Hemodynamic Changes in Fetal Lamb in Utero in Response to Asphyxia, Hypoxia, and Hypercapnia

Ketamine Reduces Inflammation Pathways in the Hypothalamus and Hippocampus Following Transient Hypoxia in the Late-Gestation Fetal Sheep

The physiological response to hypoxia in the fetus has been extensively studied with regard to redistribution of fetal combined ventricular output and sparing of oxygen delivery to fetal brain and heart. Previously, we have shown that the fetal brain is capable of mounting changes in gene expression that are consistent with tissue inflammation. The present study was designed to use transcriptomics and systems biology modeling to test the hypothesis that ketamine reduces or prevents the upregulation of inflammation-related pathways in hypothalamus and hippocampus after transient hypoxic hypoxia. Chronically catheterized fetal sheep (122 ± 5 days gestation) were subjected to 30 min hypoxia (relative reduction in P_aO₂∼50%) caused by infusion of nitrogen into the inspired gas of the pregnant ewe. RNA was isolated from fetal hypothalamus and hippocampus collected 24 h after hypoxia, and was analyzed for gene expression using the Agilent 15.5 k ovine microarray. Ketamine, injected 10 min prior to hypoxia, reduced the cerebral immune response activation to the hypoxia in both brain regions. Genes both upregulated by hypoxia and downregulated by ketamine after hypoxia were significantly associated with gene ontology terms and KEGG pathways that are, themselves, associated with the tissue response to exposure to bacteria. We conclude that the results are consistent with interruption of the cellular response to bacteria by ketamine.

Pharmacological models and approaches for pathophysiological conditions associated with hypoxia and oxidative stress

Hypoxia is the failure of oxygenation at the tissue level, where the reduced oxygen delivered is not enough to satisfy tissue demands. Metabolic depression is the physiological adaptation associated with reduced oxygen consumption, which evidently does not cause any harm to organs that are exposed to acute and short hypoxic insults. Oxidative stress (OS) refers to the imbalance between the generation of reactive oxygen species (ROS) and the ability of endogenous antioxidant systems to scavenge ROS, where ROS overwhelms the antioxidant capacity. Oxidative stress plays a crucial role in the pathogenesis of diseases related to hypoxia during intrauterine development and postnatal life. Thus, excessive ROS are implicated in the irreversible damage to cell membranes, DNA, and other cellular structures by oxidizing lipids, proteins, and nucleic acids. Here, we describe several pathophysiological conditions and in vivo and ex vivo models developed for the study of hypoxic and oxidative stress injury. We reviewed existing literature on the responses to hypoxia and oxidative stress of the cardiovascular, renal, reproductive, and central nervous systems, and discussed paradigms of chronic and intermittent hypobaric hypoxia. This systematic review is a critical analysis of the advantages in the application of some experimental strategies and their contributions leading to novel pharmacological therapies.

Oligohydramnios and associated fetal complications

During intrauterine life the fetus is bathed in amniotic fluid which provides a low resistance space for free movement and a buffer against external trauma. This fluid is produced in early pregnancy largely as a maternal dialysate, then as a fetal transudate. Fetal urine is the most important source of amniotic fluid after 16 weeks gestation. The control of amniotic fluid is complex and poorly understood; it arises from secondary partitioning of water within the fetoplacental extracellular space and reflects fetal fluid balance.

Metabolic acidosis decreases fetal myocardial isovolumic velocities in a chronic sheep model of increased placental vascular resistance

We hypothesized that acute fetal metabolic acidosis decreases fetal myocardial motion in a chronic sheep model of increased placental vascular resistance ( Rua). Eleven ewes and fetuses were instrumented at 118–122 days of gestation. After 5 days of recovery and 24 h of placental embolization to increase Rua, longitudinal myocardial velocities of the right and left ventricles and interventricular septum (IVS) were assessed at the level of the atrioventricular valve annuli via tissue Doppler imaging (TDI). Ventricular inflow (E and A waves) and outflow velocities were obtained, and cardiac outputs were calculated. All measurements were performed at baseline and during fetal acidosis caused by epidural anesthesia-induced maternal hypotension, which decreased uterine artery volume blood flow, fetal oxygenation, arterial pH, and base excess and increased lactate. Compared with baseline, the peak isovolumic myocardial contraction and relaxation velocities of the ventricles and IVS, early relaxation velocity (E′) of the ventricles, and systolic velocity of the IVS decreased during metabolic acidosis. The proportion of isovolumic contraction time of the cardiac cycle increased but the isovolumic relaxation and ejection time proportions and the TDI Tei index did not change. The E-to-E′ ratio for both ventricles was higher during metabolic acidosis than at baseline. During metabolic acidosis, right and left ventricular cardiac outputs remained unchanged compared with baseline. In sheep fetuses with increased Rua and acute metabolic acidosis, global cardiac function was preserved. However, acute metabolic acidosis impaired myocardial contractility during the isovolumic phase and relaxation during the isovolumic and early filling phases of the cardiac cycle.

