Maintenance Therapy

Children normally have large variations in their daily intake of water and electrolytes. The only exceptions are patients who receive fixed dietary regimens orally, via a gastric tube, or as intravenous total parenteral nutrition. Healthy children can tolerate significant variations in intake because of the many homeostatic mechanisms that can adjust absorption and excretion of water and electrolytes (Chapter 52). The calculated water and electrolyte needs that form the basis of maintenance therapy are not absolute requirements. Rather, these calculations provide reasonable guidelines for a starting point to estimate intravenous therapy. Children do not need to be started on intravenous fluids simply because their intake is being monitored in a hospital and they are not taking “maintenance fluids” orally, unless there is a pathologic process present that necessitates high fluid intake.

Maintenance fluids are most commonly necessary in preoperative and postoperative surgical patients; many nonsurgical patients also require maintenance fluids. It is important to recognize when it is necessary to begin maintenance fluids. A normal teenager who is given nothing by mouth (NPO) overnight for a morning procedure does not require maintenance fluids because a healthy adolescent can easily tolerate 12 or 18 hr without oral intake. In contrast, a 6 mo old child waiting for surgery should begin receiving intravenous fluids within 8 hr of the last feeding. Infants become dehydrated more quickly than older patients. A child with obligatory high urine output from nephrogenic diabetes insipidus should begin receiving intravenous fluids soon after being classified as NPO.

Maintenance fluids are composed of a solution of water, glucose, sodium, and potassium. This solution has the advantages of simplicity, long shelf life, low cost, and compatibility with peripheral intravenous administration. Such a solution accomplishes the major objectives of maintenance fluids (Table 53-1). Patients lose water, sodium, and potassium in their urine and stool; water is also lost from the skin and lungs. Maintenance fluids replace these losses and therefore avoid the development of dehydration and deficiency of sodium or potassium.

The glucose in maintenance fluids provides approximately 20% of the normal caloric needs of the patient, prevents the development of starvation ketoacidosis, and diminishes the protein degradation that would occur if the patient received no calories. Glucose also provides added osmoles, thus avoiding the administration of hypotonic fluids that may cause hemolysis.

Maintenance fluids do not provide adequate calories, protein, fat, minerals, or vitamins. This fact is typically not problematic for a patient receiving intravenous fluids for a few days. A patient receiving maintenance intravenous fluids is receiving inadequate calories and will lose 0.5-1% of weight each day. It is imperative that patients not remain on maintenance therapy indefinitely; total parental nutrition should be used for children who cannot be fed enterally for more than a few days, especially patients with underlying malnutrition.

Prototypical maintenance fluid therapy does not provide electrolytes such as calcium, phosphorus, magnesium, and bicarbonate. For most patients, this lack is not problematic for a few days, although there are patients who will not tolerate this omission, usually because of excessive losses. A child with renal tubular acidosis wastes bicarbonate in urine. Such a patient will rapidly become acidemic unless bicarbonate (or acetate) is added to the maintenance fluids. It is important to remember the limitations of maintenance fluid therapy.

Maintenance Water

Water is a crucial component of maintenance fluid therapy because of the obligatory daily water losses. These losses are both measurable (urine, stool) and not measurable (insensible losses from the skin and lungs). Failure to replace these losses leads to a child who is thirsty, uncomfortable, and, ultimately, dehydrated.

The goal of maintenance water is to provide enough water to replace these losses. Although urinary losses are approximately 60% of the total, the normal kidney has the ability to markedly modify water losses, with daily urine volume potentially varying by more than a factor of 20. Maintenance water is designed to provide enough water so that the kidney does not need to significantly dilute or concentrate the urine. It also provides a margin of safety, so that normal homeostatic mechanisms can adjust urinary water losses to prevent overhydration and dehydration. This adaptability obviates the need for absolute precision in determining water requirements. This fact is important, given the absence of absolute accuracy in the formulas for calculation of water needs. Table 53-2 provides a system for calculating maintenance water on the basis of the patient’s weight and emphasizes the high water needs of smaller, less mature patients. This approach is reliable, although calculations based on weight do overestimate the water needs of an overweight child, in whom it is better to base the calculations on the lean body weight, which can be estimated by using the 50th percentile of body weight for the child’s height. It is also important to remember that there is an upper limit of 2.4 L/24 hr in adult-sized patients. Intravenous fluids are written as an hourly rate. The formulas in Table 53-3 enable rapid calculation of the rate of maintenance fluids.

