypoglycaemia upon the cardiovascular, endocrine and metabolic responses to acute hypoxaemia in the ovine fetus

Although it is established that the fetus can successfully withstand a single, acute hypoxaemic challenge during gestation, little is known about what effects prevailing adverse intrauterine conditions might have on the fetal response to acute hypoxaemia. The aims of this study were therefore: (1) to characterise the effects of prevailing and sustained hypoxaemia, acidaemia or hypoglycaemia on the fetal cardiovascular responses to an episode of acute hypoxaemia; and (2) to determine the effects of these adverse intrauterine conditions on mechanisms mediating these cardiovascular responses. Thirty-three Welsh Mountain sheep fetuses were chronically instrumented (1-2 % halothane) between 117 and 125 days of gestation (term is ca 145 days) with amniotic and vascular catheters and with a transit-time flow probe around a femoral artery. The animals were divided retrospectively into four groups based upon post-surgical, sustained, basal blood oxygen (chronically hypoxaemic; P(a,O2), 17.3 +/- 0.5 mmHg; n = 8), glucose (chronically hypoglycaemic; blood glucose, 0.49 +/- 0.03 mmol l(-1); n = 6) and acid-base (chronically acidaemic; pH(a), 7.25 +/- 0.01; n = 5) status. Values for compromised fetuses were -2 S.D. from a group of control (n = 14) fetuses. At 130 +/- 4 days, a 1 h episode of acute, isocapnic hypoxaemia (9 % O(2) in N(2), to reduce carotid P(a,O2) to 12 +/- 1 mmHg) was induced in all fetuses by reducing the maternal inspired O(2) fraction (F(I,O2)). Fetal cardiovascular variables were recorded at 1 s intervals throughout the experimental protocol and arterial blood samples taken at appropriate intervals for biophysical (blood gases, glucose, lactate) and endocrine (catecholamines, vasopressin, cortisol, ACTH) measures. During acute hypoxaemia all fetuses elicited hypertension, bradycardia and femoral vasoconstriction. However, prevailing fetal compromise altered the cardiovascular and endocrine responses to a further episode of acute hypoxaemia, including: (1) enhanced pressor and femoral vasoconstriction; (2) greater increments in plasma noradrenaline and vasopressin during hypoxaemia; and (3) basal upward resetting of hypothalamic-pituitary-adrenal axis function. Only chronically hypoxaemic fetuses had significantly elevated basal concentrations of noradrenaline and enhanced chemoreflex function during acute hypoxaemia. These data show that prevailing adverse intrauterine conditions alter the capacity of the fetus to respond to a subsequent episode of acute hypoxaemia; however, the partial contributions of hypoxaemia, acidaemia or hypoglycaemia to mediating these responses can vary.

Alpha-adrenergic contribution to the cardiovascular response to acute hypoxemia in the chick embryo

Fetal responses to acute hypoxemia include bradycardia, increase in blood pressure, and peripheral vasoconstriction. Peripheral vasoconstriction contributes to the redistribution of the cardiac output away from ancillary vascular beds toward myocardial, cerebral, and adrenal circulations. We investigated the effect of alpha-adrenergic receptor blockade on this fetal response. Fluorescent microspheres were used to measure cardiac output distribution during basal and hypoxemic conditions with and without phentolamine treatment. Phentolamine altered basal cardiac output distribution, indicating a basal alpha-adrenergic tone, but this was mainly noted at the earlier stages of incubation. During hypoxemia, phentolamine prevented vasoconstriction in the carcass. At day 19 of incubation, the percent cardiac output distributed to the carcass increased by 20% compared with a decrease in the control group by 17%. Phentolamine markedly attenuated the subsequent redistribution of the cardiac output toward the brain (from +102% in the control group to -25% in the phentolamine-treated group) and the heart (from +196% in the control group to +69% in the phentolamine-treated group). In the chick embryo, alpha-adrenergic mechanisms contribute to the maintenance of basal vascular tone and to the redistribution of the cardiac output away from the peripheral circulations toward the brain and heart during hypoxemic conditions.

A novel method for controlled and reversible long term compression of the umbilical cord in fetal sheep

