Sustained sympathetic nervous system support of arterial blood pressure during repeated brief umbilical cord occlusions in near-term fetal sheep

Extended period of ventilation before delayed cord clamping augments left-to-right shunting and decreases systemic perfusion at birth in preterm lambs

Previous studies have suggested that an extended period of ventilation before delayed cord clamping (DCC) augments birth-related rises in pulmonary arterial (PA) blood flow. However, it is unknown whether this greater rise in PA flow is accompanied by increases in left ventricular (LV) output and systemic arterial perfusion or whether it reflects enhanced left-to-right shunting across the ductus arteriosus and/or foramen ovale (FO), with decreased systemic arterial perfusion. Using an established preterm lamb birth transition model, this study compared the effect of a short (∼40 s, n = 11), moderate (∼2 min, n = 11) or extended (∼5 min, n = 12) period of initial mechanical lung ventilation before DCC on flow probe-derived perinatal changes in PA flow, LV output, total systemic arterial blood flow, ductal shunting and FO shunting. The LV output was relatively stable during initial ventilation but increased after DCC, with similar responses in all groups. Systemic arterial flow patterns displayed only minor differences during brief and moderate periods of initial ventilation and were similar after DCC. However, an increase in PA flow was augmented with an extended initial ventilation (P < 0.001), owing to an earlier onset of left-to-right ductal and FO shunting (P < 0.001), and was accompanied by a pronounced reduction in total systemic arterial flow (P = 0.005) that persisted for 4 min after DCC (P ≤ 0.039). These findings suggest that, owing to increased left-to-right shunting and a greater reduction in systemic arterial perfusion, an extended period of ventilation before DCC does not result in greater perinatal circulatory benefits than shorter periods of initial ventilation in the birth transition. KEY POINTS: Previous studies suggest that an extended period of initial ventilation before delayed cord clamping (DCC) augments birth-related rises in pulmonary arterial (PA) blood flow. It is unknown whether this greater rise in PA flow is accompanied by an increased left ventricular output and systemic arterial perfusion or whether it reflects enhanced left-to-right shunting across the ductus arteriosus and/or foramen ovale, with decreased systemic arterial perfusion. Anaesthetized preterm fetal lambs instrumented with central arterial flow probes underwent a brief (∼40 s), moderate (∼2 min) or extended (∼5 min) period of ventilation before DCC. Perinatal changes in left ventricular output were similar in all groups, but extended initial ventilation augmented both perinatal increases in PA flow, owing to earlier onset and greater left-to-right ductal and foramen ovale shunting, and perinatal reductions in total systemic arterial perfusion. Extended ventilation before DCC does not confer a greater perinatal circulatory benefit than shorter periods of initial ventilation.

Prediction of Fetal Blood Pressure during Labour with Deep Learning Techniques

Our objective is to develop a model for the prediction of minimum fetal blood pressure (FBP) during fetal heart rate (FHR) decelerations. Experimental data from umbilical occlusions in near-term fetal sheep (2698 occlusions from 57 near-term lambs) were used to train a convolutional neural network. This model was then used to estimate FBP for decelerations extracted from the final 90 min of 53,445 human FHR signals collected using cardiotocography. Minimum sheep FBP was predicted with a mean absolute error of 6.7 mmHg (25th, 50th, 75th percentiles of 2.3, 5.2, 9.7 mmHg), mean absolute percentage errors of 17.3% (5.5%, 12.5%, 23.9%) and a coefficient of determination R2=0.36. While the model was unable to clearly predict severe compromise at birth in humans, there is positive evidence that such a model could predict human FBP with further development. The neural network is capable of predicting FBP for many of the sheep decelerations accurately but performed far from satisfactory at identifying FHR segments that correspond to the highest or lowest minimum FBP. These results indicate that with further work and a larger, more variable training dataset, the model could achieve higher accuracy.

Fetal defenses against intrapartum head compression-implications for intrapartum decelerations and hypoxic-ischemic injury

Uterine contractions during labor and engagement of the fetus in the birth canal can compress the fetal head. Its impact on the fetus is unclear and still controversial. In this integrative physiological review, we highlight evidence that decelerations are uncommonly associated with fetal head compression. Next, the fetus has an impressive ability to adapt to increased intracranial pressure through activation of the intracranial baroreflex, such that fetal cerebral perfusion is well-maintained during labor, except in the setting of prolonged systemic hypoxemia leading to secondary cardiovascular compromise. Thus, when it occurs, fetal head compression is not necessarily benign but does not seem to be a common contributor to intrapartum decelerations. Finally, the intracranial baroreflex and the peripheral chemoreflex (the response to acute hypoxemia) have overlapping efferent effects. We propose the hypothesis that these reflexes may work synergistically to promote fetal adaptation to labor.

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.

A second look at intrapartum fetal surveillance and future directions

Intrapartum fetal surveillance aims to predict significant fetal hypoxia and institute timely intervention to avoid fetal injury, and do so without unnecessary operative delivery of fetuses at no risk of intrapartum hypoxia. However, the configuration and application of current clinical guidelines inadvertently undermine these aims because of persistent failure to incorporate increased understanding of fetal cardiovascular physiology and adaptations to oxygen deprivation, advances in signal acquisition/processing, and related technologies. Consequently, the field on intrapartum fetal surveillance is stuck in rudimentary counts of the fetal R-R intervals and visual assessment of very common, but nonspecific fetal heart decelerations and fetal heart rate variability. The present authors argue that the time has come to move away from classifications of static morphological appearances of FHR decelerations, which do not assist the thinking clinician in understanding how the fetus defends itself and compensates for intrapartum hypoxic ischaemic insults or the patterns that suggest progressive loss of compensation. We also reappraise some of the controversial aspects of intrapartum fetal surveillance in modern obstetric practice, the current state of flux in training and certification, and contemplate the future of the field particularly in the context of the emerging role of artificial intelligence.

Increased variability of fetal heart rate during labour: a review of preclinical and clinical studies

Increased fetal heart rate variability (FHRV) in intrapartum cardiotocographic recording has been variably defined and poorly understood, limiting its clinical utility. Both preclinical (animal) and clinical (human) evidence support that increased FHRV is observed in the early stage of intrapartum fetal hypoxaemia but can also be observed in a subset of fetuses during the preterminal stage of repeated hypoxaemia. This review of available evidence provides data and expert opinion on the pathophysiology of increased FHRV, its clinical significance and a stepwise approach regarding the management of this pattern, and propose recommendations for standardisation of related terminology.

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.

