CHAPTER 80

Management of Dehydration in Children: Fluid and Electrolyte Therapy

Gangadarshni Chandramohan, MD, MSc, FASN, FAAP

Dehydration resulting from gastrointestinal (GI) and other disorders, especially diarrhea, is among the most common medical problems encountered in children younger than 5 years. During the past 50 years or more, the usual therapy for children who are hospitalized with dehydration has been to administer intravenous (IV) fluids starting with 1 or 2 boluses of normal saline (NS; 0.9% sodium chloride [NaCl] solution at 20 mL/kg). This is followed by the administration of a sodium (Na⁺) solution of variable concentration (usually 0.45% NaCl) mixed with 5% dextrose over the next 24 to 48 hours until the child is able to take oral fluids. The exact amount of fluid and electrolytes is calculated using complicated formulas to provide maintenance fluids and correction of remaining deficit (ie, deficit therapy). The calculation of maintenance therapy was first recommended in the 1950s, but more recently it has been suggested that dehydration management should focus on rapid restoration of extracellular fluid (ECF deficit) followed by oral rehydration therapy (ORT), and traditional calculations of fluid deficits should be abandoned. Alternatively, pediatric nephrologists and intensivists have recommended that physicians forgo sodium calculations in hospitalized children and rely solely on isotonic NS (0.9% NaCl solution) for management. Although this chapter incorporates these suggestions where appropriate, it describes the traditional approach to maintenance and deficit therapy because an understanding of the pathophysiology of dehydration helps in the treatment not only of the dehydrated child but also of children with other types of fluid and electrolyte disorders.

Epidemiology

Over the past 30 years, hospital admissions and mortality resulting from diarrhea and dehydration have decreased worldwide; nevertheless, diarrhea remains 1 of the leading medical problems in children younger than 5 years. According to the Centers for Disease Control and Prevention, more than 200,000 hospitalizations and 300 deaths of children occur each year in the United States resulting from diarrhea. Additionally, diarrhea is responsible for 2 to 3 million outpatient visits each year and contributes to 10% of all hospital admissions.

Maintenance Fluid and Electrolyte Requirements

The body has a maintenance fluid requirement to replace daily normal losses that occur through the kidney, intestines, skin, and respiratory tract. Of the various methods used to determine fluid needs, the most common is the caloric method, also called the Holliday-Segar method, which is based on the linear relationship between metabolic rate and fluid needs. For every calorie expended in metabolism, a child requires approximately 1 mL of water. Metabolic rate in children is a function of body surface area. Infants, with their higher relative surface areas per unit of body weight, have higher metabolic rates and, therefore, higher fluid requirements per unit weight compared with older children and adults. As the child grows, the relative surface area decreases, as do the metabolic rate and fluid requirement per unit weight. Using this relationship, maintenance fluid needs can be calculated for the healthy child using the method outlined in Table 80.1. These calculations of fluid needs are often used to determine the amount of IV fluids provided to a hospitalized child or to calculate the approximate amount of fluid a healthy child requires orally to maintain hydration. These calculations may not be appropriate for children who are critically ill, however, some of whom require fluid restriction and others of whom may have increased fluid needs. Moreover, the caloric method makes no allowance for extra fluid needed for weight gain, growth, activity, or pathophysiological states that increase fluid needs (eg, fever). The fluid requirement derived from this method is valid to determine the daily fluid need for an essentially healthy child. Thriving infants normally drink more fluid than indicated by this method. On average, a growing infant may take 150 to 200 mL/kg per day of milk (human milk or infant formula) as desired to support the average weight gain of 30 g (1.1 oz) per day usually observed in the first few months after birth.

Replacement of normal daily losses of electrolytes is considered when a child is not able to take adequate nutritional intake orally (as in the example in Box 80.1, in which the child receives nothing orally in preparation for surgery). Electrolyte quantities usually are expressed as milliequivalent (mEq) or millimole (mmol) amount per 100 mL of fluid required. Traditionally, the recommended sodium requirement for a healthy child is 3 mEq/100 mL fluid required (approximately 0.2% NaCl or 0.25 NS), and the potassium (K⁺) requirement is 2 to 2.5 mEq/100 mL of fluid (see Box 80.1, part B, for sample calculation and IV order). Potassium should be administered only after ensuring adequate renal function. These estimations of sodium and potassium requirements are meant to replace normal daily losses and would not be adequate in the setting of increased electrolyte losses that can occur in a number of pathologic conditions (eg, diarrhea). Additionally, increased attention has recently been given to the risk of hyponatremia and related complications in hospitalized ill children.

