Neonatal Adaptation: Sympatho-Adrenal Response to Umbilical Cord Cutting

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Abstract
Summary: The present studies were designed to assess the individual effects of delivery and umbilical cord cutting on the stimulation of the sympatho-adrenal system during parturition. Pregnant ewes with time-dated singleton pregnancies were used in an acutely exteriorized fetal lamb model with an intact umbilical circulation. We observed a minimal, transient elevation in plasma catecholamines (CAT) coincident with the operative procedures and delivery. Subsequent cord clamping was observed to evoke a rapid and marked increase of both norepinephrine and epinephrine (E), maximal at 5 min and persisting over the 4-hr study period. Animals could be grouped on the basis of the observed CAT responses, severity of postpartum acidosis, the extent of free fatty acid (FFA) mobilization and degree of postpartum hypothermia. A blunted FFA response and slower correction of hypothermia were observed in the more acidotic animals despite higher CAT concentrations. One group of four animals had high peak CAT concentrations, 32,000 pg/ml norepinephrine and 35,000 pg/ml E, a deep nadir in pH of 6.88 ± 0.09, a 2-hr delay in maximal FFA mobilization and slower correction of hypothermia. The other group of four animals had peak norepinephrine of 2800 pg/ml and E of 1100 pg/ml, a nadir in pH of 7.09 ± 0.08, maximal plasma FFA concentration by 1 hr after cord cutting and a higher nadir in body temperature 35.7 versus 32.5°C. The results demonstrate that umbilical cord cutting itself is a potent stimulus for fetal CAT release and FFA mobilization. Acidosis is capable of markedly augmenting E release in the mature fetus and obtunding chemical thermogenesis. Speculation: The marked catecholamine surge evoked by umbilical cord cutting at the time of birth appears to be a major factor triggering the metabolic events facilitating adaptation to extrauterine life. The mechanism for the catecholamine release is uncertain. However, cardiovascular and chemoreceptor mediated activation of autonomic nervous system activity probably are involved. The separate contributions of increased postganglionic neurosympathetic activity and adrenal-medullary catecholamine release to neonatal adaptation should be explored further.

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