Barbara M. Johnston

Transient umbilical cord occlusion causes hippocampal damage in the fetal sheep.

OBJECTIVE The purpose of our study was to examine the neuronal outcome after a standardized period of umbilical cord occlusion. STUDY DESIGN The umbilical cord was clamped for 10 minutes in nine experimental and four control chronically instrumented fetal sheep. Three days later the animals were killed for histologic interpretation. Systemic, electrophysiologic, and neurohistologic effects were compared by analysis of variance. RESULTS Clamping of the cord resulted in transient severe asphyxia, hypotension (24 +/- 5 mm Hg, p < 0.01), bradycardia (72 +/- 14 beats/min, p < 0.001), depressed electroencephalographic activity (-17 +/- 2 dB, p < 0.001), and an increase in cortical impedance. The electroencephalographic activity was depressed for 5 +/- 2 hours in spite of rapid recovery of arterial oxygen content. Neuronal loss was found in the hippocampus. Neither epileptiform electroencephalographic activity nor infarction were observed. Three animals with poor blood gas levels died during the occlusion. CONCLUSION An isolated and brief period of umbilical cord occlusion in utero can cause predominantly hippocampal damage without persistent functional changes in cortical activity and with rapid recovery of other potential indicators of fetal asphyxia.

The effect of cooling on breathing and shivering in unanaesthetized fetal lambs in utero.

The effects of cooling have been studied in chronically instrumented unanaesthetized fetal lambs of 106‐132 days gestation. Cold stimulation of cutaneous thermoreceptors by means of a coil placed around the fetus in the amniotic cavity induced continuous breathing which was associated with high‐voltage electrocortical activity throughout the cooling period. Direct internal cooling by means of an intragastric cooling coil did not alter fetal breathing or sleep‐state patterns. Shivering occurred in response to both external and internal cooling, and this response was present as early as 106 days gestation, well before the appearance of high‐voltage sleep. In older fetuses there was evidence of a reduction in thermoregulatory responses during low‐voltage electrocortical activity, but not of the complete suspension of thermoregulation characteristic of low‐voltage (r.e.m.) sleep seen in mature animals. It is concluded that deep regular breathing associated with high‐voltage electrocortical activity can be induced by cold stimulation of cutaneous thermoreceptors, but not by a reduction in core temperature alone. Furthermore, fetal responses to cooling are present as early as 106 days gestation.

Repeated episodes of umbilical cord occlusion in fetal sheep lead to preferential damage to the striatum and sensitize the heart to further insults

ABSTRACT: The effect of repeated episodes of asphyxia on the fetal cardiovascular system and CNS was examined. The umbilical cord was occluded for 5 min, four times, at 30-min intervals in 11 chronically instrumented fetal sheep (118–126 d). Fetal electro-corticogram (ECoG), cortical impedance, ECG, heart rate, and blood pressure were continuously recorded for 3 d, after which neuronal loss was determined histologically. Each occlusion resulted in fetal hypoxemia and bradycardia accompanied by increased T/QRS ratio. Progressively severe hypotension and lactic acidosis developed during successive occlusions. The ECoG was depressed and cortical impedance increased with each occlusion. During the final occlusion, blood pressure fell to 3.5 ± 1 kPa and heart rate to 93 ± 9 bpm, T/QRS ratio increased to 0.44 ± 0.3, and lactate rose to 7.2 ± 1.2 mM/L. Three animals died from cardiac fibrillation during recirculation after the third or fourth occlusion. After the asphyxial episodes, blood pressure and heart rate returned to normal, and the T wave was inverted for 310 ± 155 min. Lactate returned to baseline within 24 h. The ECoG remained depressed for 90 ± 35 min, and intermittent seizures developed at 3.3 ± 1.4 h after the last occlusion. Neuronal loss was primarily found in the striatum. The extent of neuronal loss correlated with the degree of hypotension, increase in T/QRS ratio, duration of postasphyxial ECoG depression, and number of seizures. These results indicate that transient asphyxial episodes compromise the ability of the heart to tolerate additional insults and further suggest that neuronal loss is a consequence of cardiovascular compromise secondary to asphyxia. Therefore, repetitive asphyxial episodes may lead to intrauterine death or striatal damage in survivors.

Role of the Cerebrovascular and Metabolic Responses in the Delayed Phases of Injury After Transient Cerebral Ischemia in Fetal Sheep

BACKGROUND AND PURPOSE Perinatal hypoxic-ischemic injuries can trigger a cascade of events leading to delayed deterioration and cell death several hours later. The objective of this study was to characterize the cerebral blood flow responses and the changes in extracellular glucose and lactate during the delayed phases of injury and to determine their relationships with the pathophysiological events after hypoxic-ischemic injury. METHODS Two groups of near-term chronically instrumented fetal sheep were subjected to 30 minutes of cerebral hypoperfusion. In the first group, regional cerebral blood flow was measured over the next 24 hours with radiolabeled microspheres. In the second, cortical extracellular glucose and lactate were measured by microdialysis. Parietal electrocorticographic activity and cortical impedance were recorded continuously in both groups, and the extent of neuronal loss was determined histologically at 72 hours after injury. RESULTS Cerebral blood flow was transiently impaired in the cortex during reperfusion, whereas during the delayed phase, there was a marked increase in cerebral blood flow. The severity of cortical neuronal loss was related to the degree of hypoperfusion in the immediate reperfusion period and inversely related to the magnitude of the delayed hyperperfusion. Cortical extracellular lactate was elevated after injury, and both glucose and lactate secondarily increased during the delayed phase of injury. CONCLUSIONS The delayed phase is accompanied by a period of hyperperfusion that may protect marginally viable tissue.