Effects of bupivacaine and calcium antagonists on human uterine arteries in pregnant and non-pregnant women

Bupivacaine is a local anesthetic commonly used in obstetrical practice. Although not generally constrictive, it has a constricting effect on blood vessels in clinically used doses, and when administered close to the uterine vasculature, as in a paracervical blockade, it can induce severe fetal bradycardia and thus be hazardous to the fetus. The bupivacaine-induced vasoconstriction on uterine arteries from pregnant and non-pregnant women was effectively reduced by two different calcium antagonists, verapamil and nifedipine. In non-pregnant women, nifedipine (2.9 x 10(-7) mol.l-1) administered simultaneously with bupivacaine (5.8 x 10(-4) mol.l-1) caused a 96% and verapamil (10(-5) mol.l-1) an 84% reduction as compared with the control vessel where only bupivacaine (5.8 x 10(-4) mol.l-1) was administered. In pregnant women, nifedipine 2.9 x 10(-7) mol.l-1 and 2.9 x 10(-6) mol.l-1 produced 66% and 79% reductions, respectively. It is possible that calcium antagonists administered together with bupivacaine in paracervical blockade could reduce the risk of fetal bradycardia.

The transitional circulation: physiology and anesthetic implications

Over the past decade, several aspects of the physiology of the transitional circulation have been elucidated. The transitional circulation may be viewed as a process divided into four phases. The concept of these phases underscores the fact that the normal transitional circulation should be viewed as an orderly process and not a single event. Within these phases, several mechanisms seem to be involved in the control of pulmonary vascular resistance (PVR). Yet even these mechanisms do not completely explain the process of the normal transition, and very little is understood about why the transition occasionally fails. No doubt other as yet undescribed mechanisms also play a role. Much work remains to be done in the study of the normal and abnormal transitional circulation. The profound hypoxia that characterizes infants with failed transitional circulation from any cause is due to a persistently high PVR, causing right-to-left shunting at the ductal and foramental levels. Clinical care of these infants is based on efforts to simultaneously decrease PVR and increase systemic vascular resistance (SVR). Appropriate measures include the use of supplemental oxygen, hyperventilation, alkalinization, and sedation to decrease PVR and intravenous (IV) fluids and pressors to increase SVR. The rapidly fluctuating nature of the physiologic processes that cause failure of the transitional circulation must be kept in mind when caring for these infants, both during initial stabilization in the delivery room and while administering anesthesia for surgical repair of congenital defects.

Hemodynamic responses to asphyxia in spontaneously breathing newborn term and premature lamb

The comparative effects of asphyxia were studied in spontaneously breathing premature and full term newborn lambs. The premature lambs exhibited significantly higher baseline blood flow to several organs. Asphyxia induced similar changes in regional organ blood flow in preterm and term newborn lambs. Regional differences were found in baseline brain blood flow with an increase from cephalad to caudad. Asphyxia resulted in preferential increases in blood flow to the lower brain structures (cerebellum, medulla, midbrain, and spinal cord). Blood flow to most organs returned to baseline 140 min after recovery from asphyxia.

The effect of fetal heart rate on cardiovascular function during hypoxemia

The effect of heart rate changes on cardiovascular function during hypoxemia was studied in lamb fetuses. In chronic preparations, we determined fetal heart rate, descending aortic blood flow (by electromagnetic flowmeter), PO2, PCO2, and pH and calculated fetal cardiac output (CO) and organ blood flows (with the use of 15 mu nuclide-labeled microspheres). Observations were made during control and hypoxemic states either alone or in the presence of parasympathetic blockade by atropine. Fetuses exposed to moderate hypoxemia (ewe breathing 10% oxygen) developed a significant bradycardia which was prevented by atropine pretreatment; no significant change in CO occurred in either case. Fetuses exposed to severe hypoxemia (8% oxygen) developed bradycardia and a significant decrease in CO; atropine pretreatment prevented the bradycardia but failed to prevent the decrease in CO. Atropine administered to the already severely hypoxemic fetus increased the heart rate but did not restore the depressed CO. The redistribution of blood flow from low-priority (e.g., viscera) to high-priority (e.g., myocardium) tissues correlated with the degree of hypoxemia. Myocardial blood flow was greater during hypoxemia when bradycardia was prevented by atropine than during hypoxemia alone; otherwise, blood flow distribution in response to hypoxemia was unaffected by atropine blockade.

