Jennifer R. Reid
EPIDEMIOLOGY
While the incidence of dehydration and electrolyte abnormalities in children is unknown, they account for more than 3 million physician visits, 220,000 hospitalizations, and hundreds of deaths annually in the United States. Worldwide, dehydration and electrolyte abnormalities, primarily from gastrointestinal illness, are a leading cause of death among children.
The most common cause of fluid and electrolyte disturbances in infants and children presenting to the emergency department is acute gastroenteritis (see Chapter 75).
Inappropriate home remedies for gastrointestinal illness may contribute to electrolyte abnormalities, especially in children <6 months of age.
DEHYDRATION
PATHOPHYSIOLOGY
During the first 2 years of life, infants and children have tremendous caloric and water maintenance requirements as a result of rapid growth and metabolism. The relative daily free water turnover is three to four times that of an adult.
Factors contributing to increased fluid requirements in children include high metabolic rate, large relative body surface area, increased insensible losses from the skin and respiratory tract (especially with fever), a relative inability to concentrate urine, and a relatively large percentage of total body water in the extracellular space.
Causes of dehydration include decreased intake, due to voluntary or involuntary causes such as anatomic or pathologic disease (eg, pharyngitis, stomatitis, cleft lip/palate or GI obstruction), neurologic disease (eg, meningitis, brain tumor, seizures), or febrile illness.
Another cause of dehydration is increased output due to insensible losses (eg, fever, heat, respiratory illness, diaphoresis, thyroid disease, cystic fibrosis), GI losses (vomiting, diarrhea), renal losses (eg, osmotic, such as diabetic ketoacidosis or nonosmotic such as renal disease or diabetes insipidus), or sodium losing conditions (eg, adrenal disease or diuretic use).
Systemic disorders such as burns, secondary ascites, respiratory disease, peritonitis, or anaphylaxis may also cause dehydration.
Dehydration can be classified as isotonic, hyponatremic, and hypernatremic, depending on the relative loss of free water and electrolytes.
Isotonic dehydration (serum sodium 130–150 mEq/L) is most common and results from a proportionately equal loss of sodium and water.
Hyponatremic dehydration (serum sodium <130 mEq/L) develops when acute fluid losses are replaced with free water (eg, tea, diluted formula). Hyponatremia may also occur in the setting of increased total body water relative to sodium (eg, syndrome of inappropriate secretion of antidiuretic hormone, and edemaforming states, such as nephrotic syndrome, cirrhosis, psychogenic or infantile water intoxication).
Hypernatremic dehydration (serum sodium >150 mEq/L) develops when there is a relatively greater loss of free water than sodium. This typically occurs when a young patient is fed salt-rich solutions (eg, inappropriately mixed formula, boiled skim milk, chicken broth).
CLINICAL FEATURES
The clinical appearance of patients depends on the degree of dehydration, the rate of fluid loss, and the age of the patient.
Older children may tolerate gradual loss of as much as 40% of intracellular volume without decompensation.
Rapid loss of large volumes in young infants can cause hypovolemic shock with cardiovascular collapse.
DIAGNOSIS AND DIFFERENTIAL
The gold standard for assessing dehydration is comparison of a pre-illness weight with post-illness weight where 1 L water = 1 kg. Based on weight lost, the fluid deficit (as a percentage of body weight) can be calculated. Dehydration is then classified as mild (<5%), moderate (5%-10%), or severe (>10%). Unfortunately, accurate pre-illness weights are rarely available in the emergency department.
In lieu of accurate weights, clinical features have been shown to provide a reliable estimation of the degree of dehydration (Table 83-1). Hypernatremic dehydration, however, may present with vital signs and skin turgor that can underestimate the total fluid losses since intravascular volume is relatively preserved.
In general, routine laboratory studies are not helpful in children older than 6 months of age unless the history or examination suggests underlying electrolyte abnormalities (see Electrolyte Disorders below).
In severe dehydration or suspected underlying systemic disease, a comprehensive metabolic panel may help classify the type of dehydration (ie, isotonic, hyponatremic, hypernatremic) and identify related problems (eg, renal failure, hypoglycemia, ketoacidosis).
Other laboratory investigations are directed by the clinical picture and aimed at diagnosis of underlying causes of dehydration (eg, infectious enteritis, metabolic disease, toxic ingestions).
TABLE 83-1 Clinical Estimate of Pediatric Dehydration
EMERGENCY DEPARTMENT CARE AND DISPOSITION
The management of fluid and electrolyte disturbances in infants and young children revolves around a few basic principles: (1) identify and treat shock; (2) identify and treat underlying causes if possible (eg, diabetic ketoacidosis, pyloric stenosis, respiratory distress); (3) administer appropriate fluids to replace fluids already lost, maintenance fluids, and ongoing fluid losses.
