Dehydration and Fluid Volume Deficit

Fluid volume deficit is a mismatch between fluid loss and replacement. The loss may involve salt and water together, as with diarrhea, vomiting, hemorrhage, or diuresis, or it may involve predominantly free water, as with poor intake, fever, or insensible losses. Those patterns are not interchangeable. Isotonic loss mainly lowers extracellular and intravascular volume; predominant free-water loss raises serum tonicity and often produces hypernatremia. The distinction matters because a patient can be profoundly hypovolemic with a nearly normal sodium concentration, while another patient can have severe hypertonicity with less obvious circulatory collapse. A patient with heart failure or cirrhosis may even have edema and excess total-body fluid while the arterial circulation remains underfilled. Treatment therefore begins with the cause, the fluid compartment affected, and the adequacy of tissue perfusion—not with the appearance of the skin alone.