After calculation of water needs and electrolyte needs,children typically receive either 5% dextrose (D5) in ¼ normal saline (NS) plus 20 mEq/L of potassium chloride (KCl)or D5 in ½ NS plus 20 mEq/L of KCl. Children weighing lessthan 10 kg do best with the solution containing ¼ NS (38.5 mEq/L) because of their high water needs per kilogram. In contrast, larger children and adults may receive the solutionwith ½ NS (77 mEq/L). These guidelines assume that thereis no disease process present that would require an adjustment in either the volume or the electrolyte composition ofmaintenance fluids. Children with renal insufficiency maybe hyperkalemic or unable to excrete potassium and maynot tolerate 20 mEq/L of KCl. In children with complicatedpathophysiologic derangements, it may be necessary toadjust the electrolyte composition and rate of maintenancefluids empirically based on electrolyte measurements andassessment of fluid balance.
There are three sources of normal water loss—the components of maintenance water (see Chapter 32): urine (60%), insensible losses from the skin and lungs (35%), and stool (5%) (Table 33-1). Sweating is not insensible and, in contrast to evaporative losses, sweat contains water and electrolytes.
A variety of clinical situations modify normal maintenance water balance (Table 33-2). Evaporative skin water losses can be higher in neonates, especially premature infants who are under radiant warmers or undergoing phototherapy. Burns can result in massive losses of water and electrolytes (see Chapter 44). Fever increases insensible losses. Tachypnea or a tracheostomy increases evaporative losses from the lungs.
The gastrointestinal tract is potentially a source of considerable water and electrolyte losses. A child who has large amounts of gastrointestinal losses should have these losses measured and replaced with an appropriate replacement solution (Table 33-3).
Urine output is normally the largest cause of water loss. Diseases such as renal failure and the syndrome of inappropriate antidiuretic hormone (SIADH) can lead to a decrease in urine volume. Maintenance fluids in a patient with oliguria or anuria produce fluid overload. In contrast, other conditions produce an increase in urine volume; these include the polyuric phase of acute tubular necrosis, diabetes mellitus, and diabetes insipidus. When the urine output is excessive, the patient must receive more than standard maintenance fluids to prevent dehydration.
The approach to decreased or increased urine output is similar (Table 33-4). Insensible losses are replaced by a solution that is administered at a rate one third of the normal maintenance rate. Placing the anuric child on “insensibles” theoretically maintains an even fluid balance, with the caveat that one third of maintenance fluid is only an estimate of insensible losses. This rate may need to be adjusted based on monitoring of the patient’s weight and hydration status. An oliguric child needs to receive a urine replacement solution. Most children with polyuria (except for children with diabetes mellitus [see Chapter 171]) should be placed on insensible fluids plus urine replacement.
Sodium: 55 mEq/L Solution: 5% dextrose in ¼ normal saline + 20 mEq/L sodium bicarbonate + 20 mEq/L potassium chloride Replace stool mL/mL every 1–6 hr
Potassium: 25 mEq/L
Bicarbonate: 15 mEq/L
Sodium: 60 mEq/L Solution: normal saline + 10 mEq/L potassium chloride Replace output mL/mL every 1–6 hr
Potassium: 10 mEq/L
Chloride: 90 mEq/L
Place the patient on insensible Place the patient on insensible fluids (⅓ maintenance) fluids (⅓ maintenance)
Replace urine output mL/mL Measure urine electrolytes with half normal saline