Table 53-2 BODY WEIGHT METHOD FOR CALCULATING DAILY MAINTENANCE FLUID VOLUME

* The maximum total fluid per day is normally 2,400 mL.

Intravenous Solutions

The components of the commonly available solutions are shown in Table 53-4. Normal saline (NS) and Ringer lactate (LR) are isotonic solutions; they have approximately the same tonicity as plasma. Isotonic fluids are generally used for the acute correction of intravascular volume depletion (Chapter 54). The usual choices for maintenance fluid therapy in children are half-normal saline () and 0.2NS. These solutions are available with 5% dextrose (D5). In addition, they are available with 20 mEq/L of potassium chloride, 10 mEq/L of potassium chloride, or no potassium. A hospital pharmacy can also prepare custom-made solutions with different concentrations of glucose, sodium, or potassium. In addition, other electrolytes, such as calcium, magnesium, phosphate, acetate, and bicarbonate, can be added to intravenous solutions. Custom-made solutions take time to prepare and are much more expensive than commercial solutions. The use of custom-made solutions is necessary only for patients who have underlying disorders that cause significant electrolyte imbalances. The use of commercial solutions saves both time and expense.

A normal plasma osmolality is 285-295 mOsm/kg. Infusing an intravenous solution peripherally with a much lower osmolality can cause water to move into red blood cells, leading to hemolysis. Thus, intravenous fluids are generally designed to have an osmolality that is either close to 285 or greater (fluids with moderately higher osmolality do not cause problems). Thus, 0.2NS (osmolality = 68) should not be administered peripherally, but D5 0.2NS (osmolality = 346) or D5 + 20 mEq/L KCl (osmolality = 472) can be administered.

There is controversy about the appropriate sodium content of maintenance fluids, considering the suggestion that excessive amounts of hypotonic fluids may cause hyponatremia, which at times may have serious sequelae. One approach to avoid water intoxication is to reduce the rate of infusion of fluids containing 0.2NS or . The other recommends that normal saline be used as the maintenance fluid; most centers have not adopted the routine use of NS as the initiating maintenance solution.

Glucose

Maintenance fluids usually contain D5, which provides 17 calories/100 mL and nearly 20% of the daily caloric needs. This level is enough to prevent ketone production and helps minimize protein degradation, but the child will lose weight on this regimen. The weight loss is the principal reason why a patient needs to be started on total parental nutrition after a few days of maintenance fluids if enteral feedings are still not possible. Maintenance fluids are also lacking in such crucial nutrients as protein, fat, vitamins, and minerals.

Selection of Maintenance Fluids

After calculation of water and electrolyte needs, children typically receive either D5 + 20 mEq/L KCl or D5 0.2NS + 20 mEq/L KCl. Children weighing less than approximately 10 kg do best with the solution containing 0.2NS because of their high water needs per kilogram. Larger children and adults may receive the solution with . These guidelines assume that there is no disease process present that would require an adjustment in either the volume or the electrolyte composition of maintenance fluids (children with renal insufficiency may be hyperkalemic or unable to excrete potassium and may not tolerate 20 mEq/L of potassium). These solutions work well in children who have normal homeostatic mechanisms for adjusting urinary excretion of water, sodium, and potassium. In children with complicated pathophysiologic derangements, it may be necessary to empirically adjust the electrolyte composition and rate of maintenance fluids on the basis of electrolyte measurements and assessment of fluid balance. In all children, it is critical to carefully monitor weight, urine output, and electrolytes to identify overhydration or underhydration, hyponatremia, and other electrolyte disturbances, and to then adjust the rate or composition of the intravenous solution accordingly.

Maintenance Fluids and Hyponatremia

Patients who are producing antidiuretic hormone (ADH) may retain water, creating a risk of hyponatremia due to water intoxication. Patients who may be producing ADH owing to subtle volume depletion or other mechanisms (respiratory disease, stress, pain, nausea, medications such as narcotics) may be more safely treated with fluids that have a higher sodium concentration, with a decrease in fluid rate, or with a combination of these strategies. Patients with persistent ADH production due to an underlying disease process (syndrome of inappropriate ADH secretion [SIADH], congestive heart failure, nephrotic syndrome, liver disease) should receive less than maintenance fluids. Treatment is individualized, and careful monitoring is critical. Special caution is needed in patients who are known to have low-normal serum sodium concentrations or hyponatremia.

