Bryan S. Richardson

Cerebral oxidative metabolism in the fetal lamb: Relationship to electrocortical state

Cerebral oxidative metabolism and blood flow were measured in 14 chronically prepared fetal lambs with changes in fetal electrocortical activity. Myocardial blood flow was also measured with changes in fetal state in order to determine whether metabolic changes observed were organ specific. Samples of preductal arterial and sagittal vein blood were analyzed for oxygen content, blood gases, and pH. Blood flow was measured with a radioactive microsphere technique. Cerebral oxidative metabolism increased significantly from 126 +/- 7 mumol/100 gm/min during the high-voltage electrocortical state to 152 +/- 7 mumol/100 gm-1/min-1 (p less than 0.05) during the low-voltage electrocortical state. The increase in cerebral oxidative metabolism was sustained by an increase in blood flow, 148 +/- 7 ml/100 gm/min to 173 +/- 10 ml/100 gm/min (p less than 0.01), whereas the arterial venous oxygen difference remained unchanged. Changes in myocardial blood flow were in the opposite direction, with a decrease noted during the low-voltage electrocortical state, and were correlated with changes in fetal heart rate. We conclude that cerebral oxidative metabolism is increased during the fetal low-voltage electrocortical state and suggests an important role for the increased incidence of this state during the accelerated growth and development of the brain during the perinatal period.

Circulatory responses to prolonged hypoxemia in fetal sheep

Abstract Experiments were conducted in 11 chronically catheterized pregnant sheep to determine the distribution of blood flow within the fetus during prolonged (48 hours) hypoxemia secondary to the restriction of uterine blood flow. Uterine blood flow was mechanically restricted with a polytetrafluoroethylene vascular clamp placed around the maternal common internal iliac artery such that mean (± SEM) fetal arterial oxygen tension decreased from 23.4 ± 1.9 to 17.3 ± 0.8 mm Hg at 1 hour of hypoxemia and remained low for 48 hours. There was an initial increase in fetal arterial carbon dioxide pressure from 48.5 ± 0.9 mm Hg during the control period to 56.2 ± 2.3 mm Hg at 1 hour; this parameter subsequently returned to control values, whereas base excess showed a transient decrease. Fetal cerebral, myocardial, and adrenal blood flows were significantly increased at 1, 24, and 48 hours of hypoxemia. In contrast, there was no change in nuchal muscle or renal blood flows with hypoxemia of this magnitude. Cotyledonary blood flow increased transiently by 38% at 1 hour of hypoxemia, but was not changed from control at 24 and 48 hours. These experiments demonstrate that the sheep fetus is able to maintain the normal protective circulatory adjustments seen with acute hypoxemia for up to 48 hours in the absence of progressive metabolic acidemia.

Metabolic and Circulatory Adaptations to Chronic Hypoxia in the Fetus

When oxygenation is compromised the fetus is capable of a number of adaptive responses, both protective and potentially pathologic, which can be categorized as those affecting fetal metabolism and those affecting fetal oxygen transport. However, both the extent and the duration of the impairment in oxygenation will bear on these adaptive responses. While fetal O2 extraction is increased when oxygenation is acutely compromised thus maintaining O2 consumption, with chronic hypoxemia there is a decrease in O2 consumption paralleling that in O2 delivery and contributed to by the resultant fall-off in growth and alterations in behavioural activity. While a redistribution of blood flow to vital organs continues to be evident, this will be less pronounced than that seen with acute hypoxemia reflecting diminished hormonal changes, underlying metabolic alterations, and the extent to which fetal blood gases are normalized. Much of this information is based on experimental data using unanesthetized fetal sheep with chronic catheterization; however, clinical outcome data and the use of investigative techniques including ultrasound scanning and cordocentesis have supported the relevance of this experimental data to the human situation.

Human fetal breathing activity during electively induced labor at term

Human fetal breathing movements were measured during the first stage of electively induced labor in 20 healthy term pregnancies. Fetuses made breathing movements 25.6% of the time during a 1 hour control period and breathing decreased significantly to 8.3% during latent-phase labor and further decreased to 0.8% during active labor (P less than 0.001). Patterns of increased fetal breathing activity accompanied by increased gross fetal body movements and increased fetal heart rate variability for periods of 20 to 60 minutes out of every 1.0 to 1.5 hours were observed, and the intermittent patterns of increased body movement and heart rate variability continued throughout the first stage of labor despite the decrease in fetal breathing activity during latent- and active-phase labor. It will be important to account for rest activity patterns when interpreting variability of heart rate during labor. The absence of fetal breathing activity during electively induced labor at term is not a clinical indicator of fetal ill health.