Blood flow to fetal organs as a function of arterial oxygen content

In a sheep preparation the blood flow to fetal organs was studied 3 to 10 days after surgery by means of the microsphere technique over a range of fetal arterial O2 content from 6 to 1 mM. Blood flows to neural tissues (cerebrum, cerebellum, brain stem), heart, and the adrenals increased in inverse relation to arterial O2 content. As a result the arterial supply of O2 to these organs tended to remain constant over the O2 range studied. Blood flow to the fetal lungs decreased progressively with hypoxia. The blood flow to kidneys, digestive tract, pancreas, and carcass had a tendency to remain constant or increase gradually in the transition from high to moderately low levels of arterial O2 content and then to decrease abruptly in more severe hypoxia. Umbilical blood flow did not change systematically in relation to arterial O2 content.

Clinical significance of fetal heart rate patterns during labor. V. Variable decelerations

Among a large series of high-risk patients who had direct electronic monitoring, 330 fetuses presented "variable" decelerations and 598 had no decelerations. Among those with variable decelerations, there were 18 who presented such associated fetal heart rate (FHR) alterations as tachycardia, fixed baseline, and slow recovery ("hypoxic components"). The fetal outcome, as well as the characteristics of FHR patterns, were compared among the three groups. Baseline changes were significantly higher among pure variable decelerations and still higher among those having variable decelerations with hypoxic components. Likewise, one- and five-minute Apgar scores were significantly lower in that order. Only 4 per cent with no decelerations had clinical fetal distress while 23 per cent with variable decelerations and 78 per cent of those with variable and hypoxic components had distress. Neonatal death was higher among groups with variable decelerations. Premature rupture of membranes seemed to be the only clearly different (and probably influencing) clinical condition, being higher among the group with variable decelerations. Variable decelerations ("cord patterns") are associated with a relatively high rate of depressed infants. This problem is much worse when elements characteristic of hypoxia are identified in FHR tracings. The degree of drop in FHR frequency is not among those elements so identified. It is thus possible to prevent significant fetal deterioration by meticulous analysis of FHR tracings and active management when "late" components are identified on variable decelerations.

Evolution of thinking in fetal respiratory physiology

An outstanding characteristic of intrauterine life is the low oxygen pressure (Po2) of fetal blood. It seemed at one time that in order to tolerate the Po2 of the uterine environment the fetus must have a relatively slow rate of oxidative metabolism or depend upon anaerobic pathways of substrate utilization. Subsequent studies have shown that the fetus has a high rate of oxygen consumption and that lactic acid is a substrate rather than a net product of normal fetal metabolism. The circulatory response of the fetus to acute hypoxia is centered on the requirement of maintaining the arterial oxygen flow to the heart and central nervous system without increasing cardiac output. The arterial oxygen flow concept helps in understanding the physiologic meaning of the high oxygen affinity of fetal blood and the boundary conditions of the fetal defense against hypoxia.

The effects of changes of maternal PaO2 and PaCO2 on the fetal PaO2 and PaCO2--in vivo study

Umbilical PaO2 and PaCO2 were continuously monitored in vivo in acute fetal lamb preparations with a semipermeable membrane connected to a mass spectrometer. The response time of this system (0 to 90% of final value) was 36 sec. In seven pregnant sheep (128--135 days gestation) the maternal inspired mixture was abruptly changed and the following changes in fetal PaO2 and PaCO2 were observed: (1) 100% O2 to room air: PaO2 decreased from 21.5 +/- 0.8 (mean +/- SEM) to 14 +/- 1.1 mm Hg at a rate of 1.63 +/- 0.33 mm Hg/min. Following return to 100% O2 the PaO2 returned to 21 +/- 1.1 mm Hg at a rate of 2.44 +/- 0.4 mm Hg/min. (2) 100% O2 to 12% O2/10% CO2: after 6 min the PaO2 fell from 19.3 +/- 1.3 to 6.3 +/- 0.3 mm Hg at a rate of 4.65 mm Hg/min and the PaCO2 rose from 37 +/- 8 to 70 +/-5 mm Hg. At 100% O2 the PaO2 returned to 19 +/- 1.0 mm Hg at a rate of 11.76 +/- 0.086 mm Hg, the PaCO2 to 39 +/- 7 mm Hg. (3) 100% O2 to 90% O2/10% CO2. The PaO2 and PaCO2 increased by 4.7 and 22 mm Hg, respectively. The changes of fetal PaO2 and PaCO2 occurred after 1 minute of changing in maternal inspired mixture except in the transition from 12% O2/10% CO2 to 100% O2 (34 +/- 12 sec). Following the reinstitution of 100% the fetal PaO2 and PaCO2 returned to their previous values within 4 and 16 min, respectively.