^a Excluding neonates and preterm infants.

Relatively healthy, well-nourished children receiving IV fluids for a brief period (ie, 1–2 days) during hospitalization do not routinely require supplementation with other electrolytes, such as calcium and magnesium. However, it is important to realize that standard IV fluids containing 5% dextrose, sodium chloride, and potassium chloride provide only minimal caloric needs and do not adequately support weight gain or provide other necessary nutrients. The child who requires prolonged IV therapy because of inadequate GI tract function should receive total parenteral nutrition to better meet the child’s caloric and nutritional needs.

Abbreviation: D5 = 5% dextrose water; IV, intravenous; K⁺, potassium; KCl = potassium chloride; Na⁺, sodium; NaCl, sodium chloride; NS, normal saline.

^a Even though in this example the calculations for sodium concentration in the IV fluid is physiologic, recent American Academy of Pediatrics guidelines recommend use of NS with 5% dextrose preoperatively to prevent potential postoperative hyponatremia.

Alterations in Fluid Needs in Illness

Several conditions can influence fluid requirements. Conditions that increase a patient’s metabolic rate (eg, fever) will also increase a patient’s fluid requirement. A child’s metabolic rate is increased 12% for every 1°C temperature elevation above normal. Most otherwise healthy children with free access to fluids will increase their own intake to account for increased needs when febrile. Other less common hypermetabolic states, such as thyrotoxicosis or salicylate poisoning, may have an even more dramatic effect, perhaps increasing metabolic rate by 25% to 50% over maintenance. In these cases and for children who are dependent on others to provide their fluids, the physician must be aware of the magnitude of increased need and provide supplemental fluids to avoid dehydration.

Other conditions may decrease a child’s fluid requirement. In hypometabolic states, such as hypothyroidism, metabolic rate and fluid needs are decreased by 10% to 25%. Fluid requirements are decreased by 10% to 25% in high environmental humidity unless the ambient temperature is also high and results in visible sweating. In these situations, a healthy child with normal renal function given extra fluid beyond what is needed can, within limits, effectively excrete any excessive intake. The child with renal failure, however, poses a special challenge for the physician in the management of fluid and electrolytes. When a child cannot adequately excrete excessive fluid intake, this fluid can accumulate and result in complications such as congestive heart failure and pulmonary edema. Without functioning kidneys, only insensible fluid losses need replacing. Insensible losses occur primarily through the skin and respiratory tract; they account for approximately 40% of maintenance fluid needs. However, fluid needs for patients with renal failure usually are estimated to be 30% of the maintenance requirement, with additional fluids provided if necessary. Limiting fluids avoids the accumulation of excessive fluids that may require dialysis for removal.

Fluid requirements may also be decreased under circumstances in which arginine vasopressin (AVP; also called antidiuretic hormone) is increased. In addition to hypovolemia or hypertonicity (ie, hyperosmolality), AVP release is also stimulated by pain, nausea, surgery (ie, in the postoperative period), central nervous system (CNS) infections (eg, meningitis, encephalitis), severe pneumonia or respirator use, and certain medications, including thiazide diuretics, chemotherapeutic agents, and selective serotonin reuptake inhibitors. Arginine vasopressin release in the absence of hypovolemia or hypertonicity results in hyponatremia and is referred to as syndrome of inappropriate antidiuretic hormone secretion. In patients with this syndrome, fluid restriction as well as administration of fluids with a higher sodium concentration may be indicated.

The most appropriate sodium concentration of IV fluids for the hospitalized child admitted to a pediatric intensive care unit or in the postoperative patient is controversial. Over the past 25 years, most such children have been maintained on a solution containing 5% dextrose water in half NS (D5 0.5 NS) or lower sodium concentrations (D5 0.25 NS). Studies suggest that because the kidney retains free water in response to excessive AVP in these children, they are at risk for hyponatremia, hyponatremic encephalopathy, brain stem herniation, permanent brain damage, or death. Recently, some pediatric nephrologists and intensivists have recommended forgoing sodium calculations in hospitalized very sick children and instead relying solely on isotonic fluids. Others have cautioned that physicians must make certain that the new recommendations to use isotonic fluids do not result in excessive congestive heart failure or hypernatremia before abandoning previous practices. Regardless of the approach used, close attention to the type and quantity of fluids provided, quantity of body fluid output, weight change, and serial electrolyte assessments are important in the management of all sick children, and fluid and electrolytes must be individualized to each patient to prevent serious complications.