Peripheral chemoreflex activation and cardiac function during hypoxemia in near-term fetal sheep without placental compromise

A drop in arterial oxygen content activates fetal chemoreflex including an increase in sympathetic activity leading to peripheral vasoconstriction and redistribution of blood flow to protect the brain, myocardium, and adrenal glands. By using a chronically instrumented fetal sheep model with intact placental circulation at near-term gestation, we investigated the relationship between peripheral chemoreflex activation induced by hypoxemia and central hemodynamics. A total of 17 Åland landrace sheep fetuses at 115-128/145 gestational days were instrumented. Carotid artery was catheterized in 10 fetuses and descending aorta in 7 fetuses. After a 4-day recovery, baseline measurements of fetal arterial blood pressures, blood gas values, and fetal cardiovascular hemodynamics by pulsed Doppler ultrasonography were obtained under isoflurane anesthesia. Comparable data to baseline were collected 10 min (acute hypoxemia) and 60 min (prolonged hypoxemia) after maternal hypo-oxygenation to saturation level of 70%-80% was achieved. During prolonged hypoxemia, pH and base excess (BE) were lower and lactate levels were higher in the descending aorta than in the carotid artery. During hypoxemia mean arterial blood pressure (MAP) in the descending aorta increased, whereas in the carotid artery, MAP decreased. In addition, right pulmonary artery pulsatility index values increased, and the diastolic component in the aortic isthmus blood flow velocity waveform became more retrograde, thus decreasing the aortic isthmus antegrade/retrograde blood flow (AoI Net Flow) ratio. Both fetal ventricular cardiac outputs were maintained even during prolonged hypoxemia when significant fetal metabolic acidemia developed. Fetal chemoreflex activation induced by hypoxemia decreased the perfusion pressure in the cerebral circulation. Fetal weight-indexed left ventricular cardiac output (LVCO) or AoI Net Flow ratio did not correlate with a drop in carotid artery blood pressure.NEW & NOTEWORTHY During fetal hypoxemia with intact placental circulation, peripheral chemoreflex was activated, as demonstrated by an increase in the descending aorta blood pressure, pulmonary vasoconstriction, and an increase in retrograde diastolic AoI blood flow, while both ventricular cardiac outputs remained stable. However, perfusion pressure in the cerebral circulation decreased. These changes were seen even during prolonged hypoxemia when significant metabolic acidosis developed. Weight-indexed LVCO or AoI Net Flow ratio did not correlate with a drop in carotid artery blood pressure.

Reducing lung liquid volume in fetal lambs decreases ventricular constraint

BACKGROUND

This study evaluated whether an increased left ventricular (LV) pump function accompanying reduction of lung liquid volume in fetal lambs was related to increased LV preload, augmented LV contractility, or both.

METHODS

Eleven anesthetized preterm fetal lambs (gestation 128 ± 2 days) were instrumented with (1) an LV micromanometer-conductance catheter to obtain LV end-diastolic volume (EDV) and end-diastolic pressure (EDP), the maximal rate of rise of LV pressure (dP/dt_max), LV output, LV stroke work, and LV end-systolic elastance (E_es), a relatively load-independent measure of contractility; (2) an endotracheal tube to measure mean tracheal pressure and to reduce lung liquid volume. LV transmural pressure was calculated as LV EDP minus tracheal pressure.

RESULTS

Reducing lung liquid volume by 16 ± 4 ml kg^-1 (1) augmented LV output (by 16%, P = 0.001) and stroke work (29%, P < 0.001), (2) increased LV EDV (12%, P < 0.001), (3) increased LV transmural pressure (2.2 mmHg, P < 0.001), (4) did not change LV dP/dt_max normalized for EDV (P > 0.7), and (5) decreased LV E_es (20%, P < 0.01).

CONCLUSION

These findings suggest a rise in LV pump function evident after reduction of lung liquid volume in fetal lambs was related to increased LV preload secondary to lessening of external LV constraint, without any associated rise in LV contractility.

IMPACT

This study has shown that reducing the volume of liquid filling the fetal lungs lessens the degree of external constraint on the heart. This lesser constraint permits a rise in left ventricular dimensions and thus greater cardiac filling that leads to increased left ventricular pumping performance. This study has defined a mechanism whereby a reduction in lung liquid volume results in enhanced pumping performance of the fetal heart. These findings suggest that a reduction in lung liquid volume which occurs during the birth transition contributes to increases in left ventricular dimensions and pumping performance known to occur with birth.

Prelabor and intrapartum Doppler ultrasound to predict fetal compromise

According to current estimates, over 20% of the 4 million neonatal deaths occurring globally every year are related to intrapartum hypoxic complications that happen as a result of uterine contractions against a background of inadequate placental function. Most of such intrapartum complications occur among apparently uncomplicated term pregnancies. Available evidence suggests that current risk-assessment strategies do not adequately identify many of the fetuses vulnerable to periods of intermittent hypoxia that characterize human labor. In this review, we discuss the data available on Doppler ultrasound for the evaluation of placental function before and during labor in appropriately grown fetuses; we also discuss the current strategies for ultrasound-based risk stratification, the physiology of intrapartum compromise, and the potential future treatments to prevent fetal distress in labor and reduce perinatal complications related to birth asphyxia.

Roles of parasympathetic outflow and sympathetic outflow in the cardiovascular response to brief umbilical cord occlusion in fetal sheep

Fetal heart rate (FHR) deceleration is the most common change seen during labor. The role of the autonomic nervous system in regulating the fetal cardiovascular response during multiple uterine contractions has been well-established. However, the mechanism underlying the hemodynamic response remains unclear and the specific reflex that mediates the cardiovascular modifications is still controversial. This study aimed to determine the role of the sympathetic and parasympathetic systems on fetal hemodynamics in complete cord occlusion. Chronically instrumented fetal sheep were randomized to receive an intravenous injection of atropine 2.5 mg (n = 8), propranolol 5 mg (n = 7), atropine and propranolol (n = 7), or a control protocol (n = 9), followed by three episodes of 1-minute umbilical cord occlusion repeated every 5 minutes. Cord compression induces a rapid decrease in the FHR and a rapid increase in MAP. The decrease in FHR is caused by an increase in parasympathetic activity, (atropine and atropine-propranolol abolish the FHR response to the occlusion). The change in FHR during occlusion was not modified by propranolol injection, showing no effect of sympathetic tone. The increase in MAP during occlusion was similar in the four protocols. After releasing occlusion, the FHR was still lower than that at baseline due to a sustained parasympathetic tone. Suppression of the parasympathetic output to the cardiovascular system unmasks an increase in the FHR above baseline values. The lower FHR with the propranolol protocol further supports an increase in myocardial β-adrenoceptor stimulation after cord release. The increase in MAP after cord release was similar in the four protocols, except after the early stage of interocclusion period in atropine protocol. Four minutes after cord release, the FHR returned to baseline irrespective of the drugs that were infused, thereby showing recovery of ANS control. Blood gases (pH, PaCO2, PaO2) and plasma lactate concentrations was similar between the four protocols at the end of three applications of UCO. Complete cord compression-induced deceleration is likely due to acute activation of parasympathetic output. β-adrenoceptor activity is involved in the increase in FHR after cord release. Understanding the reflexes involved in FHR deceleration may help us understand the mechanisms underlying fetal autonomic adaptation during cord occlusion.