1. In fetal sheep during late gestation the aims of the present study were to (1) develop a technique for inducing prolonged but reversible periods of controlled compression of the umbilical cord and (2) characterise the cardiovascular, endocrine and metabolic responses to this challenge. 2. Under 1-2 % halothane anaesthesia, 16 Welsh Mountain sheep fetuses were chronically instrumented at 118 +/- 2 days of gestation (term is ca 145 days) with an inflatable occluder cuff around the umbilical cord, amniotic and femoral vascular catheters and with transit-time flow probes around the contra-lateral femoral artery and an umbilical artery. At 125 days, umbilical blood flow was reduced by 30 % from a pre-determined 24 h baseline for 3 days by automated servo-controlled inflation of the occluder cuff (n = 8). The occluder was then deflated allowing return of umbilical blood flow to baseline. The remaining eight fetuses were used as sham-operated controls in which the occluder was not inflated throughout the protocol. Fetal cardiovascular variables were recorded at 8 s intervals and arterial blood samples taken for measurement of blood gases, glucose and lactate and plasma adrenaline, noradrenaline and vasopressin concentration throughout the study. 3. Automated servo-controlled inflation of the occluder cuff, programmed to reduce umbilical blood flow by 30 % from baseline, reduced umbilical blood flow by 30.2 +/- 1.7 %, with a coefficient of variation during compression of 6.5 +/- 1.1 %. Sustained partial compression of the umbilical cord produced falls in fetal arterial pH, P(a,O2), percentage O(2) saturation of haemoglobin, and hindlimb oxygen delivery, and increases in P(a,CO2), haemoglobin concentration, arterial blood oxygen carrying capacity and in blood glucose and lactate concentrations. While the reductions in P(a,O2), percentage saturation of haemoglobin and hindlimb oxygen delivery and the increase in P(a,CO2) were sustained throughout compression, the reduction in arterial pH and the increase in arterial oxygen carrying capacity had returned towards baseline values by 48 h compression. Fetal blood lactate concentrations reached a peak at 8 h of compression and, thereafter, were maintained at an elevated level relative to baseline. 4. Partial compression of the umbilical cord produced fetal hypertension, a reduction in femoral blood flow and, consequently, an increase in calculated fetal femoral vascular resistance for the duration of the challenge. In addition, the fall in heart rate measured in sham control fetuses by the end of the study, did not occur in cord-compressed fetuses. Cosinor analysis on 24 h rhythms of cardiovascular data indicated a significant increase in the amplitude of the 24 h rhythm in heart rate in cord-compressed fetuses relative to sham controls during the period of compression or sham-compression. Furthermore, cord compression led to an increase in fetal plasma noradrenaline, but not adrenaline and vasopressin concentrations relative to sham control fetuses. 5. In conclusion, a novel reversible method for controlled, long-term compression of the umbilical cord in sheep has been developed. The data show that sustained, partial compression of the umbilical cord produced moderate but sustained asphyxia, which resolved after the end of the compression period, and induced changes in fetal cardiovascular, endocrine and metabolic functions.

Purinergic contribution to circulatory, metabolic, and adrenergic responses to acute hypoxemia in fetal sheep

This study investigated the effects on femoral vascular resistance, blood glucose and lactate levels, and plasma catecholamine concentrations of fetal treatment with an adenosine receptor antagonist during acute hypoxemia in fetal sheep during late gestation. Under anesthesia, seven fetal sheep were instrumented between 117 and 118 days gestation (term is approximately 145 days) with vascular and amniotic catheters and an ultrasonic probe around a femoral artery. Six days after surgery, all fetuses were randomly subjected to a 3-h experiment consisting of 1 h of normoxia, 1 h of hypoxemia, and 1 h of recovery. This was done during either intravenous infusion of vehicle or the adenosine receptor antagonist [8-(p-sulfophenyl)-theophylline; 8-SPT] dissolved in vehicle. During vehicle infusion, all fetuses responded to hypoxemia with bradycardia, an increase in arterial blood pressure, and femoral vasoconstriction. Increases in blood glucose and lactate concentrations and in plasma epinephrine and norepinephrine concentrations also occurred in all fetuses during hypoxemia. Fetal treatment with 8-SPT markedly attenuated the bradycardic, hypertensive, vasoconstrictor, glycemic, and adrenergic responses to hypoxemia, but it did not affect the increase in blood lactate concentrations during hypoxemia. These data show that adenosine is involved in the mechanisms mediating fetal cardiovascular, metabolic, and adrenergic responses to hypoxemia in fetal sheep. Fetal treatment with 8-SPT mimics the effects of carotid sinus nerve section on fetal cardiovascular function during hypoxemia, suggesting a role for adenosine in mediating fetal cardiovascular chemoreflexes.

Mechano- and chemoreceptor modulation of renal sympathetic nerve activity at birth in fetal sheep

Physiological responses at birth include increases in heart rate (HR), blood pressure, sympathetic nerve activity, and circulating vasoactive peptides. The factors mediating these responses are not known. To test the hypothesis that afferent input from peripheral mechanoreceptors (arterial and cardiopulmonary baroreceptors) and chemoreceptors contribute to the sympathoexcitatory and hormonal responses at birth, we studied the effects of sinoaortic denervation (SAD) and SAD with vagotomy (Vx) on changes in HR, mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and catecholamine, arginine vasopressin (AVP), and ANG II levels at birth in term sheep. One hour after delivery by cesarean section, RSNA increased by 168 +/- 49 and 192 +/- 32% (relative to fetal values) in SAD and SAD-Vx animals, respectively. Significant increases in HR (18 +/- 5 and 20 +/- 6%) and MABP (24 +/- 4 and 20 +/- 5%) were also observed 1 h after delivery in SAD and SAD-Vx lambs, respectively. These responses are similar to those seen in intact sheep delivered at the same gestational age. AVP levels markedly increased after birth (19.8 +/- 6.7 to 136.1 +/- 75.9 pg/ml) in SAD-Vx lambs, whereas SAD animals displayed no change in AVP concentrations. Plasma ANG II also did not change after birth in either group, although levels were consistently higher (P < 0.01) in SAD compared with SAD-Vx animals. In the presence of SAD, Vx resulted in significantly greater plasma levels of norepinephrine, although levels did not change after birth in either group. The epinephrine responses at birth were similar in both groups of animals. The present data suggest that afferent input from peripheral chemoreceptors and mechanoreceptors contributes little to the hemodynamic and sympathetic responses after delivery by cesarean section. On the other hand, these peripheral mechanisms appear to be involved in modulating endocrine responses at birth.

