Myometrial contracture-related increases in plasma adrenocorticotropin in fetal sheep in the last third of gestation are abolished by maintaining fetal normoxemia.

Low amplitude, long-lasting epochs of myometrial activity, contractures, occur throughout the majority of pregnancy in all species studied to date. Contractures are associated with a fall in fetal oxygenation and changes in fetal behavioral state. In the present study we observed that contractures produced by the administration of 70 mU oxytocin iv to the pregnant ewe at 125-139 days gestational age (term 145-150 days) result in a fall in fetal carotid arterial PO2 of approximately 2.5 mm Hg and are followed by a rise in fetal carotid arterial plasma ACTH of 16.3 +/- 9.6 pg.ml-1 (mean +/- SEM). When the contracture-induced fall in fetal arterial PO2 was prevented by administration of oxygen to the ewe, fetal ACTH did not rise after the contracture. In conclusion, these observations demonstrate that the relatively small fall in fetal PO2 that accompanies a contracture can be sensed by the fetus and is an essential part of the stimulus to the increased secretion of fetal ACTH that accompanies a contracture. These findings support the view that myometrial activity is one of the factors that influence fetal ACTH secretion.

Nuchal muscle activity at different levels of hypoxemia in fetal sheep.

The effects of hypoxemia on nuchal muscle activity, a parameter of gross body movements, were studied during 12 experiments in five chronically instrumented fetal lambs. Hypoxemia was accomplished by tracheal infusion of nitrogen and did not result in significant changes in fetal arterial pH and PCO2. Fetal arterial blood gases and pH were continuously measured with an extracorporeal flow-through cuvette. Nuchal muscle activity was measured by electromyography and was expressed as the percentage of time spent moving. A decrease in fetal arterial oxygen saturation to three fourths of the initial value for 2 hours was not associated with significant changes in nuchal muscle activity. When fetal arterial oxygen saturation was further reduced for 2 hours to one half, fetal nuchal muscle activity fell significantly from 34.6% +/- 10.5% during normoxemia to 20.9% +/- 13.9% during hypoxemia. However, the most pronounced changes in fetal nuchal muscle activity occurred in the first 15 minutes after the changes in arterial oxygen saturation. A considerable transient increase in nuchal muscle activity was observed during the first 15 minutes after recovery from hypoxemia. During hypoxemia there was a relative predominance of short-lasting (less than 3 minute) movements.

Variability of continuously measured arterial pH and blood gas values in the near term fetal lamb.

In fetal sheep, arterial blood gas values show considerable spontaneous fluctuations. The aim of the present study was to obtain quantitative data on fetal blood gas variability. Accurate assessment of the intraindividual variations can hardly be obtained from intermittent blood samples, but requires continuous measurement. For this purpose we developed a small extracorporeal flow-through cuvette, containing pH and blood gas electrodes and the fiber optic probe of an oximeter. The cuvette is connected through two catheters to the fetal circulation. Blood from the carotid artery flows through the cuvette at a constant rate of 2 mL/min and is drained into the jugular vein. Continuous recordings for 6 h were obtained from five chronically instrumented sheep fetuses at 134-137-d gestational age. Intraindividual variability of fetal arterial blood pH (pHa), partial pressure of arterial blood CO2 (Paco2) and O2 (Pao2), and saturation of arterial blood (Sao2) is quantitatively described. The individual variation coefficients ranged from 0.1 to 0.4% (pHa), from 1.4 to 7.6% (Paco2), from 2.5 to 5.5% (Pao2), and from 3.6 to 7.0% (Sao2). The 5th to 95 percentile intervals (delta) of the individual blood gas values ranged from 0.03 to 0.09 (pHa), from 0.3 to 1.6 kPa (Paco2), from 0.22 to 0.5 kPa (Pao2), and from 10.5 to 26.0% (Sao2). These findings indicate that considerable fluctuations in fetal arterial blood gases occur during a 6-h period. These fluctuations are associated with uterine contractures and changes in maternal and fetal activity.