Fetal cerebral, circulatory, and metabolic responses during heart rate decelerations with umbilical cord compression

OBJECTIVE The purpose of this study was to determine the cerebral, circulatory, and metabolic responses of the ovine fetus near term to umbilical cord compression with variable-type fetal heart rate decelerations. STUDY DESIGN Nine fetal sheep, at 0.9 of gestation, were studied before, during, and after umbilical cord occlusion for 1-minute and again after repetitive 1-minute cord occlusions every 5 minutes for 1 hour, with resultant fetal heart rate decelerations of approximately 90 beats/min. Brachiocephalic arterial and sagittal venous blood was analyzed for oxygen content, blood gases and pH, glucose, and lactate. Cerebral and upper body blood flow was measured with the microsphere technique. RESULTS Umbilical cord occlusion with moderate to severe variable-type fetal heart rate deceleration resulted in an immediate drop in arterial PO2 by approximately 7 torr, an increase in PCO2 by approximately 9 torr, and a small but significant increase in lactate levels. Cerebral oxidative metabolism was well maintained but required an increase in fractional oxygen extraction because the variable change in cerebral blood flow was insufficient to maintain oxygen delivery. A redistribution of upper body blood flow was evident, with that to the bran and heart variably maintained or increased whereas that to muscle tissue was markedly decreased. Repetitive umbilical cord occlusion over 1 hour resulted in a significant drop in fetal arterial pH, with the acidemia mixed as PCO2 increased approximately 6 torr, whereas lactate levels increased almost fourfold. CONCLUSION Although cerebral oxidative metabolism appears to be well maintained during moderate to severe variable-type fetal heart rate decelerations with umbilical cord occlusion, the need to increase fractional oxygen extraction and the redistribution of blood flow from carcass tissues may contribute to an accumulation of lactic acid both within the brain and systemically when such an insult occurs repeatedly.

Systemic and cerebral inflammatory response to umbilical cord occlusions with worsening acidosis in the ovine fetus.

OBJECTIVE We hypothesized that repetitive umbilical cord occlusions (UCOs) with worsening acidosis will lead to a fetal inflammatory response. STUDY DESIGN Chronically instrumented fetal sheep underwent a series of UCOs until fetal arterial pH decreased to <7.00. Maternal and fetal blood samples were taken for blood gases/pH and plasma interleukin (IL)-1B and IL-6 levels. Animals were euthanized at 24 hours of recovery with brain tissue processed for subsequent measurement of microglia and mast cell counts. RESULTS Repetitive UCOs resulted in a severe degree of fetal acidemia. Fetal plasma IL-1B values were increased approximately 2-fold when measured at maximal fetal acidosis and again at 1-2 hours of recovery. Fetal microglia cells were increased approximately 2-fold in the white matter and hippocampus, while mast cells were increased approximately 2-fold in the choroid plexus and now evident in the thalamus when analyzed at 24 hours recovery. CONCLUSION Repetitive UCOs leading to severe acidemia in the ovine fetus near term will result in an inflammatory response both systemically and locally within the brain.

Carotid arterial blood flow in the ovine fetus as a continuous measure of cerebral blood flow

OBJECTIVE The purpose of this study was to establish a continuous measure of cerebral blood flow in the ovine fetus with a transit time flow probe. METHODS Seven chronically catheterized fetal sheep were studied near term with placement of a 3R Transonic flow probe on the external carotid artery, just proximal to the internal maxillary artery. Blood flow changes were induced by altering maternal inspired oxygen and carbon dioxide concentrations, with cerebral and extracerebral blood flows also determined by the microsphere technique. RESULTS Although absolute carotid and cerebral blood flow values were only modestly related under the present study conditions (r = 0.60, P < .05), both the percent change and the actual change in carotid arterial blood flow showed a strong linear correlation with that for cerebral blood flow (r = 0.84, P < .01, and r = 0.72, P < .02) but less so with that for extracerebral blood flow (r = 0.44 and r = 0.28). CONCLUSION Measurements of carotid blood flow as studied with a transit time flow probe and induced blood gas changes can provide for a continuous assessment of changes in blood flow to the ovine fetal brain.