Treat hypovolemic shock with prompt isotonic crystalloid: administer 20 mL/kg of normal saline or lactated Ringer’s solution by rapid IV or IO bolus and repeat until vital signs, peripheral perfusion, and mental status improve. A minimum of 60 mL/kg or more in the first hour should be given, unless contraindicated based on the patient’s underlying disease (eg, diabetic ketoacidosis).
Treat children with mild to moderate dehydration with oral rehydration therapy (ORT). See Chapter 76 for a detailed discussion of ORT in the setting of vomiting and diarrhea.
Administer ondansetron 0.15 milligram/kg PO or 0.1 milligrams/kg IV for children in whom vomiting prevents adequate oral rehydration.
Consider nasogastric rehydration with isotonic crystalloids or oral rehydration solution as an alternative to oral or parenteral rehydration.
Maintenance fluids are calculated as follows: for children ≤10 kg administer 100 mL/kg/d over 24 hours (4 mL/kg/h); for children 11 to 20 kg administer 1000 mL + 50 mL/kg for each kg >10 over 24 hours; for children >20 kg administer 1500 mL + 20 mL/kg for each kg >20 over 24 hours.
Fluid deficit is determined from the estimated percentage of dehydration (see Table 83-1). The calculations are performed in the following manner: If the patient weighs 15 kg on presentation and is estimated to be 10% dehydrated, then the estimated fluid loss is 15 kg × 10% = 1.5 kg = 1.5 L.
Administer half of the total fluid deficit over the first 8 hours and the remaining half over the following 16 hours. The hourly IV fluid rate is determined by the sum of maintenance and deficit fluid requirements for the patient.
ELECTROLYTE DISORDERS
PATHOPHYSIOLOGY
The most common electrolyte abnormalities in children involve sodium. Hyponatremia, defined as a serum sodium <130 mEq/L, is most commonly caused by GI losses or water intoxication (eg, ingestion of hypotonic replacement fluids, such as water, especially during infancy).
Hypernatremia, defined as serum sodium >150 mEq/L, is most commonly associated with diarrhea, as well as renal disease and diabetes insipidus.
Hypokalemia, serum potassium <3.4 mEq/L, is usually due to profuse vomiting ± diarrhea, or loop diuretics. In diabetic ketoacidosis, total body potassium stores may be significantly depleted due to osmotic diuresis, while serum levels remain normal or falsely elevated. Uncommon causes of hypokalemia include renal tubular acidosis and familial hypokalemiainduced paralysis.
Hyperkalemia, potassium >5.5 mEq/L, is usually due to hemolysis artifact during blood draws. True hyperkalemia can be caused by renal failure, rhabdomyolysis, and adrenal corticoid insufficiency.
Disorders of calcium are relatively uncommon in children. Hypocalcemia may be caused by hyperparathyroidism, end organ resistance to parathyroid hormone associated with vitamin D deficiency, young infant’s ingestion of cow’s milk, hyperventilation or chronic renal failure.
Hypercalcemia, serum level >11 milligrams/dL most often results from increased bone absorption.
Magnesium disorders are defined by serum levels outside of the normal range of 1.5 to 2.2 mEq/L.
Causes of hypomagnesemia include diarrhea, malabsorption, short gut, fistulas, and renal loss (osmotic diuretics, chemotherapeutics).
CLINICAL FEATURES
Signs and symptoms of various electrolyte abnormalities, as well as their causes and treatment are listed in Table 83-2.
TABLE 83-2 Causes, Symptoms, and Treatment of Electrolyte Disorders in Infants and Children
DIAGNOSIS AND DIFFERENTIAL
The diagnosis is made through a careful history, physical examination, and measurement of serum electrolytes.
Rapid bedside glucose should be obtained in all patients presenting with signs of dehydration and altered mental status.
Additional laboratory studies are directed by the initial electrolyte abnormalities and the clinical picture.
Consider toxic ingestions and occult injury in children with vomiting and altered mental status.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Treat hypovolemic shock and dehydration as above (see dehydration).
Specific therapy for various electrolyte abnormalities is presented in Table 83-2.
Avoid rapid correction of sodium abnormalities, which can cause cerebral edema or central pontine myelinolysis.
Electrolyte requirements remain constant throughout childhood. The daily requirement for sodium is 2 to 3 mEq/kg/d and for potassium is 2 mEq/kg/d. All infant formulas contain sufficient electrolytes.
For further reading in Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 7th ed., see Chapter 142, “Fluid and Electrolyte Therapy in Infants and Children,” by Alan Nager.