Measurement of total body water content in horses, using deuterium oxide dilution.

OBJECTIVE: To measure total body water (TBW) content in horses, using deuterium oxide (D2O) dilution. ANIMALS: Six 8- to 10-year-old healthy untrained mixed-breed horses, weighing (mean +/-SD) 503.4 +/- 64.0 kg. PROCEDURE: After a 12-hour nonfeeding period, 6 horses were given D2O (0.14 g/kg of body weight) via nasogastric tube. Blood samples were collected from a preplaced indwelling jugular vein catheter prior to and 1 to 8, 10, 12, 14, and 24 hours after administration of D2O. Blood samples were centrifuged immediately, and plasma was collected and stored at -70 C until analysis. The D2O content in plasma was measured by zinc reduction to deuterium gas. The resulting gas was measured, using an isotope ratio mass spectrometer. RESULTS: Deuterium oxide was rapidly absorbed from the gastrointestinal tract of all horses, and reached peak (mean +/- SD) plasma concentration (1,454.4 +/- 163 delta D/ml or parts/thousand) 1 hour after administration. Plasma concentration decreased slowly during the next 2 to 3 hours, then remained statistically constant from 2 to 5 hours (early plateau phase) and 3 to 7 hours (late plateau phase) after administration. Mean +/- SEM TBW content was 623.0 +/- 2.2 ml/kg (62.3% of body weight) for the early plateau phase and 630.3 +/- 2.2 ml/kg (63.0% of body weight) for the late plateau phase. CONCLUSION: Deuterium oxide dilution appears to be of value for measurement of TBW content in horses, and has a 4-hour plateau effect. Equilibration of D2O with large intestinal water may be the reason for the prolonged equilibrium time and plateau effect seen in these horses. CLINICAL RELEVANCE: Deuterium oxide appears safe and efficacious for determining TBW content in horses and may be helpful for determining changes in TBW content during exercise and disease.