Characteristics and physiological basis of falls in ventricular outputs after immediate cord clamping at delivery in preterm fetal lambs

KEY POINTS

Controversy exists about the physiological mechanism(s) underlying decreases in cardiac output after immediate clamping of the umbilical cord at birth. To define these mechanisms, the four major determinants of ventricular output (afterload, preload, heart rate and contractility) were measured concurrently in fetal lambs at 15 s intervals over a 2 min period after cord clamping and before ventilation following delivery. After cord clamping, right (but not left) ventricular output fell by 20% in the initial 30 s, due to increased afterload associated with higher arterial blood pressures, but both outputs then halved over 45 s, due to a falling heart rate and deteriorating ventricular contractility accompanying rapid declines in arterial oxygenation to asphyxial levels. Ventricular outputs subsequently plateaued from 75 to 120 s, associated with rebound rises in ventricular contractility accompanying asphyxia-induced surges in circulating catecholamines. These findings provide a physiological basis for the clinical recommendation that effective ventilation should occur within 60 s after immediate cord clamping.

ABSTRACT

Controversy exists about the physiological mechanism(s) underlying large decreases in cardiac output after immediate clamping of the umbilical cord at birth. To define these mechanisms, anaesthetized preterm fetal lambs (127(1)d, n = 12) were instrumented with flow probes and catheters in major central arteries, and a left ventricular (LV) micromanometer-conductance catheter. Following immediate cord clamping at delivery, haemodynamics, LV and right ventricular (RV) outputs, and LV contractility were measured at 15 s intervals during a 2 min non-ventilatory period, with aortic blood gases and circulating catecholamine (noradrenaline and adrenaline) concentrations measured at 30 s intervals. After cord clamping, (1) RV (but not LV) output fell by 20% in the initial 30 s, due to a reduced stroke volume associated with increased arterial blood pressures, (2) both outputs then halved over the next 45 s, associated with falls in heart rate, arterial blood pressures and ventricular contractility accompanying a rapid decline in arterial oxygenation to asphyxial levels, (3) reduced outputs subsequently plateaued from 75 to 120 s, associated with rebound rises in blood pressures and ventricular contractility accompanying exponential surges in circulating catecholamines. These findings are consistent with a time-dependent decline of ventricular outputs after immediate cord clamping, which comprised (1) an initial, minor fall in RV output related to altered loading conditions, (2) ensuing large decreases in both LV and RV outputs related to the combination of bradycardia and ventricular dysfunction during emergence of an asphyxial state, and (3) subsequent stabilization of reduced LV and RV outputs during ongoing asphyxia, supported by cardiovascular stimulatory effects of marked sympathoadrenal activation.

Cerebral blood volume increment after resuscitation measured by near-infrared time-resolved spectroscopy can estimate degree of hypoxic-ischemic insult in newborn piglets

Neonatal hypoxic–ischemic encephalopathy is a notable cause of neonatal death and developmental disabilities. To achieve better outcomes, it is important in treatment strategy selection to categorize the degree of hypoxia ischemia and evaluate dose response. In an asphyxia piglet model with histopathological brain injuries that we previously developed, animals survived 5 days after insult and showed changes in cerebral blood volume (CBV) that reflected the severity of injuries. However, little is known about the relationship between changes in CBV during and after insult. In this study, an HI event was induced by varying the amount and timing of inspired oxygen in 20 anesthetized piglets. CBV was measured using near-infrared time-resolved spectroscopy before, during, and 6 h after insult. Change in CBV was calculated as the difference between the peak CBV value during insult and the value at the end of insult. The decrease in CBV during insult was found to correlate with the increase in CBV within 6 h after insult. Heart rate exhibited a similar tendency to CBV, but blood pressure did not. Because the decrement in CBV was larger in severe HI, the CBV increment immediately after insult is considered useful for assessing degree of HI insult.

Lack of evidence for impaired preload or Bezold-Jarisch activation during brief umbilical cord occlusions in fetal sheep

Impaired cardiac preload secondary to umbilical cord occlusion (UCO) has been hypothesized to contribute to intrapartum decelerations, brief falls in fetal heart rate (FHR), through activation of the Bezold-Jarisch reflex. This cardioprotective reflex increases parasympathetic and inhibits sympathetic outflows triggering hypotension, bradycardia, and peripheral vasodilation, but its potential to contribute to intrapartum decelerations has never been systematically examined. In this study, we performed bilateral cervical vagotomy to remove the afferent arm and the efferent parasympathetic arm of the Bezold-Jarisch reflex. Twenty-two chronically instrumented fetal sheep at 0.85 of gestation received vagotomy (n = 7) or sham vagotomy (control, n = 15), followed by three 1-min complete UCOs separated by 4-min reperfusion periods. UCOs in control fetuses were associated with a rapid fall in FHR and reduced femoral blood flow mediated by intense femoral vasoconstriction, leading to hypertension. Vagotomy abolished the rapid fall in FHR (P < 0.001) and, despite reduced diastolic filling time, increased both carotid (P < 0.001) and femoral (P < 0.05) blood flow during UCOs, secondary to carotid vasodilation (P < 0.01) and delayed femoral vasoconstriction (P < 0.05). Finally, vagotomy was associated with an attenuated rise in cortical impedance during UCOs (P < 0.05), consistent with improved cerebral substrate supply. In conclusion, increased carotid and femoral blood flows after vagotomy are consistent with increased left and right ventricular output, which is incompatible with the hypothesis that labor-like UCOs impair ventricular filling. Overall, the cardiovascular responses to vagotomy do not support the hypothesis that the Bezold-Jarisch reflex is activated by UCO. The Bezold-Jarisch reflex is therefore mechanistically unable to contribute to intrapartum decelerations.

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.

Excess cerebral oxygen delivery follows return of spontaneous circulation in near-term asphyxiated lambs

Hypoxic-ischaemia renders the neonatal brain susceptible to early secondary injury from oxidative stress and impaired autoregulation. We aimed to describe cerebral oxygen kinetics and haemodynamics immediately following return of spontaneous circulation (ROSC) and evaluate non-invasive parameters to facilitate bedside monitoring. Near-term sheep fetuses [139 ± 2 (SD) days gestation, n = 16] were instrumented to measure carotid artery (CA) flow, pressure, right brachial arterial and jugular venous saturation (SaO2 and SvO2, respectively). Cerebral oxygenation (crSO2) was measured using near-infrared spectroscopy (NIRS). Following induction of severe asphyxia, lambs received cardiopulmonary resuscitation using 100% oxygen until ROSC, with oxygen subsequently weaned according to saturation nomograms as per current guidelines. We found that oxygen consumption did not rise following ROSC, but oxygen delivery was markedly elevated until 15 min after ROSC. CrSO2 and heart rate each correlated with oxygen delivery. SaO2 remained > 90% and was less useful for identifying trends in oxygen delivery. CrSO2 correlated inversely with cerebral fractional oxygen extraction. In conclusion, ROSC from perinatal asphyxia is characterised by excess oxygen delivery that is driven by rapid increases in cerebrovascular pressure, flow, and oxygen saturation, and may be monitored non-invasively. Further work to describe and limit injury mediated by oxygen toxicity following ROSC is warranted.