The effects of corticotrophin‐releasing factor and two antagonists on breathing movements in fetal sheep.

1. The respiratory effects of corticotrophin‐releasing factor (CRF) and the CRF antagonists alpha‐helical CRF 9‐41 (alpha hCRF) and [DPhe 12, Nle 21‐38] rCRF (12‐41) (DPhe CRF) have been studied in unanaesthetized fetal lambs of 125‐140 days gestation. 2. CRF when given as a 10 micrograms bolus followed by a 5 micrograms h‐1 infusion into a lateral cerebral ventricle caused prolonged continuous fetal breathing movements which were stimulated in both amplitude and frequency but which did not persist during hypoxia. 3. Lower doses of CRF (20 ng bolus followed by 10 ng h‐1) increased the amplitude but not the frequency of fetal breathing movements which did not become continuous. 4. At higher doses (20 micrograms bolus followed by 10‐15 micrograms h‐1) CRF induced cerebral convulsions which were also associated with fetal breathing movements of increased amplitude and frequency. 5. The CRF antagonists alpha hCRF and DPhe CRF both inhibited fetal breathing movements and induced a prolonged apnoea which was resistant to the stimulatory effects of 5‐6% hypercapnia. 6. We conclude that CRF stimulates breathing movements in the fetal lamb. The finding that administration of the CRF antagonists alone cause apnoea suggests that CRF may have a tonic role in the regulation of fetal breathing movements.

Surface Cooling Rapidly Induces Coordinated Activity in the Upper and Lower Airway Muscles of the Fetal Lamb in Utero

ABSTRACT: Stimulation of cutaneous thermoreceptors may be an important factor in the initiation of continuous breathing at birth. Maintenance of a patent airway is also important in the continuation of effective ventilation after birth, but whereas in the adult the principal pharyngeal dilator is the genioglossus muscle, in the fetus genioglossus phasic activity is not synchronous with that of the fetal diaphragm. To ascertain whether a cold stimulus that initiates continuous breathing would also induce synchronized inspiratory activity in the upper airway muscles of the fetus, we have cooled fetal lambs in utero and examined the response of the genioglossus and alae nasi muscles in relation to diaphragm activity. Deep regular breathing as recorded by diaphragmatic activity and tracheal pressure started within 1–102 s after cooling commenced and coordinated inspiratory activity was seen in the alae nasi within 2–356 s. Genioglossus activity became synchronized with that of the diaphragm within 11–356 s. Cooling was continued for 4 h but although in one fetus continuous breathing with associated inspiratory activity in the genioglossus and alae nasi muscles lasted throughout the 4-h period, in the others continuous breathing was not sustained and adaptation to the cold stimulus occurred after periods ranging from 27–218 min. The relationship between breathing, upper airway muscle activity, and sleep state passed through one or more different phases, including breathing through high voltage electrocortical activity, before the normal fetal pattern of episodic breathing restricted to the low voltage electrocortical state resumed. As breathing reverted to the normal fetal pattern, inspiratory activity was lost in the genioglossus. These observations suggest that afferent input arising from cutaneous thermoreceptors may be an important factor in initiating respiration at birth and in ensuring coordinated activity of the upper and lower airway and reducing the possibility of airway obstruction in the neonate.

The Central Effects of Thyrotropin-Releasing Hormone on the Breathing Movements and Electrocortical Activity of the Fetal Sheep

ABSTRACT. The fetal respiratory and electrocortical effects of thyrotropin-releasing hormone (TRH) administered into the lateral cerebral ventricles, have been investigated in chronically catheterized unanesthetized fetal sheep at 125–140 days of gestation. Stimulatory effects on fetal breathing movements were seen at doses as low as a lug bolus. TRH given as a 5-μg bolus followed by a 10 μg/h infusion for 2 h induced a rapid switch to significantly faster, deeper, and continuous fetal breathing movements, while the electrocorticogram remained episodic. Fetal breathing movements did not stop during hypoxia. TRH given as a 2-μg bolus followed by a 4 μg/h infusion or as a 5-μg bolus followed by a 5 μg/h infusion induced the same stimulation of FBMs, but breathing essentially remained episodic, state related and inhibited by hypoxia. As hypothermia presumably induces a surge in TRH secretion at birth it is possible that TRH has some role in the switch from fetal to postnatal breathing patterns.

The Spatial and Temporal Distribution of Insulin-Like Growth Factor-1 Following Experimental Myocardial Infarction in the Rat

Insulin-like growth factor-1 (IGF-1) is believed to be involved in the repair and adaptation that follow ischemic injury to the myocardium. The aim of this study was to elucidate the role of IGF-1 by defining the changes that occur in its distribution following regional myocardial infarction. The left anterior descending coronary artery was ligated in adult male Wistar rats, and hearts were examined microscopically from 6 hours to 20 days later. IGF-1 was identified histochemically using the avitin-biotin-peroxidase method with a polyclonal antibody to IGF-1 and was quantified by optical density measurements of standard fields in sections of hearts prepared in a single batch. Immunoreactivity was located in the cytoplasm of viable myocytes, vascular smooth-muscle cells, mast cells, leukocytes, endothelial cells, and fibroblasts. The zone of viable myocardium immediately adjacent to infarcts reacted significantly more intensely for IGF-1 than all other regions at all stages, with a maximum optical density (617% higher than sham-operated control myocardium, p < .001) 24 hours after coronary artery ligation. Immunoreactivity in myocardium tissue distant from the infarcts also increased during the first day (382% at 24 hours), but this increase was not statistically significant thereafter. These temporal and spatial changes in the distribution and amount of IGF-1 indicate that this finding is predominantly associated with a localized response to injury by the viable myocytes adjacent to infarcts.