Diminished limb blood flow in infants with transposition of the great vessels: An adaptation to chronic hypoxia?

Calf blood flow was measured by venous occlusion plethysmography using a mercury in rubber strain gauge in infants with transposition of the great vessels. (TGV), and in comparable infants free from cardiopulmonary disease. Resting calf blood flow in the infants with TGV was 3.6 +/- 0.8 ml/100 ml/min, while in the control group flow was 6.8 +/- 2.3 ml/100 ml/min, a highly significant difference. We postulate that newborns with TGV decrease their resting calf flow in response to chronic hypoxia.

Acute reduction of uterine blood flow and fetal heart rate changes in pregnant sheep near term

Five merino sheep near term were used for acute preparations to investigate the influence of basal uterine blood flow (UBF), basal fetal oxygenation and basal fetal heart rate (FHR) on FHR changes elicited by acute reduction of UBF. The ewes were anaesthesized with pentobarbital and Alloferin was given for relaxation. Ventilation was maintained via a tracheal tube by a Starling pump. Maternal heart rate, arterial pressure and intrauterine pressure as well as fetal heart rate and umbilical artery pressure were recorded continously. Initially blood flow was recorded by cuff flow meter sequentially in both uterine arteries for a short time in order to determine total UBF and the ratio of flows. During the UBF reduction sequence flow was recorded in the artery of the gravid horn continously and measurement was corrected according to the initial ratio of flows in both arteries, presuming that this ratio would remain constant throughout the course of the experiment. Repetitive and progressive acute reduction of UBF (approximately 25%, 50% and 100% from basal UBF value) was achieved three times, lasting 120 sec each and each approximatly 12 min apart, by flow meter controlled partial or total occlusion of the aorta abdominalis of the ewe with a balloon catheter inserted into the aorta. (In some cases the lateral abdominal section was left open and the aorta was compressed manually). Before, 90 sec after the beginning and 10 min after the end of UBF reduction blood samples were taken from the fetal catheters (Fig. 3). In two preparations this sequence was repeated once after complete recovery of FHR.

Response of human uterine arteries to local anesthetics

The in vitro effects of local anesthetics and norepinephrine upon strips of early gestation and term pregnancy uterine arteries were studied in eight cases. In another case, the effect upon uterine veins was studied with a standard organ bath used to record isometric contractions. Histologic preparations were made to verify the type of vessel studied. An artery obtained from an eight-week gestation did not respond to either lidocaine or mepivacaine. All other arterial specimens (radial and helicoidal strips) responded with slow, rising, strong contractions to diluted concentrations of both of these substances. Likewise they responded with rapid contractions when exposed to norepinephrine. Alpha blockers were unable to prevent the contractions triggered by the local anesthetics. The vein specimens did not respond to local anesthetics but contracted when stimulated by norepinephrine. Based on these observations and after brief review of some hypotheses advanced to explain post-paracervical anesthesia fetal bradycardia, it is postulated that this bradycardia is probably due to uterine artery spasm, causing decreased intervillous space blood flow and fetal hypoxia.

Renal response of the lamb fetus to partial occlusion of the umbilical cord

The role of the fetal kidney during impairment of placental exchange was studied in eight fetal lambs, intact in utero; a standard asphyxial insult for a period of 1 hour was produced by occluding the umbilical cord sufficiently to lower the fetal heart rate by 35 +/- 5 beats/min and the pH by 0.15 units. This asphyxial stress caused a fall in urine output from a control of 0.17 to 0.03 ml/kg/min and of glomerular filtration rate from 1.2 to 0.3 ml/min; release of the occlusion was followed by mild diuresis. The fall in urine output was accompanied by a rise in total primary solutes including concentrations of electrolytes; this rise continued for 1 to 2 hours following the release of the occlusion. Because of the low urinary output during the period of occlusion, there was a fall in excretion of electrolyte; an increase in net acid excretion occurred only after the release of the cord. These experiments show that, although the fetal kidney is capable of contributing to elimination of acid following compression of the cord, there may be an associated net loss of water and electrolytes.