Changes in fetoplacental Doppler indices following intrapartum maternal sildenafil citrate treatment

OBJECTIVE

Uterine contractions during labour can result in a 60 % decline in fetoplacental perfusion, predisposing the fetus to hypoxic brain injury. Sildenafil citrate (SC) has shown promise in increasing uteroplacental perfusion as well as reducing the risk of operative birth for intrapartum fetal compromise (IFC). The aim of this study was to investigate the effect of intrapartum SC administration on fetoplacental blood flow indices.

METHODS

This was a subgroup analysis from an earlier Phase II double blind randomized controlled trial; assessing the effect of intrapartum SC administration (50 mg orally 8 -hly in labour) compared to placebo for the reduction of operative birth for IFC. An ultrasound scan measuring fetoplacental Doppler indices was performed prior to and 1-4 h after the administration of the first treatment dose.

RESULTS

Of the 300 women randomized to the main study, pre-treatment ultrasound scans were performed in 261 participants who received the study medication; paired pre- and post-treatment scans were performed in 70 (26.8 %). SC resulted in an increase in the middle cerebral artery pulsatility index (PI) z-score [+0.08 (1.35) vs.-0.12 (1.15)], a decline in the umbilical artery-PI z-score [-0.07 (0.96) vs. + 0.04 (1.25)] and an increase in the cerebroplacental ratio [MCA-PI/UA-PI] (CPR) z-score [+0.10 (1.13) vs.-0.26 (1.14)] although these failed to reach statistical significance. Amongst those with a pre-treatment CPR above the 5th centile, SC significantly reduced the risk of operative birth for IFC compared to placebo [logrank p = 0.02; hazards ratio 0.48, 95 % CI 0.29-0.77, p = 0.003].

CONCLUSION

Although the differences in Doppler indices pre- and post- SC treatment were non-significant, there was a clear trend towards a reduction in the UA PI, a corresponding increase in the MCA PI and a rise in post treatment CPR values suggesting potential improvement in fetoplacental Dopplers with intrapartum SC treatment. However this study was limited by the small sample size. The results of this hypothesis generating study suggest that it may be possible to stratify women that would most benefit from this intervention based upon their pre-labour CPR.

Parasympathetic activity is the key regulator of heart rate variability between decelerations during brief repeated umbilical cord occlusions in fetal sheep

Fetal heart rate variability (FHRV) is a widely used index of intrapartum well being. Both arms of the autonomic system regulate FHRV under normoxic conditions in the antenatal period. However, autonomic control of FHRV during labor when the fetus is exposed to repeated, brief hypoxemia during uterine contractions is poorly understood. We have previously shown that the sympathetic nervous system (SNS) does not regulate FHRV during labor-like hypoxia. We therefore investigated the hypothesis that the parasympathetic system is the main mediator of intrapartum FHRV. Twenty-six chronically instrumented fetal sheep at 0.85 of gestation received either bilateral cervical vagotomy (n = 7), atropine sulfate (n = 7), or sham treatment (control, n = 12), followed by three 1-min complete umbilical cord occlusions (UCOs) separated by 4-min reperfusion periods. Parasympathetic blockade reduced three measures of FHRV before UCOs (all P < 0.01). Between UCOs, atropine and vagotomy were associated with marked tachycardia (both P < 0.005), suppressed measures of FHRV (all P < 0.01), and abolished FHRV on visual inspection compared with the control group. Tachycardia in the atropine and vagotomy groups resolved over the first 10 min after the final UCO, in association with evidence that the SNS contribution to FHRV progressively returned during this time. Our findings support that SNS control of FHRV is acutely suppressed for at least 4 min after a deep intrapartum deceleration and takes 5-10 min to recover. The parasympathetic system is therefore likely to be the key mediator of FHRV once frequent FHR decelerations are established during labor.

Effects of β-adrenergic stimulation on fetal heart rate, heart rate variability, and T-wave elevation during brief umbilical cord occlusions in fetal sheep

Circulating catecholamines are critical for fetal adaptation to hypoxia by regulating fetal heart rate (FHR) and promoting myocardial contractility and peripheral vasoconstriction. They have been hypothesized to contribute to changes in FHR variability (FHRV) and T-wave morphology, clinical indexes of fetal well-being during labor. β-Adrenergic blockade with propranolol does not affect FHRV during labor-like hypoxemia and only attenuated the increase in T-wave height between the episodes of hypoxemia. To further investigate the potential role of catecholamines, we investigated whether pharmacological β-adrenergic stimulation could increase FHRV and T-wave elevation during intermittent labor-like hypoxemia. Nineteen chronically instrumented fetal sheep at 0.85 of gestation received isoprenaline hydrochloride (n = 7) or saline (control, n = 12), followed by three 1-min complete umbilical cord occlusions (UCOs) separated by 4-min reperfusion periods. Before the UCOs, infusion of isoprenaline increased FHR (P < 0.001), absolute-T/QRS ratio (P < 0.001), and one measure of FHRV [root-mean-square of successive RR interval differences (RMSSD), P < 0.05]. UCOs triggered deep FHR decelerations. During UCOs, isoprenaline was associated with increased FHR (P < 0.001) and absolute-T/QRS ratio (P < 0.05), but no effect on T/QRS ratio was observed when normalized to baseline before UCOs (normalized-T/QRS ratio). Between UCOs, isoprenaline increased FHR (P < 0.001) and absolute-T/QRS ratio (P < 0.05) but did not affect normalized-T/QRS ratio or any measures of FHRV. Arterial pressure was not affected by isoprenaline at any point. Our findings indicate that circulating catecholamines regulate FHR but not FHRV during labor-like hypoxemia and promote T-wave elevation between but not during intermittent fetal hypoxemia.

