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Fetal circulatory response to oxygen lack

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Abstract

The knowledge on fetal and neonatal circulatory physiology accumulated by basic scientists and clinicians over the years has contributed considerably to the recent decline of perinatal morbidity and mortality. This review will summarize the peculiarities of the fetal circulation, the distribution of organ blood flow during normoxemia, and that during oxygen lack caused by various experimental perturbations. Furthermore, the relation between oxygen delivery and tissue metabolism during oxygen lack as well as evidence to support a new concept will be presented along with the principal cardiovascular mechanisms involved. Finally, blood flow and oxygen delivery to the principal fetal organs will be examined and discussed in relation to organ function. The fetal circulatory response to hypoxemia and asphyxia is a centralization of blood flow in favour of the brain, heart, and adrenals and at the expense of almost all peripheral organs, particularly of the lungs, carcass, skin and scalp. This response is qualitatively similar but quantitatively different under various experimental conditions. However, at the nadir of severe acute asphyxia the circulatory centralization cannot be maintained. Then there is circulatory decentralization, and the fetus will experience severe brain damage if not expire unless immediate resuscitation occurs. Future work in this field will have to concentrate on the important questions, what factors determine this collapse of circulatory compensating mechanisms in the fetus, how does it relate to neuronal damage, and how can the fetal brain be pharmacologically protected against the adverse effects of asphyxia.
... Impairment of the utero-placental circulation and perfusion during the active phase of the 2nd stage of labor (2STG), as a consequence of synergic action of uterine contractions and maternal expulsive efforts, hold the potential of determining fetal hypoxia and acidemia. However, the fetus is relatively able to cope with evolving hypoxic insults by compensation mechanisms [3,4]. ...
... In this phase, there is an increase in the intensity of contractions and greater pressure due to maternal pushing [8]. Moreover, with advancing labor, the fetus employs part of its base's reserves achieving progressively a lower tolerance to hypoxic insults [3,4,9]. Normal fetal capillary-blood pH ranges from 7.25 to 7.45 during labor. ...
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Purpose The aim of the study was to estimate by a survival analysis model the hazard function (HF) for neonatal metabolic acidemia (MA) throughout the 2nd stage of labor (2STG) at the time of occurrence of a terminal bradycardia ≥ 10 min requiring expedited delivery, and the cumulative incidence function (CIF) for MA according with the duration of bradycardia stratified in 10–12 min and > 12 min. Methods Singleton pregnancies experiencing terminal fetal bradycardia requiring expedited delivery in the 2STG at 38 + 0–41 + 3 weeks and delivering in the year 2019, were identified. The presence of MA (pH < 7 and/or BE ≤ − 12 mmol/L) was determined based on the acid–base status in the umbilical artery cord blood. Survival analysis was used to assess the hazard function (HF) and the cumulative incidence function (CIF) for MA occurring after terminal fetal bradycardia, at the 2STG. Results Out of a non-consecutive population of 12,331 pregnancies, there were 52 cases that fit the inclusion criteria. Twenty-four (46.2%) of those develop MA. Abnormal quantitative pH values and the HF for MA correlated with the duration of 2STG at the time of bradycardia onset, but not with bradycardia duration. After 60 min of duration of 2STG, the HF (or instantaneous rate of failure) increased dramatically (from 1.2 to 20 about at 120 min). At paired duration of 2STG, a higher CIF was observed for the terminal bradycardia > 12 min. Conclusion Forty-six percent of term fetuses with terminal bradycardia had MA at birth. Despite the low sensitivity and a non-significant association with quantitative pH values, the duration of terminal bradycardia in the 2STG is associated with a higher CIF for MA.
... These shunts help deliver oxygenated blood from the placenta to the fetal brain and myocardium, with flows partially bypassing the fetal lungs and liver [1]. Fetal pathologies, such as fetal growth restriction and congenital heart disease, disrupt this distribution, causing injury to critical fetal organs and increasing risk of fetal mortality [2], [3]. Accurate measurement of this flow distribution is important because it may identify impairment of organs and thereby help to guide appropriate therapy and monitor efficacy. ...