Pathophysiology

Dehydration is among the most common pathophysiological alterations in fluid balance encountered in pediatrics. Although strictly speaking, dehydration means deficit of water only, most children with dehydration have lost water and electrolytes. Dehydration can result from diminished intake, excessive losses through the GI tract (eg, diarrhea, vomiting), excessive losses from the kidney or skin (eg, polyuria resulting from osmotic diuresis in uncontrolled diabetes), or a combination of these factors.

Children are at increased risk for episodes of dehydration for many reasons. Infants and young children have 2 to 4 times the body surface area per unit body weight compared with adults and as a result have relatively higher fluid needs. It is therefore much easier for children to become dehydrated in the setting of decreased intake or increased losses that often accompany common childhood illnesses. For example, acute gastroenteritis, which is common in young children, often results in anorexia, recurrent vomiting, and frequent or large-volume stools, with proportionately more severe fluid loss than in older children and adults. Additionally, infants and young children are dependent beings who are unable to increase their own fluid intake in response to thirst and must rely on others to provide their fluid needs. If these fluid needs are not met or are underestimated, a child can easily become dehydrated.

Dehydration is classified as isotonic, hypotonic, or hypertonic. These terms often are used interchangeably with isonatremic, hyponatremic, and hypernatremic, respectively. The latter terms reflect the sodium content of the ECF that largely determines serum osmolality in the otherwise healthy dehydrated child. Acute isotonic or isonatremic dehydration (serum Na⁺ 135–145 mEq/L), which is the most common type of dehydration, involves net loss of isotonic fluid containing sodium and potassium (Figure 80.1, top). In diarrhea-related dehydration, sodium, the primary ECF cation, is not only lost from the body but also shifts into the intracellular fluid (ICF) compartment to balance the loss of potassium, because potassium losses from cells generally are not accompanied by intracellular anionic losses in acute dehydration. The sodium that has shifted into the ICF compartment will return to the ECF compartment during rehydration as potassium is being replenished, by the action of sodium/potassium adenosinetriphosphatase (ATPase). No net loss of fluid from the ICF occurs in this process; the total water deficit in dehydration comes primarily from the ECF, although some investigators have suggested, in the absence of valid data, that two-thirds of the losses come from ECF and one-third from ICF.

Figure 80.1. Pathophysiology of various types of dehydration. Top, Isotonic/ isonatremic dehydration. Middle, Hypotonic dehydration. Bottom, Hypertonic dehydration. Abbreviations: ECF, extracellular fluid; H₂O, water; ICF, intracellular fluid; K⁺, potassium; Na⁺, sodium.

A variety of mechanisms exist by which hyponatremia (serum Na⁺ <135 mEq/L) occurs in association with dehydration. In hypotonic or hyponatremic dehydration, the ECF volume is compromised to a greater degree than in isotonic dehydration because of osmotic shifts of ECF into the cells, resulting in more severe signs of dehydration (Figure 80.1, middle). Hypotonic dehydration typically occurs in children with gastroenteritis in the setting of excessive sodium losses in stool and oral fluids replacement with a reduced amount of sodium (ie, water, low-sodium beverages [eg, juice, tea]). Furthermore, the kidneys often retain free water (ie, excrete a concentrated urine) despite hyponatremia, because AVP is stimulated by the decreased effective circulating volume in such settings. Intravascular volume depletion seems to be a potent stimulus for AVP release, overriding the AVP suppressive effect of hypotonicity/hyponatremia. The result is that serum sodium levels are further decreased because of dilution. Hypotonic dehydration also may occur as the result of excessive loss of sodium (relative to body water) in stool (eg, cholera) or in the urine (eg, adrenogenital syndrome, cerebral salt wasting, pseudohypoaldosteronism, other salt-wasting renal disorders). This is the most dangerous form of dehydration because it can result in cerebral edema and eventually, brain herniation.

