Isotonic dehydration

Surgical patients are most prone to this type of dehydration; water and sodium are lost proportionally, so serum sodium remains within the normal range, and extracellular fluid osmolality is also maintained normal. It causes a rapid reduction in extracellular fluid volume (including circulating blood volume); since the lost fluid is isotonic, it basically does not change the osmolality of the extracellular fluid. Initially, intracellular fluid does not shift to the extracellular fluid compartment to compensate for the reduction in extracellular fluid, so intracellular fluid volume does not change. However, if this fluid loss persists for a longer duration, intracellular fluid will gradually shift outward and be lost together with the extracellular fluid, thereby causing cellular dehydration.