Hyponatremia as a complication of intravenous fluids is particularly a concern in the postoperative patient who is intravascularly volume-depleted from surgical losses, third space losses (discussed later), and venous pooling (due to lying supine and the effects of anesthesia and sedation). Surgical patients typically receive isotonic fluids (NS, LR) during surgery and in the recovery room for 6-8 hr postoperatively; the rate is typically approximately of the calculated maintenance rate. Subsequent maintenance fluids should contain , even in smaller patients, unless there is a specific indication to use 0.2NS. Electrolytes should be measured at least daily.

Patients with other potential causes of ADH production must have careful monitoring of their electrolytes and fluid input and output. Patients with possible subtle volume depletion (Chapter 54) should receive 20 mL/kg (maximum of 1 L) of isotonic fluid (NS, LR) over 1-2 hr to restore their intravascular volume before maintenance fluids are initiated. The patient can then be switched to D5 + 20 mEq/L KCl at a standard maintenance rate. Patients of any weight with possible volume depletion should not routinely receive fluids with 0.2NS, unless there is a specific indication. Patients who are at risk for producing ADH owing to etiologies other than volume depletion may need to receive less than maintenance fluids to avoid hyponatremia.

Variations in Maintenance Water and Electrolytes

The calculation of maintenance water is based on standard assumptions regarding water losses. There are patients, however, in whom these assumptions are incorrect. To identify such situations, it is helpful to understand the source and magnitude of normal water losses. Table 53-5 lists the 3 sources of normal water loss.

Urine is the most important contributor to normal water loss. Insensible losses represent approximately of total maintenance water (40% in infants and closer to 25% in adolescents and adults). Insensible losses are composed of evaporative losses from the skin and lungs that cannot be quantitated. The evaporative losses from the skin do not include sweat, which would be considered an additional (sensible) source of water loss. Stool normally represents a minor source of water loss.

Maintenance water and electrolyte needs may be increased or decreased, depending on the clinical situation. This may be obvious, in the case of the infant with profuse diarrhea, or subtle, in the case of the patient who has decreased insensible losses while receiving mechanical ventilation. It is helpful to consider the sources of normal water and electrolyte losses and to determine whether any of these sources is being modified in a specific patient. It is then necessary to adjust maintenance water and electrolyte calculations.

Table 53-6 lists a variety of clinical situations that modify normal water and electrolyte losses. The skin can be a source of very significant water loss, particularly in neonates, especially premature infants, who are under radiant warmers or are receiving phototherapy. Very low birthweight infants can have insensible losses of 100-200 mL/kg/24 hr. Burns can result in massive losses of water and electrolytes, and there are specific guidelines for fluid management in children with burns (Chapter 68). Sweat losses of water and electrolytes, especially in a warm climate, can also be significant. Children with cystic fibrosis have increased sodium losses from the skin. Some children with pseudohypoaldosteronism also have increased cutaneous salt losses.

Table 53-6 ADJUSTMENTS IN MAINTENANCE WATER

Fever increases evaporative losses from the skin. These losses are somewhat predictable, leading to a 10-15% increase in maintenance water needs for each 1°C increase in temperature above 38°C. These guidelines are for a patient with a persistent fever; a 1-hr fever spike does not cause an appreciable increase in water needs.

Tachypnea or a tracheostomy increases evaporative losses from the lungs. A humidified ventilator causes a decrease in insensible losses from the lungs and can even lead to water absorption via the lungs; a ventilated patient has a decrease in maintenance water requirements. It may be difficult to quantify the changes that take place in the individual patient in these situations.

Replacement Fluids

The gastrointestinal (GI) tract is potentially a source of considerable water loss. GI water losses are accompanied by electrolytes and thus may cause disturbances in intravascular volume and electrolyte concentrations. GI losses are often associated with loss of potassium, leading to hypokalemia. Because of the high bicarbonate concentration in stool, children with diarrhea usually have a metabolic acidosis, which may be accentuated if volume depletion causes hypoperfusion and a concurrent lactic acidosis. Emesis or losses from an NG tube can cause a metabolic alkalosis (Chapter 52).

In the absence of vomiting, diarrhea, or NG drainage, GI losses of water and electrolytes are usually quite small. All GI losses are considered excessive, and the increase in the water requirement is equal to the volume of fluid losses. Because GI water and electrolyte losses can be precisely measured, it is possible to use an appropriate replacement solution.

It is impossible to predict the losses for the next 24 hr; it is better to replace excessive GI losses as they occur. The child should receive an appropriate maintenance fluid that does not consider the GI losses. The losses should then be replaced after they occur, with use of a solution with the same approximate electrolyte concentration as the GI fluid. The losses are usually replaced every 1-6 hr, depending on the rate of loss, with very rapid losses being replaced more frequently.

