Disorders of water and sodium balance.

The serum sodium concentration reflects a patient's relative water balance and thus determines if the patient needs more or less water than normal maintenance needs would dictate. The serum sodium concentration has no bearing on whether a patient requires sodium restriction or saline administration. Rather, the sodium/saline balance is determined by clinical assessment of the patient's extracellular fluid volume. Physicians must clearly understand the principles involved when assessing a patient's water and sodium needs. After maintenance water needs and any foreseeable ongoing losses have been factored in, an intelligent plan of fluid management can be initiated. Frequent reassessments, with serial weight measurements, clinical evaluations of extracellular fluid volume status, and determinations of serum sodium concentration, are essential to determine if the desired therapeutic goals are being reached.