Article
Fetal development relies on a complex circulatory network. Accurate assessment of flow distribution is important for understanding pathologies and potential therapies. In this paper, we demonstrate a method for volumetric imaging of fetal flow with magnetic resonance imaging (MRI). Fetal MRI faces challenges: small vascular structures, unpredictable motion, and inadequate traditional cardiac gating methods. Here, orthogonal multislice stacks are acquired with accelerated multidimensional radial phase contrast (PC) MRI. Slices are reconstructed into flow sensitive time-series images with motion correction and image-based cardiac gating. They are then combined into a dynamic volume using slice-to-volume reconstruction (SVR) while resolving interslice spatiotemporal coregistration. Compared to prior methods, this approach achieves higher spatiotemporal resolution (1x1x1 mm3, ~30 ms) with reduced scan time - important features for the quantification of flow through small fetal structures. Validation is demonstrated in adults by comparing SVR with 4D radial PCMRI (flow bias and limits of agreement: -1.1 ml/s and [-11.8 9.6] ml/s). Feasibility is demonstrated in late gestation fetuses by comparing SVR with 2D Cartesian PCMRI (flow bias and limits of agreement: -0.9 ml/min/kg and [-39.7 37.8] ml/min/kg). With SVR, we demonstrate complex flow pathways (such as parallel flow streams in the proximal inferior vena cava, preferential shunting of blood from the ductus venosus into the left atrium, and blood from the brain leaving the heart through the main pulmonary artery) for the first time in human fetal circulation. This method allows for comprehensive evaluation of the fetal circulation and enables future studies of fetal physiology.
... The depolarised neuronal membrane releases high concentrations of glutamate, which are typically cleared via the glia reuptake pumps during aerobic respiration, establishing an excito-oxidative cascade (Rocha-Ferreira and Hristova, 2016) causing neurotoxicity (Sanders et al., 2010) and mostly necrotic cell death (Rocha-Ferreira and Hristova, 2016). After successful re-oxygenation, a latent recovery phase takes place, where respiration switches back to aerobic and homoeostasis is recovered (Vannucci, 1990;Jensen and Berger, 1991;Gunn et al., 1992;Jensen et al., 1999). Depending on the severity of the HI insult, primary energy failure might not be compensated and would lead to secondary energy failure (Rocha-Ferreira and Hristova, 2016). ...
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Neonatal hypoxic–ischaemic brain damage is a leading cause of child mortality and morbidity, including cerebral palsy, epilepsy, and cognitive disabilities. The majority of neonatal hypoxic–ischaemic cases arise as a result of impaired cerebral perfusion to the foetus attributed to uterine, placental, or umbilical cord compromise prior to or during delivery. Bacterial infection is a factor contributing to the damage and is recorded in more than half of preterm births. Exposure to infection exacerbates neuronal hypoxic–ischaemic damage thus leading to a phenomenon called infection-sensitised hypoxic–ischaemic brain injury. Models of neonatal hypoxia–ischaemia (HI) have been developed in different animals. Both human and animal studies show that the developmental stage and the severity of the HI insult affect the selective regional vulnerability of the brain to damage, as well as the subsequent clinical manifestations. Therapeutic hypothermia (TH) is the only clinically approved treatment for neonatal HI. However, the number of HI infants needed to treat with TH for one to be saved from death or disability at age of 18–22 months, is approximately 6–7, which highlights the need for additional or alternative treatments to replace TH or increase its efficiency. In this review we discuss the mechanisms of HI injury to the immature brain and the new experimental treatments studied for neonatal HI and infection-sensitised neonatal HI.
... It has the capacity to detect changes in the hemodynamics of compromised fetuses [2,3]. In case of hypoxemia, such fetuses are known to redistribute blood to vital organs, i.e. brain, heart and adrenal glands [4,5]. On the arterial side, Doppler velocimetry of the umbilical and middle cerebral arteries is often used to identify this phenomenon [6]. ...
