# Maintenance Fluids and Replacement Fluids in Children

Pediatric fluid management

## 1. Description of the problem

#### What every clinician needs to know

The majority of children in an ICU receive intravenous fluids, especially during their initial course. Enteral feeds are the ultimate goal, but may be precluded due to specific contraindications (e.g. abdominal surgery, respiratory distress). Patients who are NPO require "maintenance" fluids, unless severe volume overload is present. Patients may require additional "replacement" fluids to address excessive ongoing losses (e.g. NG output). In addition, many patients require volume resuscitation due to intravascular volume depletion.

#### Maintenance fluids

Maintenance fluids are used when a patient is NPO. Maintenance fluids consist of water, glucose, sodium, and potassium. The glucose prevents starvation ketoacidosis and decreases the likelihood of hypoglycemia. Water, sodium and potassium protect the patient from dehydration and electrolyte disorders.

#### Rate of maintenance fluids

The wide variation in size of children dictates that the rate of maintenance fluids is adjusted based on the patient's size. While there are formulas based on body surface area, weight is generally used to determine the rate of intravenous fluids. Ideal body weight (adjusting for increased adiposity) or dry weight (adjusting for volume overload or volume depletion) have theoretical advantages, but the patient's actual weight is generally used for practical reasons. Moreover, these differences are generally clinically inconsequential given the fact that the calculated rate is an estimate that relies on normal kidney function for maintaining the patient in a euvolemic state. Table I provides formulas for calculating maintenance fluids as a total volume per 24 hours or as a rate per hour:

#### Table I.

###### Maintenance Fluids
Weight Volume/24 hours Rate/hour
0-10 kg 100 ml/kg 4 ml/kg
11-20 kg 1000 ml + 50 ml/kg for each kg > 10 kg 40 ml + 2 ml/kg for each kg >10 kg
>20 kg 1500 + 20 ml/kg for each kg > 20 kg 60 ml + 1 ml/kg for each kg >20 kg
Maximum 2400-3000 ml 100-120 ml

Maintenance fluid rates are not adequate for patients with volume depletion or excessive ongoing fluid/electrolyte losses. They provide too much fluid for the patient with volume overload, especially if there is impaired kidney function.

#### Composition of maintenance fluids

Traditional maintenance fluids in children are quite hypotonic, with 0.2 NS or 1/4 NS being used in children less than 10-20 kg and 1/2 NS in larger children. This is based on theoretical arguments, including the fact that breast milk and term infant formula have a sodium content of less than 10 mEq/L.

However, the use of hypotonic fluids has been associated with iatrogenic hyponatremia in children. This is thought to be secondary to the multiple stimuli for ADH production in the hospitalized child (e.g. volume depletion, pain, nausea, medications). The production of ADH inhibits water excretion by the kidney, the usual mechanism for protection from hyponatremia.

Since almost all children in an ICU will have at least one stimulus for ADH production, isotonic fluid is generally the preferred initial "maintenance" fluid (NS or LR), unless there is a specific contraindication such as hypernatremia or volume overload.

The sodium content of the maintenance fluid may be decreased if the patient develops volume overload or hypertension. It is prudent to monitor the sodium concentration at least daily in an ICU patient to detect increases or decreases in the serum sodium concentration and then adjust the rate or composition of the maintenance fluid, depending on the clinical situation.

D5 is the standard dextrose composition of maintenance fluids. There are a few indications for providing a lower dextrose delivery. These include severe hyperglycemia or traumatic brain injury. A higher dextrose concentration may be appropriate as a strategy for insuring adequate dextrose delivery in specific situations, such as in a patient with an inborn error of metabolism.

The traditional potassium composition of maintenance fluids is 20 mEq/L. This is generally effective in preventing hypokalemia, but not causing hyperkalemia. However, this strategy was developed for the healthy child with normal kidney function. In the ICU, the potassium concentration of maintenance fluids should be carefully considered. In the child with pre-existing or new onset renal failure, potassium is usually initially withheld unless the patient has hypokalemia.

The same is true in the child with tumor lysis or rhabdomyolysis. The presence of hyperkalemia is another indication for withholding potassium initially, unless the patient has a condition where the potassium concentration is likely to decrease rapidly, usually due to a concurrent intervention (e.g. diabetic ketoacidosis).

