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Arginine Vasopressin-Induced Changes in Blood Flow to the Ovine Chorion, Amnion, and Placenta Across Gestation

William M. Gilbert, MD

Robert A. Brace, PhD

Division of Perinatal Medicine, Department of Reproductive Medicine, University of California at San Diego, La Jolla, California

Objectives: To determine whether physiological increases in fetal plasma arginine vasopressin (AVP) concentration alter blood flow rates to the ovine chorion, amnion or placenta, and to determine whether these AVP-induced changes in flow are dependent on gestational age.

Methods: Colored microspheres (15.10 ± 0.02 µm (standard deviation)) were infused into the superior vena cava before and at 30, 75, and 125 minutes of an intravenous AVP infusion (3 ng/min/kg) in 9 chronically catheterized fetal sheep between 103 to 141 days gestation (term = 147 days). Chorion, amnion, and placental cotyledons were removed, and microspheres were counted to determine blood flow rates.

Results: Fetal arterial pressure (FAP) increased (analysis of variance, P < .0001) and heart rate (FHR) decreased (P < .0001) during the infusion, with responses greater in older (> 125 days) compared to younger (<125 days) fetuses (P < .001). Chorionic blood floow rate increased by 19 ± 7% at 30 min of the AVP infusion, and declined by 18 ± 6% at 75 min, and 32 ± 5% at 125 min (P < .0001). Similarly, fetal placental blood flow rate increased by 20 ± 7% at 30 min of infusion, and declined, in parallel with chorionic blood flow rates, by 6 ± 4% at 75 min, and 17 ± 4% at 125 min (P < .0001). Amniotic blood flow rate did not change significantly during the infusion. The membranous and placental blood flow rate responses to AVP infusion did not depend upon gestational age.

Conclusions: Physiologic increases in plasma AVP concentration induce an early increase in chorionic blood flow followed by a gradual decrease which parallels similar changes in placental blood flow rate. Unlike FAP and FHR, these blood flow changes are not gestational age-dependent. AVP-induced blood flow changes could play an important role in determining abnormalities amniotic fluid volume observed clinically in some stressed fetuses.

Key Words: Membranes • fetus • arginine vasopressin • amniotic fluid volume • intramembranous pathway

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