SYMPATHOADRENAL, METABOLIC, AND REGIONAL BLOOD-FLOW RESPONSES TO COLDIN FETAL SHEEP

Because environmental temperature falls when the fetus is delivered fr om the uterus, the role of cold in stimulating cardiovascular and symp athoadrenal responses at the time of birth was examined in fetal lambs . In eight fetuses (gestational age 140 +/- 2 d), catheters were inser ted into hind-limb and neck arteries and veins, and into an umbilical vein. After returning the fetus to the uterus and administering a musc le relaxant (succinylcholine chloride), blood gases, glucose and lacta te concentrations, and plasma catecholamine and atrial natriuretic pep tide concentrations were measured. Fetal combined ventricular output a nd organ blood flows were measured by the radionuclide-labeled microsp here technique. Measurements were repeated after delivering the fetus into a warm water bath (40-degrees-C) and at 15 and 30 min after the b ath temperature was cooled to 25-degrees-C. Fetal cooling stimulated a sympathoadrenal response, which was manifested by an immediate increa se in heart rate and arterial blood pressure, a rise in atrial natriur etic peptide, and a modest increase in norepinephrine concentration. C ontrary to earlier reports that investigated the effect of cold on fet al lambs using an intrauterine cooling coil, we did not find a decreas e in fetal arterial oxygen tensions and pH, an increase in plasma gluc ose and lactate concentrations, or changes in fetal oxygen consumption . Combined ventricular output did not increase with cooling, and blood flow to most organs did not change significantly; however, blood flow to the skin decreased markedly, particularly in the lower body. The d ramatic increases in catecholamine concentrations often noted after bi rth do not appear to be related to surface cooling; it is suggested th at hypoxemia and acidemia occurring at the time of delivery could be r esponsible.