Approach to the Patient with Decreased Extracellular Fluid Volume

As in the patient with an increased ECF volume, a careful history, physical examination, and selected laboratory tests often reveal the cause and extent of ECF volume depletion. Clinical signs and symptoms of total-body sodium deficit are shown in Table 2.4. The history focuses on identification of potential sources of sodium loss. The patient is questioned regarding polydipsia and diuretic use (kidney), diarrhea and vomiting (gastrointestinal tract), and sweating (skin). Physical examination can reveal the extent of ECF volume depletion (postural changes in blood pressure and pulse, degree of hypotension) as well its cause (intestinal obstruction or gastrointestinal fistula). Laboratory tests also aid in determining whether the sodium loss is renal or extrarenal. The presence of a decreased urine sodium concentration, a decreased FENa, concentrated urine, and a BUN-to-creatinine ratio greater than 20:1 suggests that sodium losses are extrarenal and the kidney is responding appropriately. The one exception to this caveat is the patient in whom diuretics were recently discontinued. Even though sodium losses occurred via the kidney, once the diuretic effect has dissipated, the kidneys reabsorb salt and water appropriately in order to restore ECF volume. Conversely, an elevated urine sodium concentration suggests that the kidney is the source of the sodium loss.