Neonatal adaptation: greater sympathoadrenal response in preterm than full-term fetal sheep at birth.

A marked increase in sympathoadrenal activity at birth has been described in animals and humans. Studies to determine whether the magnitude and duration of the catecholamine surge at birth in preterm lambs is similar to full-term lambs were undertaken using an acutely exteriorized fetal lamb. To maintain a physiologically stable preparation, all preterm lambs were given natural sheep surfactant intratracheally before the first breath. Base-line catecholamine values were similar in the full-term and preterm lambs. After umbilical cord cutting there was a marked increase in circulating norepinephrine (NE) and epinephrine (E) in both full-term and preterm animals. The preterm animals exhibited a delayed but exaggerated elevation of both NE and E relative to term animals. The peak preterm value for NE (3.8 +/- 1.2 ng/ml) occurred at 60 min and exceeded the peak NE value 1.2 ng/ml observed at 15 min in full-term animals. The peak E concentration in preterm animals was over 9 ng/ml between 2 and 3 h of age, whereas full-term animals reached a peak value of 1.1 ng/ml at 5 min. Heart rate and blood pressure rose abruptly to peak values by 5-15 min in full-term animals. Changes in heart rate and mean arterial pressure were less profound and more gradual in preterm animals. Full-term animals also demonstrated a five-to sevenfold increase in plasma free fatty acids, whereas concentrations in preterm animals increased only two- to threefold. There was a similarly blunted response in blood glucose in preterm animals. The catecholamine surge at birth may be an important adaptive phenomenon with physiological implications.