Edema is the common term associated with fluid overload found in the interstitial or lung tissue. When an overabundance of fluid occurs in the intravascular compartment, the condition is known as hypervolemia. The cause for overhydration may be associated with excess sodium.

Situations exist, however, when sodium and water remain in equal proportions with each other. This type of fluid overload, known as isotonic fluid volume excess, results from a decreased elimination of sodium and water. Normally, the body compensates and attempts to restore the fluid balance through the homeostatic mechanisms associated with antidiuretic hormone, atrial natriuretic peptide, or aldosterone. If these mechanisms malfunction due to diseases or failing organs, hypervolemia develops. The extra volume of fluid puts an excessive strain on the left
side of the heart, which over time causes the heart to fail, allowing blood to back up into the pulmonary system and causing pulmonary edema. In other areas of the body dependent edema appears first in the sacrum and lower extremities and then becomes generalized throughout the body, a condition known as anasarca (Table 9-1).