Susan A. Kecskes
• Fluid therapy is guided by knowledge of the composition, distribution, and movement of body water.
• Fluid requirements are divided into three parts:
• maintenance fluids.
• deficit replacement.
• replacement of ongoing losses.
• Correction of circulatory failure with isotonic crystalloid or appropriate colloid is the first step in fluid management.
The initial approach to acutely ill children includes an assessment of their fluid and electrolyte status. The ability to maintain homeostasis and correct disturbances requires knowledge of the composition of the fluid spaces of the body and their changes with age and disease. This chapter discusses the physiologic basis of fluid management, some of the common disturbances, and an approach to management.
Total body water (TBW) is divided into the intracellular and extracellular compartments, with the extracellular compartment subdivided into intravascular and extravascular compartments. The relative size of these compartments varies with age (Fig. 80-1).1 TBW is approximately 79% of body weight at birth, decreasing to the adult proportion of 55% to 60% over the first year of life. This primarily relates to a drop in extracellular fluid (ECF). Postnatal diuresis, as well as growth in cellular tissue, is responsible for the majority of the change. In addition, blood volume decreases from 80 mL/kg at birth toward the adult value of 60 mL/kg. By the time the child is 1 year of age, TBW comprises approximately 60% of body weight and is approaching the adult distribution of one-third in the extra-cellular compartments and two-thirds in the intracellular compartments. Body water exists as a complex solution of salts, organic acids, and proteins. The exact composition varies with body compartment (Fig. 80-2).2
FIGURE 80-1. Distribution of total body water based on body weight at various ages. Both total body water and extracellular water decline significantly over the first year of life.
FIGURE 80-2. Electrolyte composition of intracellular space vs. extracellular space.
MOVEMENT OF FLUID
Cellular membranes form the barrier between the extracellular and intracellular spaces (Fig. 80-3). They are freely permeable to water, but impermeable to electrolytes and proteins, except by active transport. Although the specific osmoles differ in the two compartments, the osmolality is equal. Water distributes across this barrier by osmotic pressure. A rise in extracellular osmolality, as occurs with a sodium load, results in movement of water from the intracellular space to the extracellular space. Conversely, water intoxication leads to the movement of water from the extracellular space to the intracellular space.
FIGURE 80-3. Osmotic pressure controls the movement of water across cell walls. Oncotic pressure promotes water retention in the vascular space. Hydrostatic pressure promotes water movement from the vascular space to the interstitial space.
The vascular endothelium forms the barrier between the intravascular and interstitial spaces. It is permeable to water and electrolytes, but not to protein. Two forces regulate fluid movement. Hydrostatic pressure, created by the propulsion of blood through vessels, favors movement of fluid from the intravascular space to the interstitial space. This pressure falls as blood travels from the arterioles through the capillary bed to the lower-pressure veins. Oncotic pressure, exerted primarily by albumin found in the vascular space, favors water movement from the interstitium into the vascular space. Under normal conditions, there is a balance in the movement of water and electrolytes from the vascular space to the endothelium at the arteriolar side and in the reverse direction at the venous side.
These factors guide the selection of fluid to be administered to a patient. Free water added to the vascular space will distribute proportionally to all three compartments. Isotonic crystalloid distributes throughout the extracellular space. Isoncotic fluid will remain in the vascular space, with the exception of a small distribution to the interstitial space because of the increase in hydrostatic pressure.
Fluid requirements can be divided into three categories:
• Maintenance fluids, which replace routine daily fluid losses.
• Replacement of a fluid deficit, if needed.
• Ongoing excessive losses.
Maintenance fluids include insensible losses and routine outputs of urine and stool. These are proportional to the body surface area. Since infants and children have a higher body surface area per kilogram, they also have proportionally higher fluid requirements.3 There are four common methods to calculate maintenance fluids (Table 80-1). Patients in renal failure should have maintenance fluids calculated as insensible loss plus urine replacement.