Circulating catecholamines partially regulate T-wave morphology but not heart rate variability during repeated umbilical cord occlusions in fetal sheep

Fetal heart rate (FHR) variability (FHRV) and ST segment morphology are potential clinical indices of fetal well-being during labor. β-Adrenergic stimulation by circulating catecholamines has been hypothesized to contribute to both FHRV and ST segment morphology during labor, but this has not been tested during brief repeated fetal hypoxemia that is characteristic of labor. Near-term fetal sheep (0.85 gestation) received propranolol (β-adrenergic blockade; n = 10) or saline (n = 7) 30 min before being exposed to three 2-min complete umbilical cord occlusions (UCOs) separated by 3-min reperfusions. T/QRS ratio was calculated throughout UCOs and reperfusion periods, and measures of FHRV (RMSSD, SDNN, and STV) were calculated between UCOs. During the baseline period, before the start of UCOs, propranolol was associated with reduced FHR, SDNN, and STV but did not affect RMSSD or T/QRS ratio. UCOs were associated with rapid FHR decelerations and increased T/QRS ratio; propranolol significantly reduced FHR during UCOs and was associated with a slower rise in T/QRS ratio during the first UCOs, without affecting the maximal rise or T/QRS ratio during the second and third UCO. Between UCOs propranolol reduced FHR and T/QRS ratio but did not affect any measure of FHRV. These data demonstrate that circulating catecholamines do not contribute to FHRV during labor-like hypoxemia. Furthermore, circulating catecholamines did not contribute to the major rise in T/QRS ratio during labor-like hypoxemia but may regulate T/QRS ratio between brief hypoxemia.

Brief asphyxial state following immediate cord clamping accelerates onset of left-to-right shunting across the ductus arteriosus after birth in preterm lambs

Reversal of shunting across the ductus arteriosus from right-to-left to left-to-right is a characteristic feature of the birth transition. Given that immediate cord clamping (ICC) followed by an asphyxial cord clamp-to-ventilation (CC-V) interval may augment left ventricular (LV) output and central blood flows after birth, we tested the hypothesis that an asphyxial CC-V interval accelerates the onset of postnatal left-to-right ductal shunting. High-fidelity central blood flow signals were obtained in anesthetized preterm lambs (gestation 128 ± 2 days) after ICC followed by a nonasphyxial (∼40 s, n = 9) or asphyxial (∼90 s, n = 9) CC-V interval before mechanical ventilation for 30 min after birth. Left-to-right ductal flow segments were related to aortic isthmus and descending aortic flow profiles to quantify sources of ductal shunting. In the nonasphyxial group, phasic left-to-right ductal shunting was initially minor after birth, but then rose progressively to 437 ± 164 ml/min by 15 min (P < 0.001). However, in the asphyxial group, this shunting increased from 24 ± 21 to 199 ± 93 ml/min by 15 s after birth (P < 0.001) and rose further to 471 ± 190 ml/min by 2 min (P < 0.001). This earlier onset of left-to-right ductal shunting was supported by larger contributions (P < 0.001) from direct systolic LV flow and retrograde diastolic discharge from an arterial reservoir/windkessel located in the descending aorta and its major branches, and associated with increased pulmonary arterial blood flow having a larger ductal component. These findings suggest that the duration of the CC-V interval after ICC is an important modulator of left-to-right ductal shunting, LV output and pulmonary perfusion at birth.NEW & NOTEWORTHY This birth transition study in preterm lambs demonstrated that a brief (∼90 s) asphyxial interval between umbilical cord clamping and ventilation onset resulted in earlier and greater left-to-right shunting across the ductus arteriosus after birth. This greater shunting 1) resulted from an increased left ventricular output associated with a higher systolic left-to-right ductal flow and increased retrograde diastolic discharge from a lower body arterial reservoir/windkessel, and 2) was accompanied by greater lung perfusion after birth.

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.

Blunted sympathoadrenal activation accompanies hemodynamic stability after early ventilation and delayed cord clamping at birth in preterm lambs

BACKGROUND

As surges in circulating norepinephrine and epinephrine have chronotropic, pressor, and inotropic effects, we tested the hypothesis that blunted rises in these catecholamines during preterm birth accompanied hemodynamic stability observed after early ventilation and delayed cord clamping (DCC), with findings compared to immediate cord clamping (ICC) and a non-asphyxial cord clamp-to-ventilation interval.

METHODS

Anesthetized preterm fetal lambs were instrumented with arterial micromanometers to obtain pressure and the maximal rate of pressure rise (dP/dt_max) as a surrogate of ventricular contractility and an aortic catheter to obtain blood samples for catecholamine assay. Fetuses were delivered and mechanically ventilated before cord clamping ∼1.5 min later (DCC, n = 9) or subjected to ICC with ventilation started ∼40 s later (n = 8).

RESULTS

Perinatal hemodynamics were stable after DCC, with greater fluctuations evident following birth after ICC (P ≤ 0.05). With DCC, circulating norepinephrine and epinephrine were unchanged after early ventilation but rose following cord clamping (P ≤ 0.01), with concentrations below the threshold for hemodynamic effects. Norepinephrine was higher in the ICC group after cord clamping and immediately after ventilation (P < 0.025), but catecholamine levels were otherwise similar between groups.

CONCLUSION

Hemodynamic stability at birth after DCC is accompanied by sub-threshold rises in circulating norepinephrine and epinephrine and thus blunted sympathoadrenal activation.

The Effect of Antenatal Betamethasone on White Matter Inflammation and Injury in Fetal Sheep and Ventilated Preterm Lambs

Antenatal administration of betamethasone (BM) is a common antecedent of preterm birth, but there is limited information about its impact on the acute evolution of preterm neonatal brain injury. We aimed to compare the effects of maternal BM in combination with mechanical ventilation on the white matter (WM) of late preterm sheep. At 0.85 of gestation, pregnant ewes were randomly assigned to receive intra-muscular (i.m.) saline (n = 9) or i.m. BM (n = 13). Lambs were delivered and unventilated controls (UVCSal, n = 4; UVCBM, n = 6) were humanely killed without intervention; ventilated lambs (VentSal, n = 5; VentBM, n = 7) were injuriously ventilated for 15 min, followed by conventional ventilation for 75 min. Cardiovascular and cerebral haemodynamics and oxygenation were measured continuously. The cerebral WM underwent assessment of inflammation and injury, and oxidative stress was measured in the cerebrospinal fluid (CSF). In the periventricular and subcortical WM tracts, the proportion of amoeboid (activated) microglia, the density of astrocytes, and the number of blood vessels with protein extravasation were higher in UVCBM than in UVCSal (p < 0.05 for all). During ventilation, tidal volume, mean arterial pressure, carotid blood flow, and oxygen delivery were higher in -VentBM lambs (p < 0.05 vs. VentSal). In the subcortical WM, microglial infiltration was increased in the VentSal group compared to UVCSal. The proportion of activated microglia and protein extravasation was higher in the VentBM group compared to VentSal within the periventricular and subcortical WM tracts (p < 0.05). CSF oxidative stress was increased in the VentBM group compared to UVCSal, UVCBM, and VentSal groups (p < 0.05). Antenatal BM was associated with inflammation and vascular permeability in the WM of late preterm fetal sheep. During the immediate neonatal period, the increased carotid perfusion and oxygen delivery in BM-treated lambs was associated with increased oxidative stress, microglial activation and microvascular injury.