Similarly, hypokalemia may be an indication of a higher potassium concentration. Again, careful monitoring of the serum potassium concentration and kidney function permit empiric adjustment in the potassium concentration.

#### Adjustment in maintenance fluids in oliguria or polyuria

Maintenance fluid is based on replacing fluid/electrolytes due to insensible losses (principally water losses from the skin and lungs) and urinary losses of water and electrolytes. Gastrointestinal losses are considered negligible in the absence of a pathologic process, such as diarrhea. Providing maintenance fluids in the presence of anuria due to chronic renal failure will quickly cause volume overload. Similarly, the patient with nephrogenic diabetes insipidus will rapidly become dehydrated if only given maintenance fluids.

#### Oliguria or anuria

Kidney failure is the most common cause of oliguria or anuria that warrants reducing the maintenance fluid rate. This must be distinguished from oliguria/anuria due to intravascular volume depletion, which typically requires increased fluid rates or another intervention (e.g. pressors) to prevent the development of AKI. SIADH is another potential cause of oliguria that requires adjustment in maintenance fluids.

The basic strategy for managing the patient with oliguria is to provide maintenance fluids at a rate that replaces insensible losses (typically 1/3 of maintenance in children less than 20 kg and 1/4 of maintenance in children >20 kg). D5 or D10 1/2 NS is a good choice, with the caveat that potassium can be added in the child without renal failure or if the potassium level is low. D10 1/2 NS may be appropriate if the child has no other source of dextrose, since low rate will decrease the dextrose if the patient only receives traditional D5.

Additional fluid is necessary in the child who is not anuric. Hence, urine replacement solution (D5 1/2 NS) should be given to replace urinary losses as they occur (ml/ml every 4 hours). This is especially important in the child with AKI, since it prevents the development of intravascular volume depletion if the child's urine output increases.

Naturally, the child who is volume overloaded or volume depleted will require an adjustment in these strategies until he or she reaches a euvolemic state (e.g. less than insensible fluids in the volume overloaded patient).

#### Polyuria

Polyuria has multiple potential etiologies. These include the polyuric phase of ATN, post-obstructive diuresis and diabetes insipidus. There are also a variety of inherited disorders associated with polyuria in children (e.g. Bartter's syndrome, cystinosis, Gitelman's syndrome).

The patient with polyuria should receive D5 1/2 NS + 20 mEq/L of KCl at a rate that replaces insensible losses (typically 1/3 of maintenance in children less than 20 kg and 1/4 of maintenance in children more than 20 kg). The potassium concentration may be adjusted based on the clinical situation (e.g. serum potassium, kidney function). In addition, the patient should receive urine replacement (ml/ml every 2-4 hours).

The composition of the urine replacement should be based on the Na concentration of the urine (e.g. use 1/4 NS if the urine sodium is 40 mEq/L). There may be a need to empirically adjust the urine replacement sodium concentration based on the patient's serum sodium concentration and its change in value over time.

#### Adjustment in maintenance fluids with altered insensible losses

There are a variety of situations where insensible losses may be increased or decreased. Skin losses may be increased due to fever, sweating, burns, phototherapy (hyperbilirubinemia) and burns. Lung losses are increased due to tachypnea or a tracheostomy. In most of these situations, the losses cannot be quantified and the clinician must simply consider these losses when prescribing fluids and assessing the child who develops dehydration while on maintenance fluids.

Fever is expected to increase maintenance water needs by 10-15% for each degree over 38 degrees. There are also formulas for adjusting fluid therapy in burns based on the size of the patient and size of the burn. Pulmonary losses are decreased in the intubated child, who may thus need a lower insensible fluid rate in the presence of anuria/oliguria.

#### Excessive losses

There are a variety of situations beyond polyuria when a patient may have excessive losses of fluids and electrolytes. Diarrhea and NG losses are especially common in the ICU, but some patients may have very high losses from surgical drains or chest tubes. Finally, "third space" losses of fluid, most commonly into the abdomen, may cause intravascular volume depletion.