Diarrhea is a common cause of fluid loss in children. It can cause dehydration and electrolyte disorders. In the unusual patient with significant diarrhea and a limited ability to take oral fluid, it is important to have a plan for replacing excessive stool losses. The volume of stool should be measured, and an equal volume of replacement solution should be given. Data are available on the average electrolyte composition of diarrhea in children (Table 53-7). With use of this information, it is possible to design an appropriate replacement solution. The solution shown in Table 53-7 replaces stool losses of sodium, potassium, chloride, and bicarbonate. Each 1 mL of stool should be replaced by 1 mL of this solution. The average electrolyte composition of diarrhea is just an average, and there may be considerable variation. It is therefore advisable to consider measuring the electrolyte composition of a patient’s diarrhea if the amount is especially excessive or if the patient’s serum electrolyte levels are problematic.

Loss of gastric fluid, via either emesis or NG suction, is also likely to cause dehydration, in that most patients with either condition have impaired oral intake of fluids. Electrolyte disturbances, particularly hypokalemia and metabolic alkalosis, are also common. These complications can be avoided by judicious use of a replacement solution. The composition of gastric fluid shown in Table 53-8 is the basis for designing a replacement solution.

Patients with gastric losses frequently have hypokalemia, although the potassium concentration of gastric fluid is relatively low. The associated urinary loss of potassium is an important cause of hypokalemia in this situation (Chapter 52). These patients may need additional potassium either in their maintenance fluids or in their replacement fluids to compensate for prior or ongoing urinary losses. Restoration of the patient’s intravascular volume, by decreasing aldosterone synthesis, lessens the urinary potassium losses.

Urine output is normally the largest cause of water loss. Diseases such as renal failure and SIADH can lead to a decrease in urine volume. The patient with oliguria or anuria has a decreased need for water and electrolytes; continuation of maintenance fluids produces fluid overload. In contrast, postobstructive diuresis, the polyuric phase of acute tubular necrosis, diabetes mellitus, and diabetes insipidus increase urine production. To prevent dehydration, the patient must receive more than standard maintenance fluids when urine output is excessive. The electrolyte losses in patients with polyuria are variable. In diabetes insipidus, the urine electrolyte concentration is usually low, whereas children with diseases such as juvenile nephronophthisis and obstructive uropathy usually have increased losses of both water and sodium.

The approach to decreased or increased urine output is similar (Table 53-9). The patient receives fluids at a rate to replace insensible losses. This is accomplished by a rate of fluid administration that is 25-40% of the normal maintenance rate, depending on the patient’s age. Replacing insensible losses in the anuric child will theoretically maintain an even fluid balance, with the caveat that 25-40% of the normal maintenance rate is only an estimate of insensible losses. In the individual patient, this rate is adjusted on the basis of monitoring of the patient’s weight and volume status. Most children with renal insufficiency receive little or no potassium because the kidney is the principal site of potassium excretion.

For the oliguric child, it is important to add a urine replacement solution to prevent dehydration. This issue is especially important in the patient with acute renal failure, in whom output may increase slowly, potentially leading to volume depletion and worsening of renal failure if the patient remains on only insensible fluids. A replacement solution of D5 is usually appropriate initially, although its composition may have to be adjusted if urine output increases significantly.

Most children with polyuria (except in diabetes mellitus; Chapter 583) should be started on replacement of insensible fluid plus urine losses. This approach avoids the need to attempt to calculate the volume of urine output that is “normal” so that the patient can be given replacement fluid for the excess. In these patients, urine output is, by definition, excessive, and it is important to measure the sodium and potassium concentrations of the urine to help in formulating the urine replacement solution.

Surgical drains and chest tubes can produce measurable fluid output. These fluid losses should be replaced when they are significant. They can be measured and replaced with an appropriate replacement solution. Third space losses, which manifest as edema and ascites, are due to a shift of fluid from the intravascular space into the interstitial space. Although these losses cannot be quantitated easily, third space losses can be large and may lead to intravascular volume depletion, despite the patient’s weight gain. Replacement of third space fluid is empirical but should be anticipated in patients who are at risk, such as children who have burns or abdominal surgery. Third space losses and chest tube output are isotonic; thus, they usually require replacement with an isotonic fluid, such as NS or LR. Adjustments in the amount of replacement fluid for third space losses are based on continuing assessment of the patient’s intravascular volume status. Protein losses from chest tube drainage can be significant, occasionally necessitating that 5% albumin be used as a replacement solution.