Understanding Fetal Heart Rate Patterns That May Predict Antenatal and Intrapartum Neural Injury

Electronic fetal heart rate (FHR) monitoring is widely used to assess fetal well-being throughout pregnancy and labor. Both antenatal and intrapartum FHR monitoring are associated with a high negative predictive value and a very poor positive predictive value. This in part reflects the physiological resilience of the healthy fetus and the remarkable effectiveness of fetal adaptations to even severe challenges. In this way, the majority of "abnormal" FHR patterns in fact reflect a fetus' appropriate adaptive responses to adverse in utero conditions. Understanding the physiology of these adaptations, how they are reflected in the FHR trace and in what conditions they can fail is therefore critical to appreciating both the potential uses and limitations of electronic FHR monitoring.

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.

Magnesium sulfate and sex differences in cardiovascular and neural adaptations during normoxia and asphyxia in preterm fetal sheep

Magnesium sulfate (MgSO₄) is recommended for preterm neuroprotection, preeclampsia, and preterm labor prophylaxis. There is an important, unmet need to carefully test clinical interventions in both sexes. Therefore, we aimed to investigate cardiovascular and neurophysiological adaptations to MgSO₄ during normoxia and asphyxia in preterm male and female fetal sheep. Fetuses were instrumented at 98 ± 1 days of gestation (term = 147 days). At 104 days, unanesthetized fetuses were randomly assigned to intravenous MgSO₄ ( n = 12 female, 10 male) or saline ( n = 13 female, 10 male). At 105 days fetuses underwent umbilical cord occlusion for up to 25 min. Occlusions were stopped early if mean arterial blood pressure (MAP) fell below 8 mmHg or asystole occurred for >20 s. During normoxia, MgSO₄ was associated with similar reductions in fetal heart rate (FHR), EEG power, and movement in both sexes ( P < 0.05 vs. saline controls) and suppression of α- and β-spectral band power in males ( P < 0.05 vs. saline controls). During occlusion, similar FHR and MAP responses occurred in MgSO₄-treated males and females compared with saline controls. Recovery of FHR and MAP after release of occlusion was more prolonged in MgSO₄-treated males ( P < 0.05 vs. saline controls). During and after occlusion, EEG power was lower in MgSO₄-treated females ( P < 0.05 vs. saline controls). In conclusion, MgSO₄ infusion was associated with subtle sex-specific effects on EEG spectral power and cardiac responses to asphyxia in utero, possibly reflecting sex-specific differences in interneuronal connectivity and regulation of cardiac output.

Impact of chronic maternal stress during early gestation on maternal-fetal stress transfer and fetal stress sensitivity in sheep

Acute stress-induced reduction of uterine blood flow (UBF) is an indirect mechanism of maternal-fetal stress transfer during late gestation. Effects of chronic psychosocial maternal stress (CMS) during early gestation, as may be experienced by many working women, on this stress signaling mechanism are unclear. We hypothesized that CMS in sheep during early gestation augments later acute stress-induced decreases of UBF, and aggravates the fetal hormonal, cardiovascular, and metabolic stress responses during later development. Six pregnant ewes underwent repeated isolation stress (CMS) between 30 and 100 days of gestation (dGA, term: 150 dGA) and seven pregnant ewes served as controls. At 110 dGA, ewes were chronically instrumented and underwent acute isolation stress. The acute stress decreased UBF by 19% in both the CMS and control groups (p < .05), but this was prolonged in CMS versus control ewes (74 vs. 30 min, p < .05). CMS increased fetal circulating baseline and stress-induced cortisol and norepinephrine concentrations indicating a hyperactive hypothalamus-pituitary-adrenal (HPA)-axis and sympathetic-adrenal-medullary system. Increased fetal norepinephrine is endogenous as maternal catecholamines do not cross the placenta. Cortisol in the control but not in the CMS fetuses was correlated with maternal cortisol blood concentrations; these findings indicate: (1) no increased maternal-fetal cortisol transfer with CMS, (2) cortisol production in CMS fetuses when the HPA-axis is normally inactive, due to early maturation of the fetal HPA-axis. CMS fetuses were better oxygenated, without shift towards acidosis compared to the controls, potentially reflecting adaptation to repeated stress. Hence, CMS enhances maternal-fetal stress transfer by prolonged reduction in UBF and increased fetal HPA responsiveness.

Cardiovascular Supportive Therapies for Neonates With Asphyxia - A Literature Review of Pre-clinical and Clinical Studies

Asphyxiated neonates often have hypotension, shock, and poor tissue perfusion. Various "inotropic" medications are used to provide cardiovascular support to improve the blood pressure and to treat shock. However, there is incomplete literature on the examination of hemodynamic effects of these medications in asphyxiated neonates, especially in the realm of clinical studies (mostly in late preterm or term populations). Although the extrapolation of findings from animal studies and other clinical populations such as children and adults require caution, it seems appropriate that findings from carefully conducted pre-clinical studies are important in answering some of the fundamental knowledge gaps. Based on a literature search, this review discusses the current available information, from both clinical studies and animal models of neonatal asphyxia, on common medications used to provide hemodynamic support including dopamine, dobutamine, epinephrine, milrinone, norepinephrine, vasopressin, levosimendan, and hydrocortisone.

An investigation of fetal behavioural states during maternal sleep in healthy late gestation pregnancy: an observational study

KEY POINTS

Fetal behavioural state in healthy late gestation pregnancy is significantly affected by maternal position overnight. Maternal left lateral position is the one most frequently adopted at sleep onset. The maternal position at sleep onset is maintained the longest overnight. Fetal state 1F is more common in maternal supine positions overnight. Fetal state 4F is less common in maternal supine sleep positions. Fetal state and maternal sleep position are independently associated with fetal heart rate variability. Maternal sleep position significantly affects fetal heart rate and heart rate variability and affects circadian fetal heart rate patterns.

ABSTRACT

Fetal behavioural states (FBS) are measures of fetal wellbeing. Maternal position affects FBS with supine position being associated with an increased likelihood of fetal quiescence consistent with the human fetus adapting to a lower oxygen consuming state. Several studies have now confirmed the association between sleep position and risk of late intrauterine death. We designed this study to observe the effects of maternal sleep positions overnight in healthy late gestation pregnancy. Twenty-nine healthy women had continuous fetal ECG recordings overnight. Two blinded observers assigned fetal states in 5 min blocks. Measures of fetal heart rate variability (FHRV) were calculated from ECG beat to beat data. Maternal position was determined from infrared video recording. Compared to state 2F (active sleep), 4F (active awake-high activity) occurred almost exclusively when the mother was in a left or right lateral position. State 1F (quiet sleep) was more common when the mother was supine [odds ratio (OR) 1.30, 95% confidence interval (CI) 1.11-1.52] and less common on the maternal right side with the left being the referent position (OR 0.81, 95% CI, 0.70-0.93). State 4F was more common between 21.00 and 01.00 h than between 01.00 and 07.00 h (OR 2.83, 95% CI 2.32-3.47). In each fetal state, maternal position had significant effects on fetal heart rate and measures of FHRV. In healthy late gestation pregnancy, maternal sleep position affects FBS and heart rate variability. These effects are probably fetal adaptations to positions which may produce a mild hypoxic stress.