#### Diarrheal losses

Severe, ongoing diarrheal losses typically need to be replaced to avoid volume depletion and electrolyte derangements. However, most patients with diarrhea do not need stool replacement solutions. Stool losses have sodium, potassium, and base and thus typically produce hypokalemia and metabolic acidosis, with the sodium concentration dictated by the patient's intake of sodium and water.

Stool replacement solution should contain about 55 mEq/L of sodium, 20 mEq/L of potassium and 15 mEq/L of bicarbonate (e.g. D5 1/4 NS + 20 mEq/L sodium bicarbonate and 20 mEq/L KCl). Losses are replaced ml/ml every 2-6 hours as they occur. The composition of stool is different and requires an adjustment in replacement solution composition in specific situations (e.g. chloride losing diarrhea or cholera).

#### Gastric losses

Severe gastric losses (typically NG losses since emesis is seldom protracted) need to be replaced to avoid volume depletion and electrolyte derangements. Gastric losses have sodium, hydrogen ions, chloride and a small amount of potassium. The patient typically develops alkalosis and hypokalemia, which is mostly due to urinary potassium losses. An appropriate gastric fluid replacement solution is NS + 10 mEq/L of KCl. Losses are replaced ml/ml every 2-6 hours as they occur.

#### Drains and third space losses

Patients may lose significant fluid from surgical drains and chest tubes. Intravascular losses may occur due to third space losses. These losses are typically isotonic and thus should be replaced with NS or LR.

#### Dehydration

Dehydration may occur due to decreased intake, excessive losses, or a combination of these mechanisms. The first priority is volume resuscitation with fluid boluses. This is followed by a plan for rehydrating the patient, typically over 24 hours.

In addition, many patients in the ICU have intravascular volume depletion due to other mechanisms (e.g. capillary leak). These patients often require fluid resuscitation but may also benefit from other interventions (pressors). They do not typically require a plan for rehydration unless they also have total body volume depletion.

#### Fluid resuscitation

Isotonic fluid boluses (NS) are the initial approach to the child with moderate to severe dehydration. A bolus is 20 ml/kg (maximum 1 liter). This is typically given over 20 minutes in the child with moderate dehydration and as fast as possible in the child with severe dehydration. Boluses should be repeated until the child has restoration of intravascular volume.

There is a much lower likelihood of overhydrating a pediatric patient than an adult patient since cardiac dysfunction is a rare cause of intravascular volume depletion. Nevertheless, this possibility should be considered when there are clinical signs of cardiac dysfunction or the patient is not responding to multiple fluid boluses.

#### Rehydration

A plan for rehydration should be developed after the initial fluid resuscitation. The first step is to calculate the fluid deficit. This is determined by multiplying the percentage dehydration times the patient's weight (e.g. 10% dehydration in a 10 kg child: 10% of 10 Kg = 1 kg = 1 liter). Subtract any boluses from this volume (e.g. 1 liter - 400 ml of boluses = 600 ml). Calculate the patient's maintenance fluid needs over 24 hours (e.g. 10 kg = 1000 ml). The two volumes are added together (e.g., 1000 ml + 600 ml = 1600 ml). The patient then receives this amount of fluid over 24 hours.

In addition, excessive losses must also be replaced and the plan would be adjusted in the setting of renal failure. An appropriate fluid is D5 1/2 NS + 20 mEq/L KCL. The potassium content should be adjusted based on the patient's renal function and initial serum potassium value. Regular monitoring of electrolytes is appropriate, with the frequency dictated by the severity of the patient's condition and initial laboratory assessment.

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## What's the evidence?

Colletti, JE, Brown, KM, Sharieff, GQ. "The management of children with gastroenteritis and dehydration in the emergency department". J Emerg Med. vol. 38. 2010. pp. 686-98.

Friedman, A. "Fluid and electrolyte therapy: A primer". Pediatr Nephrol. vol. 2010. 25. pp. 843-6.

Greenbaum, LA, Kliegman, RM, Stanton, BF, St. Geme, JW. "Pathophysiology of body fluids and fluid therapy". Nelson Textbook of Pediatrics. Elsevier Science. 2011. pp. 212-49.

Simpson, JN, Teach, SJ. "Pediatric rapid fluid resuscitation". Curr Opin Pediatr. vol. 23. 2011. pp. 286-92.

## Hypo/hyperkalemia

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