Hemodynamic responses of the sheep fetus to vasopressin infusion.

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Abstract
The effects of i.v. vasopressin on the circulation of the fetus were studied in lambs in utero with chronically maintained intravascular catheters. Vasopressin infused in doses of 0.91-2.26 mU(unit)/min per kg fetal weight resulted in plasma levels of arginine vasopressin of 6.8-36.4 .mu.U/ml; these levels are similar to those achieved during fetal hypoxia. Fetal mean arterial blood pressure increased from control levels of 47 .+-. 1.7-56 .+-. 1.9 mm Hg, and heart rate fell from 174 .+-. 4.1 to 144 .+-. 4.4 beats/min. Fetal cardiac output and its distribution and actual organ blood flows were measured before and during vasopressin infusion by the radionuclide-labeled microsphere technique. Combined ventricular output did not change significantly, but there was a redistribution of flow, with a marked reduction of the proportion of cardiac output to the gastrointestinal and peripheral circulations and an increase in the percent of cardiac output to the umbilical-placental, myocardial and cerebral circulations. This redistribution was associated with a significant increase in fetal arterial O2 partial pressure (PO2) from 22 to 24 torr. Changes in heart rate, cardiac output, and distribution of cardiac output to various fetal organs are similar to those seen during fetal hypoxia and suggest that vasopressin release may play an important role in the fetal cardiovascular response to stress.