Fluid Volume Deficit

Fluid volume deficit is a loss of water and electrolytes from the extracellular fluid compartment that becomes clinically significant when circulating volume falls. The result is hypovolemia: less venous return to the heart, lower stroke volume, and potentially inadequate tissue perfusion. Common causes include vomiting, diarrhea, fever, diaphoresis, hemorrhage, burns, diuretic therapy, osmotic diuresis, renal salt loss, and movement of fluid into a third space such as the peritoneum. The bedside question is not simply, “Does this patient look dry?” It is, “Is the circulating volume adequate for the organs right now, and is it getting better or worse?” A patient can have a normal blood pressure early in the process while compensatory tachycardia and vasoconstriction preserve pressure. Conversely, edema does not exclude intravascular depletion when fluid has shifted into tissues or a third space. Dehydration is often used interchangeably with fluid volume deficit, but the terms are not identical. Dehydration more specifically describes loss of water relative to solute; fluid volume deficit includes loss of isotonic or hypotonic extracellular fluid. The sodium concentration may therefore be high, low, or...