Hypertonic or hypernatremic dehydration (serum Na⁺ >145 mEq/L) occurs when net loss of water exceeds that of solute loss (Figure 80.1, bottom). It usually is seen in clinical conditions in which rapid loss of hypotonic fluid in stool, vomit, or urine occurs, accompanied by failure of adequate water intake because of anorexia or vomiting. Fever or hyperventilation, if present, may intensify the disproportionate loss of water. Occasionally, hypertonic dehydration may be caused by excessive solute intake. Urinary excretion of excess solute obligates loss of large volumes of water, resulting in dehydration. The history may reveal that the child was accidentally fed a high sodium solution because of incorrect mixing of oral rehydration packets or concentrated formula. In hypertonic dehydration, shift of fluid from the ICF to the ECF occurs to attain osmotic balance. As such, the ECF volume is somewhat spared at the expense of the ICF, and signs of dehydration may be delayed. However, fluid loss from the ICF results in intracellular dehydration, the most serious effect of which can occur in the brain. If hypernatremia occurs rapidly, not only does a decrease in brain size occur, but a fall in cerebrospinal fluid pressure occurs as well resulting from diffusion of water from cerebrospinal fluid to the blood. As the brain shrinks, the bridging veins within the skull may stretch and even tear, resulting in intracranial hemorrhage or other complications. If the hypertonic state manifests more slowly, brain cell size may initially shrink minimally but will gradually return to normal size even with continued hypernatremia. Preservation of the brain cell volume despite hypernatremia is thought to be caused by the generation of idiogenic osmoles (eg, myoinositol, trimethylamines, taurine, and other amino acids) that prevent water loss and attract water back into the cell and thus maintain cell volume. Rehydration of the patient with hypernatremia must occur slowly and cautiously to avoid brain cell swelling (Figure 80.2). In the child with hypertonic dehydration, on clinical examination the skin may sometimes feel “doughy” because of intracellular dehydration. This finding, however, is inconsistent even when evaluation is done by an experienced pediatrician; thus, clinical examination of the skin should not substitute for serum sodium measurements in the diagnosis of hypernatremia.

Evaluation

History

In addition to signs and symptoms of the current illness, the history should focus on the cause of dehydration. The parent or guardian should be questioned about the type and amount of oral intake; the duration, quality, and frequency of vomiting or diarrhea; whether blood is present in the stool; the presence or absence of fever; frequency of urination; and whether a recent pre-illness weight is known for the child (Box 80.2). Changes of mental status reported by a parent or guardian is of particular concern because this can occur as the result of significant electrolyte (eg, sodium) disturbance, marked dehydration, or other serious infection or illness. The most accurate means of assessing the degree of dehydration is to compare current weight with a recent pre-illness weight. In acute dehydration, weight loss is primarily the result of fluid loss. The difference between pre-illness and current weight can be used to determine the degree of the fluid deficit.

Figure 80.2. Illustration of the steps in managing hypertonic dehydration showing the effects on brain volume of rapid versus slow development of hypernatremia and the results of rapid versus slow correction of hypernatremia.

Physical Examination

An important goal of the physical examination of the dehydrated child is to assess the degree of dehydration. In the process, vital signs, including blood pressure and a current weight, should be obtained. Specific attention should be paid to the general appearance of the child and, in particular, whether the child is ill-appearing, listless, or less reactive. In addition to the usual components of the physical examination, it is important to assess for the following factors: whether the oral mucosal membranes appear tacky or dry, whether tears are present or absent, if tenting of the skin is present (ie, tenting remains after the skin is pinched between 2 fingers), and perfusion status of the extremities. In the process, it is important that the physician recognize whether shock is present, because this is a life-threatening condition requiring emergent treatment (see Chapter 74).

If comparison to an accurate recent pre-illness weight is not possible, the physician must rely on vital signs as well as clinical signs and symptoms to assess the degree of dehydration (Table 80.2). In infants and young children (ie, younger than 5 years), the estimated fluid loss for mild dehydration is less than or equal to 5% deficit of body water; moderate dehydration, 6% to 9%; and severe dehydration, 10% to 15%. The corresponding numbers for estimated fluid loss in children 5 years and older are 3%, 6%, and 9%, respectively. A variety of clinical signs have been proposed to evaluate the degree of dehydration—some more valid and reliable than others. A systematic review found that assessment of capillary refill was the most useful single sign in detecting dehydration of 5% or more. Capillary refill is assessed by placing brief pressure on the distal palmar aspect of a fingertip and assessing the amount of time for the blanched area to refill; normal is considered less than or equal to 2 seconds. Two other single signs found to be important in predicting dehydration of 3% to 5% or more were abnormal skin turgor (ie, tenting) and respiratory disturbance, in particular hyperpnea (ie, deep, rapid breathing without other signs of respiratory distress) suggestive of acidosis. Generally, the more signs that are present, the greater the severity of dehydration; however, a combination of prolonged capillary refill, absent tears dry mucosal membranes, and general ill appearance may be more diagnostic than tenting and hyperpnea combined in identifying the child with more than mild to moderate dehydration.

Abbreviations: ++, certain to occur; +, likely to occur; ±, may occur.

^a In mild dehydration, may be only a history of fluid loss in the form of diarrhea or vomiting without any of the signs of dehydration listed in this table.

^b Often, such patients present with hypovolemic shock, need more intense treatment, and may require additional volume expanders (eg, colloids, blood products).

^c Usually corrects with restoration of intravascular volume.

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