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Intravenous fluids for the hospitalized patient can be divided into two categories: resuscitation fluid and maintenance fluid. Resuscitation fluid is for hypovolemic patients to correct volume loss (see Chapter 12-12). Maintenance fluid is for a euvolemic patient who is unable to ingest enough electrolytes and water to keep up with ongoing losses.

Historically 0.45% saline with or without potassium supplementation has been used as a maintenance fluid. Hypotonic fluid replacement, however, has been strongly associated with the development of hyponatremia in hospitalized patients. Isotonic fluids may be acceptable, or even preferable, in most patients requiring maintenance fluids (eTable 21-1). The choice of fluid replacement should be individualized based on serum electrolytes and the rate and type of ongoing fluid losses, which can vary between patients. In most patients, ongoing fluid losses are fairly low. Some patients, however, have massive GI, renal or skin losses, and substantial maintenance fluid rates may be required. Obtaining electrolyte studies on the fluid losses can help predict appropriate repletion (Table 21–18). Adjustments to intravenous fluid replacement should be made based on frequent electrolyte and volume assessments. If intravenous fluids are the only source of water, electrolytes, and calories for longer than approximately 1 week, parenteral nutrition containing amino acids, lipids, trace metals, and vitamins may be indicated (see Chapter 29-28).

eTable 21–1.Electrolyte concentrations in common isotonic crystalloids.
Table 21–18.Approximate electrolyte concentration of common gastrointestinal losses.
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Semler  MW  et al; SMART Investigators and the Pragmatic Critical Care Research Group. Balanced crystalloids versus saline in critically ill adults. N Engl J Med. 2018 Mar 1;378(9):829–39.
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