Overhydration and Fluid Volume Excess

Fluid volume excess is expansion of the extracellular fluid compartment caused by retaining more sodium and water than the body can excrete. Impaired renal excretion is a central mechanism, particularly with chronic kidney disease or acute kidney injury, but heart failure, excessive intravenous fluid, and high sodium intake can add to the burden. Retained sodium holds water in the extracellular space. Intravascular volume rises, venous and hydrostatic pressures increase, and fluid moves into dependent tissues, producing peripheral edema. When pressure rises in the pulmonary circulation, fluid enters the lung interstitium and alveoli; lung compliance falls and oxygen diffusion becomes more difficult. The same process explains weight gain, swollen ankles, jugular venous distension, crackles, and worsening breathlessness. “Overhydration” does not necessarily mean the patient drank too much. A patient may receive an ordinary amount of fluid but still become overloaded when the kidneys cannot clear it. Serum sodium may be low, normal, or high depending on the relative amounts of sodium and water retained, so a normal sodium result does not exclude fluid volume excess. Fluid excess is also not the...