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Background Fetal superior vena cava (SVC) is essentially the single vessel returning blood from the upper body to the heart. With approximately 80-85% of SVC blood flow representing cerebral venous return, its interrogation may provide clinically relevant information about fetal brain circulation. However, normal reference values for fetal SVC Doppler velocities and pulsatility index are lacking. Our aim was to establish longitudinal reference intervals for blood flow velocities and pulsatility index of the SVC during the second half of pregnancy. Methods This was a prospective study of low-risk singleton pregnancies. Serial Doppler examinations were performed approximately every 4 weeks to obtain fetal SVC blood velocity waveforms during 20–41 weeks. Peak systolic (S) velocity, diastolic (D) velocity, time-averaged maximum velocity (TAMxV), time-averaged intensity-weighted mean velocity (TAMeanV), and end-diastolic velocity during atrial contraction (A-velocity) were measured. Pulsatility index for vein (PIV) was calculated. Results SVC blood flow velocities were successfully recorded in the 134 fetuses yielding 510 sets of observations. The velocities increased significantly with advancing gestation: mean S-velocity increased from 24.0 to 39.8 cm/s, D-velocity from 13.0 to 19.0 cm/s, and A-velocity from 4.8 to 7.1 cm/s. Mean TAMxV increased from 12.7 to 23.1 cm/s, and TAMeanV from 6.9 to 11.2 cm/s. The PIV remained stable at 1.5 throughout the second half of pregnancy. Conclusions Longitudinal reference intervals of SVC blood flow velocities and PIV were established for the second half of pregnancy. The SVC velocities increased with advancing gestation, while the PIV remained stable from 20 weeks to term.
Article
Despite improvements in clinical management, pregnancies complicated by pre-existing diabetes mellitus, gestational diabetes mellitus or obesity carry substantial risks for parent and offspring. Some of the endocrine and metabolic changes in parent and fetus in diabetes mellitus and obesity lead to fetal oxygen deficit, mostly due to insulin-induced accelerated fetal metabolism. The human fetus deals with reduced oxygenation through a wide range of adaptive responses that act at various levels in the placenta as well as the fetus. These responses ensure adequate oxygen delivery to the fetus, increase the oxygen transport capacity of fetal blood and redistribute oxygen-rich blood to vital organs such as the brain and heart. The liver has a central role in adapting to reduced oxygenation by increasing its oxygen extraction and stimulating erythropoietin synthesis to increase haematocrit. The type of adaptive response depends on the onset and duration of hypoxia and the severity of the metabolic disturbance. In pregnancies characterized by diabetes mellitus or obesity, these adaptive systems come under additional strain owing to the increased maternal supply of glucose and resultant fetal hyperinsulinaemia, both of which stimulate oxidative metabolism. In the rare situation that the adaptive responses are overwhelmed, stillbirth can ensue.
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The antioxidant defense system is involved in the pathogenesis of neonatal hypoxic-ischemic encephalopathy (HIE). To analyze the relationship between first serum blood glucose levels and outcomes in neonatal HIE, seventy-four patients were divided, based on the first glucose level, into group 1 (>0 mg/dL and <60 mg/dL, n =11), group 2 (≥60 mg/dL and <150 mg/dL, n = 49), and group 3 (≥150 mg/dL, n = 14). Abnormal glucose levels had poor outcomes among three groups in terms of the clinical stage (p = 0.001), brain parenchymal lesion (p = 0.004), and neurodevelopmental outcomes (p = 0.029). Hearing impairment was more common in group 3 than in group 1 (p = 0.062) and group 2 (p = 0.010). The MRI findings of group 3 exhibited more thalamus and basal ganglion lesions than those of group 1 (p = 0.012). The glucose level was significantly correlated with clinical staging (p< 0.001), parenchymal brain lesions (p = 0.044), hearing impairment (p = 0.003), and neurodevelopmental outcomes (p = 0.005) by Pearson’s test. The first blood glucose level in neonatal HIE is an important biomarker for clinical staging, MRI findings, as well as hearing and neurodevelopment outcomes. Hyperglycemic patients had a higher odds ratio for thalamus, basal ganglia, and brain stem lesions than hypoglycemic patients with white matter and focal ischemic injury. Hyperglycemia can be due to prolonged or intermittent hypoxia and can be associated with poor outcomes.
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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.
Article
Umbilical cord rupture (UCR) in utero is a very rare and critical emergency that can cause fetal death within minutes. A 38-year-old nulliparous woman was admitted at 39 weeks in labour. Sudden watery vaginal discharge and bleeding with a rapid drop in the fetal heart rate to 60 beats/min necessitated an emergency caesarean section. A male infant weighing 2632 g was delivered 21 min after the onset of bradycardia; Apgar scores were 0 and 1 at 1 and 5 min, respectively. He was extremely pale; the umbilical arterial blood pH was 6.89 and haemoglobin was 9.0 g/dL. The umbilical cord had a velamentous insertion and was lacerated, with haemorrhage in the outer layer of an umbilical artery close to the placental end. The presentation was typical of UCR: vaginal bleeding following the rupture of membranes. Prompt diagnosis of UCR and termination of pregnancy are essential for fetal survival.