Apoptosis in the preterm and near term ovine fetal brain and the effect of intermittent umbilical cord occlusion

Programmed cell death or apoptosis plays a central role during the development of the brain, but can also be activated by hypoxic/ischemic insult. The purpose of the present study was to determine the regional distribution of apoptotic cells in the preterm and near term ovine fetal brain and thus in relation to the maturation of neurobehavioural activity, and the effect of intermittent umbilical cord occlusion (UCO), which might then contribute to adverse neurodevelopment. Fetal sheep (control and experimental groups at 0.75 and 0.90 of gestation) were studied over 4 days with UCOs performed in the experimental group animals by complete inflation of an occluder cuff for 90 s every 30 min for 3 to 5 h each day. Animals were then euthanized and the fetal brain perfusion-fixed and prepared for subsequent histology and apoptosis staining using the TUNEL assay method. The number of TUNEL positive cells for both the preterm and near term control group animals was low but with a significant regional hierarchy whereby values were higher in the cerebellar peduncle and cortex and lower in the cortical grey and white matter, hippocampus, and pons. While the apoptotic indices (expressed as TUNEL positive cells/1000 cells or high powered field) for most brain regions were not significantly changed between the preterm and near term control group animals, that for the hippocampus and pons were increased approximately 5- and 4-fold, respectively, (both P<0.05), in the near term animals. Intermittent UCO with severe but limited hypoxemia and no cumulative acidosis to ensure longer term survival, had no significant effect on apoptotic indices in the brains of either the preterm or near term animals, although hippocampal values for both occlusion groups were increased approximately 2-3-fold. Levels of apoptosis noted for the ovine fetal brain at 0.75 to 0.90 of gestation are thus low and likely approaching the basal levels of later life, but there are regional differences and changes over this period although little change in response to intermittent cord occlusion as studied, with implications for behavioural state activity and antenatal hypoxic insults in the brains development.

Central chemical regulation of breathing movements in fetal lambs

In 12 chronically prepared fetal lambs between 126 and 136 days of gestation, 17 ventriculocisternal perfusions (123 microliter/min) were performed. The concentration of bicarbonate ([HCO3-]) in the mock cerebrospinal fluid (CSF) perfusate was altered to manipulate the [HCO3-] in the cerebral ventricles. These perfusions did not systematically alter fetal arterial PCO2, PO2, pH, heart rate or mean blood pressure. Fetal breathing movements (FBM) were noted as rhythmic negative intrathoracic pressures with reference to amniotic fluid pressure. The incidence of FBM was determined and expressed as the percent of the 3 h observation period during which breathing movements were present. Perfusions with mock CSF having an approximately normal [HCO3-], resulted in cisternal [HCO3-] of 23.9 +/- 0.8 (SEM) meq/L and an incidence of fetal breathing movements (FBM) of 22.1 +/- 10.0%. Perfusions with an elevated [HCO3-] in the mock CSF increased the cisternal [HCO3-] to 30.7 +/- 0.4 meq/L and lowered the incidence of FBM to 0.6 +/- 0.2%. Perfusions with a lowered [HCO3-] mock CSF decreased the cisternal [HCO3-] to 17.3 +/- 0.8 meq/L and increased the incidence of FBM to 45.7 +/- 6.8%. These perfusions did not alter the relative incidence of low voltage (LV) versus high voltage (HV) electrocortical (ECOG) state. In 4 animals, low [HCO3-] perfusions induced FBM in the normally apneic HV ECOG state. We conclude that in the near-term fetal lamb, central chemoreceptors can modulate the incidence of rhythmic respiratory efforts, that tonic afferent stimuli arising from these receptors are critical for the generation of spontaneous FBMs and that central acidosis is capable of evoking FBMs in the normally apneic HV ECOG state.

Chronic intrauterine hypoxia interferes with aortic development in the late gestation ovine fetus

Non‐technical summary  It is now known that adverse events in the womb leading to fetal growth impairment increase the risk of cardiovascular disease (CVD) in adulthood. We show perturbed arterial development in sheep fetuses subjected to oxygen deprivation by placental dysfunction, which accounts for the majority of fetal growth restriction in developed countries. The altered structure and composition of the vascular wall exhibited by growth restricted sheep fetuses are akin to those changes present in the preclinical stage of CVD. Therefore, we reveal a plausible mechanism of CVD susceptibility in individuals growth restricted by placental dysfunction. In addition, we identified molecular factors involved in aberrant arterial formation and thus highlight the need for further investigation into these molecular pathways and possible prenatal interventions such as antioxidants.