Figure 2. Estimates of body water and the osmotic and sodium volume of distribution in acute hypernatremia.

Shown here are the average estimates of body water (VH₂O) measured by tritiated water dilution, the osmotic volume of distribution (VOsm) calculated by formula 11, and sodium volume of distribution (VNa) calculated by formula 12 in a study of anuric dogs infused with hypertonic saline [26]. The figure makes two points: (a) Even in acute hypertonicity, VOsm is substantially lower than VH₂O and (b) VNa is equal to VH₂O. The equality of VNa and VH₂O provides the basis for applying equations 9 or 10 in the management of dysnatremias.