Greater sympathoadrenal activation with longer preventilation intervals after immediate cord clamping increases hemodynamic lability at birth in preterm lambs

This study tested the hypothesis that varying degrees of hemodynamic fluctuations seen after birth following immediate cord clamping were related to development of asphyxia with longer cord clamp-to-ventilation intervals, resulting in higher perinatal circulating levels of the catecholamines norepinephrine (NE) and epinephrine (Epi), and thus increased heart rate, blood pressures, and cardiac contractility after birth. Anesthetized preterm fetal lambs were instrumented with 1) aortic (AoT) and pulmonary trunk (PT) micromanometers to obtain pressures and the maximal rate of pressure rise (dP/dt_max) as a surrogate measure of ventricular contractility, and 2) an AoT catheter to obtain samples for blood gas and catecholamine analyses. After delivery, immediate cord clamping was followed by ventilation ∼40 s (n = 7), ∼60 s (n = 8), ∼90 s (n = 9), or ∼120 s later (n = 8), with frequent blood sampling performed before and after ventilation. AoT O₂ content fell rapidly after immediate cord clamping (P < 0.001), with an asphyxial state evident at ≥60 s. Plasma NE and Epi levels increased progressively with longer cord clamp-to-ventilation intervals, with an exponential relation between falling AoT O₂ content and rising catecholamines (R² = 0.64-0.67). Elevated circulating catecholamines persisted for some minutes after ventilation onset, with postbirth surges in heart rate, AoT and PT pressures, and AoT and PT dP/dt_max linearly related to log_e of catecholamine levels (R² = 0.41-0.54, all P < 0.001). These findings suggest that 1) a greater degree of asphyxia-induced sympathoadrenal activation (reflected in elevated circulating catecholamine levels) occurs with longer intervals between immediate cord clamping and subsequent ventilation, and 2) this activation is a major determinant of hemodynamic fluctuations evident with birth.

Sex, drugs and rock and roll: tales from preterm fetal life

Premature fetuses and babies are at greater risk of mortality and morbidity than their term counterparts. The underlying causes are multifactorial, but include exposure to hypoxia. Immaturity of organs and their functional control may impair the physiological defence responses to hypoxia and the preterm fetal responses, or lack thereof, to moderate hypoxia appear to support this concept. However, as this review demonstrates, despite immaturity, the preterm fetus responds to asphyxia in a qualitatively similar manner to that seen at term. This highlights the importance in understanding metabolism versus homeostatic threat when assessing fetal responses to adverse challenges such as hypoxia. Data are presented to show that the preterm fetal adaptation to asphyxia is triphasic in nature. Phase one represents the rapid institution of maximal defences, designed to maintain blood pressure and central perfusion at the expense of peripheral organs. Phase two is one of adaptive compensation. Controlled reperfusion partially offsets peripheral tissue oxygen debt, while maintaining sufficient vasoconstriction to limit the fall in perfusion. Phase three is about decompensation. Strikingly, the preterm fetus generally performs better during phases two and three, and can survive for longer without injury. Paradoxically, however, the ability to survive can lead to longer exposure to hypotension and hypoperfusion and thus potentially greater injury. The effects of fetal sex, inflammation and drugs on the triphasic adaptations are reviewed. Finally, the review highlights the need for more comprehensive studies to understand the complexity of perinatal physiology if we are to develop effective strategies to improve preterm outcomes.

Effect of maternal position on fetal behavioural state and heart rate variability in healthy late gestation pregnancy

KEY POINTS

Fetal behavioural state in healthy late gestation pregnancy is affected by maternal position. Fetal state 1F is more likely to occur in maternal supine or right lateral positions. Fetal state 4F is less likely to occur when the woman lies supine or semi-recumbent. Fetal state change is more likely when the woman is supine or semi-recumbent. Fetal heart rate variability is affected by maternal position with variability reduced in supine and semi-recumbent positions.

ABSTRACT

Fetal behavioural states (FBS) are measures of fetal wellbeing. In acute hypoxaemia, the human fetus adapts to a lower oxygen consuming state with changes in the cardiotocograph and reduced fetal activity. Recent studies of late gestation stillbirth described the importance of sleep position in the risk of intrauterine death. We designed this study to assess the effects of different maternal positions on FBS in healthy late gestation pregnancies under controlled conditions. Twenty-nine healthy women had continuous fetal ECG recordings under standardized conditions in four randomly allocated positions, left lateral, right lateral, supine and semi-recumbent. Two blinded observers, assigned fetal states in 5 min blocks. Measures of fetal heart rate variability were calculated from ECG beat to beat data. Compared to state 2F, state 4F was less likely to occur when women were semi-recumbent [odds ratio (OR) = 0.11, 95% confidence interval (95% CI) 0.02, 0.55], and supine (OR = 0.27, 95% CI 0.07, 1.10). State 1F was more likely on the right (OR = 2.36, 95% CI 1.11, 5.04) or supine (OR = 4.99, 95% CI 2.41, 10.43) compared to the left. State change was more likely when the mother was semi-recumbent (OR = 2.17, 95% CI 1.19, 3.95) or supine (OR = 2.67, 95% CI 1.46, 4.85). There was a significant association of maternal position to mean fetal heart rate. The measures of heart rate variability (SDNN and RMSSD) were reduced in both semi-recumbent and supine positions. In healthy late gestation pregnancy, maternal position affects FBS and heart rate variability. These effects are likely fetal adaptations to positions which may produce a mild hypoxic stress.

Cholinergic and β-adrenergic control of cardiovascular reflex responses to brief repeated asphyxia in term-equivalent fetal sheep

The role of cholinergic and β-adrenergic activity in mediating fetal cardiovascular recovery from brief repeated episodes of asphyxia consistent with established labor, remains unclear. In this study, we tested the effect of cholinergic and β-adrenergic blockade on the fetal chemoreflex and fetal heart rate (FHR) overshoot responses during brief repeated asphyxia at rates consistent with early or active labor. Chronically instrumented fetal sheep at 0.85 of gestation received either i.v. atropine sulfate (cholinergic blockade, n=8) or vehicle (n=7) followed by 3 x 1-minute umbilical cord occlusions repeated every 5 minutes (1:5; consistent with early labor), or i.v. propranolol hydrochloride (β-adrenergic blockade, n=6) or vehicle (n=6) followed by 3 x 2-minute occlusions repeated every 5 minutes (2:5; consistent with active labor). In vehicle-controls, 1:5 occlusions were associated with rapid and sustained FHR decelerations followed by rapid return of FHR to baseline values after release of the occlusion. Cholinergic blockade abolished FHR decelerations during occlusions and caused FHR overshoot after release of the occlusion (P<0.05 vs. control 1:5). In vehicle-controls, 2:5 occlusions caused rapid and sustained FHR decelerations followed by FHR overshoot after release of the occlusion. β-adrenergic blockade was associated with greater reduction in FHR during occlusions and attenuated FHR overshoot (P<0.05 vs. control 2:5). These data demonstrate that the FHR overshoot pattern after asphyxia is mediated by a combination of attenuated parasympathetic activity and increased β-adrenergic stimulation of the fetal heart.