Article
Zusammenfassung Neugeborene sind mit einer Reihe natürlicher Anpassungsmechanismen ausgestattet, die sie trotz ihres (körpergrößenabhängig) hohen Energiebedarfs vor Mangelversorgung schützen. Hierzu gehört der aus einer bradykarden Kreislaufzentralisation mit begleitender Apnoe bestehende, von wasserlebenden Säugetieren bekannte Tauchreflex, der einen sparsamen Umgang mit den O2-Reserven gewährleistet und sich in einer nachlaufenden Laktateinschwemmung aus der Körperperipherie äußert. Metabolisch verhalten sich Säugetierfeten „wie ein Organ der Mutter“ und zeigen damit eine Winterschlaf-artige Abweichung von der üblichen Körpergröße-Energieumsatz-Beziehung, durch die sie an das limitierte intrauterine O2-/Substratangebot angepasst sind. Im Falle einer Mangelversorgung können sie ihren Energiebedarf weiter drosseln, indem sie auf Wachstum verzichten, wobei der Plazenta eine Gatekeeper-Funktion zukommt. Ein postnataler O2-Mangel hat nicht nur eine Suppression der zitterfreien Thermogenese, sondern auch einen hypoxischen Hypometabolismus zur Folge, wie er sonst nur von poikilothermen Tierarten bekannt ist. Nach prolongierter Apnoe setzen Schnappatemzüge ein, die durch kurze pO2-Anstiege eine rudimentäre Herzaktion aufrechterhalten (Selbstwiederbelebung). Insgesamt verzögern diese Mechanismen ein kritisches O2-Defizit und bedingen so eher eine „Resistenz“ als eine „Toleranz“ gegenüber einer Hypoxie. Da sie auf einer (aktiven) Drosselung des Energiebedarfs beruhen, sind sie nicht leicht von dem (passiven) Zusammenbruch des Stoffwechsels aufgrund einer Hypoxie zu unterscheiden.
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Introduction: Cerebroplacental ratio (CPR) and umbilicocerebral ratio (UCR) are clinically used as a measure of fetal brain sparing. These are calculated as the ratios between the pulsatility indices (PIs) of middle cerebral (MCA) and umbilical (UA) arteries, and are an indirect representation of the balance between cerebral and placental perfusion. Volume blood flow (Q)-based ratios, ie Q-CPR or Q-UCR, would directly reflect the distribution of fetal cardiac output to the placenta and brain. Thus, we aimed to determine the development pattern of Q-CPR and Q-UCR during the second half of pregnancy, construct reference intervals, and evaluate their association with CPR and UCR. Material and methods: In a longitudinal cohort study of low-risk pregnancies, the inner diameter of the fetal superior vena cava (SVC) and umbilical vein (UV) was measured and velocity waveforms were obtained from the MCA, UA, UV and SVC using ultrasound at approximately 4-weekly intervals from 20 to 41 weeks. The CPR was calculated as PIMCA/PIUA and the inverse ratio was the UCR. Cerebral and placental blood flows were estimated as the product of mean velocity and cross-sectional area of the SVC and UV, respectively. Q-CPR was calculated as QSVC/QUV and the inverse as the Q-UCR. Gestational age-specific reference intervals were calculated and associations between variables were tested using multilevel regression modeling. Results: Longitudinal reference intervals of Q-CPR and Q-UCR were established based on 471 paired measurements of QSVC and QUV obtained serially from 134 singleton pregnancies. The mean Q-CPR increased from 0.4 to 0.8 during the second half of pregnancy and Q-UCR declined from 2.5 to 1.3, while the CPR and UCR had U-shaped curves but in opposite directions. No significant correlation was found between CPR and Q-CPR (R = 0.10; P = .051), or UCR and Q-UCR (R = 0.09; P = .11), and the agreement between PI-based and Q-based indices of fetal brain sparing was poor. Conclusions: Indices of fetal brain sparing based on placental and cerebral volume blood flow differ from those calculated from UA and MCA PIs. They correlated poorly with conventional CPR and UCR, indicating that they may provide additional/different physiological information. Reference values of Q-CPR and Q-UCR established here can be useful to investigate their clinical value further.
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