Overhydration and Fluid Volume Excess

Fluid volume excess is an expansion of extracellular fluid volume. The added fluid may collect in the vascular space, the interstitium, or both. Edema appears when Starling forces no longer favor keeping fluid inside the capillaries: capillary hydrostatic pressure rises, plasma oncotic pressure falls with low albumin, capillary permeability increases, lymphatic drainage is obstructed, or several of these mechanisms occur together. This is not the same as simply drinking too much water. An isotonic sodium-and-water gain expands circulating and interstitial volume while the serum sodium may remain normal, producing weight gain, jugular venous distention, and edema. Free-water excess adds proportionally more water than sodium and dilutes serum sodium. Heart failure, cirrhosis, and advanced kidney disease commonly produce hypervolemic hyponatremia: total body sodium and water are both increased, but water increases more. The underlying disease determines why the excess persists. Reduced cardiac output activates the renin-angiotensin-aldosterone system and antidiuretic hormone, causing the kidneys to retain sodium and water even though the body is already congested. Kidney disease limits excretion directly. Cirrhosis promotes portal hypertension, low albumin, and sodium retention. NSAIDs can...