The myths and physiology surrounding intrapartum decelerations: the critical role of the peripheral chemoreflex

A distinctive pattern of recurrent rapid falls in fetal heart rate, called decelerations, are commonly associated with uterine contractions during labour. These brief decelerations are mediated by vagal activation. The reflex triggering this vagal response has been variably attributed to a mechanoreceptor response to fetal head compression, to baroreflex activation following increased blood pressure during umbilical cord compression, and/or a Bezold-Jarisch reflex response to reduced venous return from the placenta. Although these complex explanations are still widespread today, there is no consistent evidence that they are common during labour. Instead, the only mechanism that has been systematically investigated, proven to be reliably active during labour and, crucially, capable of producing rapid decelerations is the peripheral chemoreflex. The peripheral chemoreflex is triggered by transient periods of asphyxia that are a normal phenomenon associated with all uterine contractions. This should not cause concern as the healthy fetus has a remarkable ability to adapt to these repeated but short periods of asphyxia. This means that the healthy fetus is typically not at risk of hypotension and injury during uncomplicated labour even during repeated brief decelerations. The physiologically incorrect theories surrounding decelerations that ignore the natural occurrence of repeated asphyxia probably gained widespread support to help explain why many babies are born healthy despite repeated decelerations during labour. We propose that a unified and physiological understanding of intrapartum decelerations that accepts the true nature of labour is critical to improve interpretation of intrapartum fetal heart rate patterns.

Magnesium sulphate and cardiovascular and cerebrovascular adaptations to asphyxia in preterm fetal sheep

Magnesium sulphate is a standard therapy for eclampsia in pregnancy and is widely recommended for perinatal neuroprotection during threatened preterm labour. MgSO4 is a vasodilator and negative inotrope. Therefore the aim of this study was to investigate the effect of MgSO4 on the cardiovascular and cerebrovascular responses of the preterm fetus to asphyxia. Fetal sheep were instrumented at 98 ± 1 days of gestation (term = 147 days). At 104 days, unanaesthetised fetuses were randomly assigned to receive an intravenous infusion of MgSO4 (n = 6) or saline (n = 9). At 105 days all fetuses underwent umbilical cord occlusion for 25 min. Before occlusion, MgSO4 treatment reduced heart rate and increased femoral blood flow (FBF) and vascular conductance compared to controls. During occlusion, carotid and femoral arterial conductance and blood flows were higher in MgSO4-treated fetuses than controls. After occlusion, fetal heart rate was lower and carotid and femoral arterial conductance and blood flows were higher in MgSO4-treated fetuses than controls. Femoral arterial waveform height and width were increased during MgSO4 infusion, consistent with increased stroke volume. MgSO4 did not alter the fetal neurophysiological or nuchal electromyographic responses to asphyxia. These data demonstrate that a clinically comparable dose of MgSO4 increased FBF and stroke volume without impairing mean arterial pressure (MAP) or carotid blood flow (CaBF) during and immediately after profound asphyxia. Thus, MgSO4 may increase perfusion of peripheral vascular beds during adverse perinatal events.

Sympathetic neural activation does not mediate heart rate variability during repeated brief umbilical cord occlusions in near-term fetal sheep

Changes in fetal heart rate variability (FHRV) and ST segment elevation (measured as the T/QRS ratio) are used to evaluate fetal adaptation to labour. The sympathetic nervous system (SNS) is an important contributor to FHRV under healthy normoxic conditions, and is critical for rapid support of blood pressure during brief labour-like asphyxia. However, although it has been assumed that SNS activity contributes to FHRV during labour; this has never been tested, and it is unclear whether the SNS contributes to the rapid increase in T/QRS ratio during brief asphyxia. Thirteen chronically instrumented fetal sheep at 0.85 of gestation received either chemical sympathectomy with 6-hydroxydopamine (6-OHDA; n = 6) or sham treatment (control; n = 7), followed 4-5 days later by 2 min episodes of complete umbilical cord occlusion repeated every 5 min for up to 4 h, or until mean arterial blood pressure fell to <20 mmHg for two successive occlusions. FHRV was decreased before occlusions in the 6-OHDA group (P < 0.05) and 2-4.5 h during recovery after occlusions (P < 0.05) compared to the control group. During each occlusion there was a rapid increase in T/QRS ratio. Between successive occlusions the T/QRS ratio rapidly returned to baseline, and FHRV increased above baseline in both groups (P < 0.05), with no significant effect of sympathectomy on FHRV or T/QRS ratio. In conclusion, these data show that SNS activity does not mediate the increase in FHRV between repeated episodes of brief umbilical cord occlusion or the transient increase in T/QRS ratio during occlusions.

Onset of asphyxial state in nonrespiring interval between cord clamping and ventilation increases hemodynamic lability of birth transition in preterm lambs

Experimentally, a typical ∼2-min cord clamp-to-ventilation interval in preterm lambs is accompanied by increased hemodynamic lability of the birth transition. However, whether this lability is related to development of asphyxia after cord clamping, or can be avoided with a shorter clamp-to-ventilation interval, is unknown. To address these questions, anesthetized preterm fetal lambs (gestation 127 ± 2 days) were instrumented with ductus arteriosus and left pulmonary artery flow probes to obtain right ventricular (RV) output, brachiocephalic trunk and aortic isthmus flow probes to measure left ventricular (LV) output, and aortic trunk catheters for pressure measurement and blood gas analysis. With hemodynamics recorded continuously, fetuses were delivered onto the ewe's abdomen and the cord clamped for 1.5 min before ventilation (n = 8), with aortic sampling at 15, 30, 45, and 60 s, or for 0.5 min, with sampling at 15 s (n = 4). With 1.5-min cord clamping, an asphyxial state (Po2 < 10 mmHg) was evident at ≥45 s, with bradycardia and marked falls in LV and RV outputs (by 60% and 50%, P < 0.001), followed after ventilation onset by tachycardia and LV and RV output surges (4- and 3-fold, P < 0.001). By contrast, heart rate and outputs remained stable after 0.5-min cord clamping, with no postventilation change in heart rate or RV output, and a lesser rise in LV output (22%, P < 0.005). In preterm lambs, rapid development of an asphyxial state within 45 s in the cord clamp-to-ventilation interval increased hemodynamic lability of the birth transition, which was reduced with a shorter (∼0.5 min) cord clamp-to-ventilation interval.