Umbilical venous blood flow and its distribution before and during autonomic blockade in fetal lambs

In nine chronically catheterized fetal lambs (120 to 135 days; term 147 days), umbilical venous blood flow and its distribution were measured by the radionuclide-labeled microsphere technique before and during autonomic blockade with atropine (0.20 to 0.25 mg/kg; seven studies) or phentolamine (0.10 to 0.12 mg/kg; six studies). Atropine significantly increased mean fetal heart rate (182 to 207 beats/min), descending aortic blood pressure (49 to 55 mm Hg), and umbilical venous blood flow (210 to 239 ml/min/kg fetus), without changing umbilical venous blood pressure. Phentolamine decreased mean descending aortic pressure (48 to 45 mm Hg), but did not affect heart rate, umbilical venous blood pressure, or umbilical venous blood flow. Neither atropine nor phentolamine altered the distribution of umbilical venous blood flow to the ductus venousus, the liver, or the other fetal organs. These data indicate that the cholinergic nervous system only indirectly affects the basal umbilical venous blood flow in the near-term fetal lamb. This effect is small and is secondary to associated changes in heart rate and arterial blood pressure. Neither the cholinergic nor the α-adrenergic systems influence the basal distribution of umbilical venous blood flow.