Objective: Maternal 1-deamino-[8-D-arginine] vasopressin (a selective antidiuretic agonist) and oral water loading decrease maternal and fetal plasma osmolality and markedly increase fetal urine flow in sheep over 24 hours. We hypothesized that a sustained reduction in plasma osmolality would increase the amniotic fluid volume.
Study design: Pregnant sheep (130 +/- 1 days, n = 6) were given oral water loading (2000 ml) and intravenous 1-deamino-[8-D-arginine] vasopressin (20 microgram injection followed by 4 micrograms/hr) to induce maternal urinary antidiuresis and to maintain a maternal plasma osmolality decrease of approximately 20 mOsm/kg for 48 hours. Fetal plasma osmolality, electrolytes, atrial natriuretic factor, and arginine vasopressin levels, fetal swallowing activity, and fetal urine flow and composition were measured each study day, and amniotic fluid volume was measured by tracer dilution before and at 48 hours of plasma hypoosmolality.
Results: In response to 1-deamino-[8-D-arginine] vasopressin and water loading, maternal plasma osmolality decreased (298 +/- 1 to 275 +/- 3 mOsm/kg), mirrored by fetal plasma hypoosmolality (293 +/-1 to 271 +/- 3 mOsm/kg). Fetal urine osmolality (155 +/- 16 to 117 +/- 10 mOsm/kg) and amniotic fluid osmolality significantly decreased (281 +/- 3 to 269 +/- 5 mOsm/kg). Fetal urine flow rate significantly increased (0.33 +/- 0.06 to 0.51 +/- 0.04 ml/min), whereas fetal swallowing activity decreased (56 +/- 7 to 33 +/-5 swallows per hour). In response to alterations in fetal fluid dynamics, amniotic fluid volume significantly increased from 493 +/- 80 to 839 +/- 160 ml.
Conclusion: Chronic maternal 1-deamino-[8-D-arginine vasopressin-induced plasma hypoosmolality induces a sustained reduction in fetal swallowing activity and an increase in fetal urine production, which results in a marked increase in amniotic fluid volume. Use of maternal 1-deamino-[8-D-arginine] vasopressin and oral fluid loading has the potential for manipulation of amniotic fluid volume homeostasis and may prove useful in the treatment of oligohydramnios.