Natriuresis and the extracellular volume expansion by hypertonic saline.
BACKGROUND
The mechanisms governing the duration of the extracellular fluid volume (ECF) expansion as a result of intravenous infusion of hypertonic saline solution are poorly understood. We hypothesized that the duration is closely related to the sodium excretion.


METHODS
Six conscious splenectomized ewes with a mean body weight of 30 kg were given an intravenous infusion of 4 ml x kg(-1) of 7.5% saline solution on two occasions, one over a period of 5 min and another over a period of 20 min. Mass balance and volume kinetic calculations of the distribution and elimination of fluid were performed after repeated sampling of the plasma sodium concentration and the urinary excretion of water and sodium during 3 h.


RESULTS
On considering the addition of sodium to and its excretion from the body, the plasma sodium concentration indicated a 10% dilution of the extracellular space. The volume expansion decayed at an average rate of 20% of the volume expansion per hour, which, however, varied greatly in the animals, depending on their capacity to excrete sodium. After 1 h, increasing natriuresis promoted translocation of water into the cells, which amounted to 25-35% of the total elimination. Computer simulations indicated that tripled natriuresis (up to approximately 750 mmol l(-1)) would increase the rate of elimination to 45% of the volume expansion per hour.


CONCLUSIONS
The sodium excretion was inversely proportional to the duration of the extracellular volume expansion by 7.5% saline.
Svensén
Christer H
CH
Department of Anesthesiology, University of Texas Medical Branch, Galveston, Texas 77550-0591, USA. chsvense@utmb.edu
Waldrop
Kimberly S
KS
Edsberg
Lennart
L
Hahn
Robert G
RG
D000818:Animals; D001826:Body Fluids / Q000187:drug effects / Q000502:physiology; D005110:Extracellular Space / Q000187:drug effects* / Q000502:physiology; D005260:Female; D006439:Hemodynamics / Q000187:drug effects* / Q000502:physiology; D007262:Infusions, Intravenous; D023421:Models, Animal; D008962:Models, Theoretical; D009318:Natriuresis / Q000187:drug effects* / Q000502:physiology; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage / Q000493:pharmacokinetics*; D012756:Sheep; D012964:Sodium / Q000097:blood / Q000502:physiology / Q000652:urine; D014882:Water-Electrolyte Balance / Q000187:drug effects / Q000502:physiology
D012462:Saline Solution, Hypertonic; D012964:Sodium