A unique hypertonic response to hypotonic infusion in the pregnant ewe.
OBJECTIVE
The purpose of this study was to compare the responses of the maternal ewe to intravenous volume expansion with either sufficient lactated Ringer's solution to elevate maternal venous pressure or sufficient hypotonic fluid to reduce blood osmolality.


METHODS
Chronically catheterized pregnant sheep were intravenously infused over 4 hours with either commercial lactated Ringer's solution (5.55 +/- 0.50 L/hr, 255 mOsm/kg, mildly hypotonic) or diluted Ringer's solution (2.04 +/- 0.27 L/hr, 150 mOsm/kg, markedly hypotonic). Data were statistically analyzed with two- and three-factor analyses of variance and bivariate regression analysis.


RESULTS
During the mildly hypotonic infusion (n = 8) the maternal blood osmolality changes were -5.1 +/- 1.2, +2.7 +/- 1.0 and +6.8 +/- 1.1 mOsm/kg at 1 and 4 hours of infusion and 1 hour after the infusion. In four of the eight animals in this group profuse diarrhea developed. During the markedly hypotonic infusion (n = 11) the maternal blood osmolality changes were -9.9 +/- 1.1, -15.9 +/- 2.5, and -10.4 +/- 2.2 mOsm/kg at 1 and 4 hours of infusion and 1 hour after the infusion. Although urine osmolalities were significantly less than the osmolality of the infusate in both groups, only during the mildly hypotonic infusion was there a net loss of free water by the kidneys. The renal free water loss, the venous pressure increase, and the blood osmolality decrease were not significantly different whether diarrhea did or did not develop.


CONCLUSIONS
The infusion of large volumes of mildly hypotonic Ringer's solution to the pregnant ewe produces a paradoxic increase in maternal plasma osmolality as a result of the excretion of large volumes of free water by the kidneys, and if the venous pressure is increased more than about 6 mm Hg with this infusion, diarrhea develops in the animals.
Powers
D R
DR
Department of Reproductive Medicine, University of California, San Diego, La Jolla 92093-0802.
Brace
R A
RA
D000818:Animals; D002319:Cardiovascular System / Q000187:drug effects; D003967:Diarrhea / Q000139:chemically induced; D004064:Digestive System / Q000187:drug effects; D005260:Female; D007038:Hypotonic Solutions / Q000494:pharmacology*; D007262:Infusions, Intravenous; D007552:Isotonic Solutions / Q000494:pharmacology; D009994:Osmolar Concentration; D010952:Plasma Substitutes / Q000494:pharmacology; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000097:blood / Q000378:metabolism* / Q000652:urine; D000077325:Ringer's Lactate; D012756:Sheep; D014690:Venous Pressure / Q000187:drug effects
D007038:Hypotonic Solutions; D007552:Isotonic Solutions; D010952:Plasma Substitutes; D000077325:Ringer's Lactate