Four Methods for Maintenance Fluid Calculations
Common maintenance fluids are D5 0.2% NaCl with 20 mEq/L of KCl in infants and young children and D5 0.45% NaCl with 20 mEq/L of KCl in older children and adults. There is, however, increasing evidence that hospitalized patients receiving parenteral fluids are vulnerable to the development of hyponatremia.4 This is likely related to antidiuretic hormone (ADH) excess that may be seen in many conditions seen in the ER including hypovolemia, pneumonia, asthma, congestive heart failure (CHF), meningitis, and head injury. There have also been numerous reports of significant morbidity and mortality related to iatrogenic hyponatremia following surgery. For this reason, a number of experts are advocating the use of isonatremic solutions (with or without dextrose, tailored to individual patient needs) as maintenance parenteral fluid outside the neonatal period. This remains an area of controversy.4,5
Many patients have fluid deficits that require replacement. In pediatrics, the most common cause is gastrointestinal disease associated with vomiting and diarrhea. The first priority in patients with fluid deficits is to restore circulation. To begin, the adequacy of the patient’s perfusion is determined (Table 80-2). Mental status, urine output, skin character, capillary refill, and vital signs are assessed. Serum electrolytes, blood urea nitrogen, creatinine, acid–base status, urinalysis, and urine sodium concentration may be useful. If the patient’s perfusion is inadequate, fluid resuscitation should be initiated. An initial bolus of 20 mL/kg of isotonic crystalloid (0.9% NaCl or lactated Ringer’s [LR] solution) is given intravenously over 20 minutes or faster if required by patient condition. The patient is reassessed and further boluses are given until perfusion is adequate. Pediatric patients commonly require >60 mL/kg of resuscitation fluid to restore perfusion. If required, blood products may be substituted for some of the bolus fluid. Additional therapy, such as inotropes or pressors, may be added if the circulatory failure is not solely related to fluid deficit.
Signs and Symptoms of Dehydration
Once circulation has been stabilized, the remaining deficit needs to be replaced. The magnitude of dehydration is divided into mild (water loss <5% TBW), moderate (water loss 5%–10% TBW), and severe (water loss >10% TBW). Fluid deficit can also be estimated from changes in body weight (assuming all loss is due to fluid loss):
Resuscitation fluids may be subtracted from the calculated deficit and the remainder replaced over 24 hours if the patient is in a normal osmotic state. Typically, the remaining deficit would be replaced with a hypotonic fluid, such as D5 0.45% NaCl with 20 mEq/L of KCl or an oral rehydration solution. Replacement solutions should be adjusted to the electrolyte status of the individual patient.
Some patients may require replacement of ongoing fluid losses not included in normal maintenance requirements (Table 80-3). Continuing emesis and diarrheal losses require replacement along with losses through external drains. Fever increases the water requirement by 10% for each degree elevation over 37.8°C. Ongoing third-space loss should be estimated and replaced. The type of fluid should be tailored to the content of the fluid lost. A standard solution with a composition close to the fluid being replaced is usually adequate to maintain homeostasis in patients with intact renal function. If more precision is required, the electrolyte content of the fluid being lost may be measured and replaced.
Adjustments to Maintenance Fluids
1. Friis-Hansen BJ. Body water compartments in children. Pediatrics. 1961;28:171.
2. Hill LL. Body composition and normal electrolyte concentrations. Pediatr Clin North Am. 1990;37:244.
3. Holliday MA, Segar WE. The maintenance need for water in parenteral fluid therapy. Pediatrics. 1957;19:823.
4. Moritz ML, Ayus JC. Prevention of hospital-acquired hyponatremia: a case for using isotonic crystalloid. Pediatrics. 2003;111:227
5. Holliday MA, Ray PE, Friedman AL. Fluid therapy for children: facts, fashions and questions. Arch Dis Child. 2007;92:546.