Adrenergic and vasopressinergic contributions to the cardiovascular response to acute hypoxaemia in the llama fetus

1. The effects of fetal intravenous treatment with phentolamine or a vasopressinergic V1-receptor antagonist on the fetal cardiovascular responses to acute hypoxaemia in the llama were investigated. 2. Six llama fetuses were surgically prepared between 60 and 70 % of gestation under general halothane anaesthesia with vascular catheters and transit-time ultrasonic flow probes around a carotid artery and a femoral artery. At least 4 days after surgery all fetuses were subjected to a 3 h experiment: 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery while on slow i.v. infusion with saline. On separate days this experiment was repeated with fetal i.v. treatment with either phentolamine or a V1-receptor antagonist dissolved in saline. 3. During saline infusion all llama fetuses responded to acute hypoxaemia with intense femoral vasoconstriction. Phentolamine during normoxia produced hypotension, tachycardia and vasodilatation in both the carotid and the femoral circulations. During hypoxaemia, fetuses treated with phentolamine did not elicit the pronounced femoral vasoconstriction and all died within 20 min of the onset of hypoxaemia. A V1-receptor antagonist produced a femoral vasodilatation during normoxia but did not affect the fetal cardiovascular responses to acute hypoxaemia. 4. In conclusion, alpha-adrenergic and V1-vasopressinergic mechanisms contribute to a basal vasoconstrictor tone in the femoral circulation in the llama fetus. The enhanced femoral vasoconstriction during acute hypoxaemia in the llama fetus is not mediated by stimulation of V1-vasopressin receptors, but is dependent on alpha-adrenergic receptor stimulation. Such alpha-adrenergic efferent mechanisms are indispensable to fetal survival during hypoxaemia in the llama since their abolition leads to cardiovascular collapse and death.

Angiotensin II and cardiovascular chemoreflex responses to acute hypoxia in late gestation fetal sheep

1. In six intact and nine carotid sinus denervated (CSD) fetal sheep (125-128 days gestation) we measured heart rate (FHR), mean systemic arterial blood pressure (MAP), femoral and carotid blood flows (FBF and CBF), and femoral and carotid vascular resistances (FVR and CVR). Three experiments were conducted on successive days: normoxia followed by acute isocapnic hypoxia (Pa,O2 to ca 12 mmHg) with infusion of vehicle (HV experiment), the same protocol but with infusion of the angiotensin converting enzyme (ACE) inhibitor, captopril (HC experiment), and normoxia alone with captopril infusion (NC experiment). Plasma angiotensin II concentration ([AII]) was measured in these fetuses, and in a separate group of fetuses (n = 5) that were infused with the nitric oxide (NO) synthesis inhibitor N G-nitro-L-arginine methyl ester (L-NAME) or saline vehicle. 2. During normoxia, cardiovascular parameters and plasma [AII] were unaltered by captopril infusion, apart from a fall in MAP (NC experiment only, P < 0.05) and FHR (HC experiment only, P < 0.05) in intact and CSD fetuses, respectively. No differences were observed between intact and CSD groups. 3. At the onset of hypoxia the rapid initial fall in FHR and rise in FVR was attenuated in CSD fetuses. In all fetuses FHR returned towards prehypoxic levels as hypoxia continued. In contrast, during hypoxia with vehicle infusion (HV experiment) plasma [AII] rose to a similar level in intact and CSD fetuses. 4. In both intact and CSD fetuses, the rise in [AII] during hypoxia was blocked by captopril or L-NAME infusion. In CSD, but not intact, fetuses infused with captopril the rise in MAP was absent, and the fall in FBF and rise in FVR did not reach significance during hypoxia. 5. Thus, during normoxia CSD alone, or combined with ACE inhibition, does not consistently alter basal cardiovascular control in the late gestation fetus. The rise in [AII] during hypoxia is not mediated by carotid reflexes but may involve NO-dependent mechanisms. In intact fetuses, AII does not appear to be pivotal in cardiovascular control during hypoxia. It is only when carotid reflex mechanisms are removed that a role for AII in the regulation of MAP and peripheral blood flow during hypoxia becomes apparent. These findings lend weight to the idea of multiple mechanisms of fetal cardiovascular control during hypoxia.

Do we now understand the control of the fetal circulation?

Physiological research has now given us a reasonably complete picture of fetal cardiovascular control in late gestation, especially with respect to the responses to acute hypoxaemia. Reflex, endocrine and auto/paracrine mechanisms all play a part. This review questions how complete our knowledge is when we address the clinically important issues of the influence of gestational age, sustained hypoxia and the effects of nutrition in altering the programming of cardiovascular development.