What Causes Dehydration and Electrolyte Imbalance?

Dehydration most commonly develops when diarrhea and/or vomiting cause excessive water and electrolyte loss. It may also develop from inadequate fluid intake or from other causes of excessive water and electrolyte loss (see Chapters 28, 33, 41, and 60). Severe dehydration can cause hypovolemic shock if fluid loss compromises the circulatory system. Disturbances of water and electrolyte balance may develop without dehydration, such as severe hyponatremia caused by excessive water intake (“water poisoning”) and severe hypernatremia caused by excess sodium intake. Disturbances of acid/base balance may also accompany fluid and electrolyte disorders and are common in diabetic ketoacidosis.

What Are the Basics of Fluid and Electrolyte Management?

The total daily fluid requirement is the sum of maintenance + deficit + ongoing losses.

Maintenance fluids are required daily to replace physiologic fluid and electrolytes losses (Table 26-1). Holliday and Segar demonstrated that daily water and electrolyte needs depend on metabolic rate, which is highest in infants and young children. The healthy child’s maintenance fluids are provided orally. Maintenance needs are increased by several mechanisms, including fever. A fluid deficit may result if adequate maintenance is not provided.

Table 26-1 Physiologic Fluid Losses

Deficit develops when a pathologic process, such as vomiting or diarrhea, causes excessive fluid loss, leading to dehydration. If a child does not drink an amount adequate to meet maintenance needs, a deficit develops. Fever can lead to a deficit because it causes an increase of insensible losses by approximately 13% for each degree above 38° C. Burns cause a marked loss of fluid from skin surfaces.

Ongoing losses from surgical drains or nasogastric tubes must be replaced regularly with fluid that has the appropriate water and electrolyte composition.

How Is Dehydration Classified?

Dehydration is classified by the serum sodium concentration:

Isonatremic dehydration is defined by serum sodium between 130 to 150 mEq/L, although careful monitoring of serum sodium is important when it is below 135 mEq/L, especially in conditions that cause vascular contraction, metabolic stress, and pain, all of which stimulate antidiuretic hormone (ADH) secretion that promotes water retention and a further drop in serum sodium to dangerous levels. Isonatremia is identified in approximately 85% of dehydrated children. Water and sodium are lost in amounts equivalent to the serum concentrations.

Hyponatremic dehydration is defined by serum sodium below 130 mEq/L, although some use a cutoff of 132 mEq/L. Hyponatremia develops in approximately 10% to 15% of dehydrated children. As mentioned, conditions that stimulate ADH secretion promote hyponatremia. Infants with cystic fibrosis or salt-losing congenital adrenal hyperplasia are particularly likely to develop hyponatremic dehydration. Dehydrated patients with hyponatremia demonstrate a greater degree of vascular insufficiency than do those with isonatremic dehydration, and hypovolemic shock is common. Seizures may occur if serum sodium falls below 120 mEq/L. Hypoxemia may occur in severe hyponatremia and poses a high risk of brain dysfunction and damage. Severe hyponatremia requires prompt correction of the sodium deficit using a 3% solution of NaCl.

Hypernatremic dehydration occurs uncommonly but represents a medical emergency in many cases. When serum sodium concentration increases above 150 mEq/L, there is the potential for cerebral edema and central pontine myelinolysis. Fluid and sodium abnormalities associated with hypernatremia must be corrected slowly, usually over a 48-hour time span.

Diabetic ketoacidosis is a special case and demands careful assessment of dehydration, metabolic state, and mental status. Assessment of serum electrolytes, acid/base status, and glucose are key to successful management. Intravenous fluids and insulin must be administered with caution: Rapid rehydration and/or rapid correction of hyperglycemia may trigger development of cerebral edema.

How Do I Determine Maintenance Fluid Needs?

Calculation of maintenance based on body weight most often suffices beyond the newborn period (> 28 days of age) (Table 26-2). In this formulation adapted from Holliday and Segar’s work, age and weight reflect metabolic rate.

Table 26-2 Daily Maintenance Water and Electrolytes Beyond the Newborn Period

How Does the History Help Identify Dehydration?

Dehydration risk is estimated from information about the fluids consumed, the fluids lost, and the underlying illness. Knowledge of the child’s maintenance fluid requirement helps you assess the adequacy of fluid intake. Remember that fever increases insensible water loss. The duration and severity of fever, diarrhea, and/or vomiting give clues to the potential severity of dehydration. History of decreased urine output suggests impending or actual dehydration. If a child has gone long periods without urine output, severe dehydration is likely. If a child has diarrhea, decreased fluid intake magnifies stool water loss and increases risk for hypernatremia. Loss of bicarbonate in diarrheal stool promotes metabolic acidosis. Persistent vomiting quickly depletes an infant’s total body water, and the HCl loss from the stomach results in metabolic alkalosis and hypochloremia. Such vomiting is classically seen with pyloric stenosis. Burns, colostomy tubes, recent surgery, and diabetes all promote fluid and electrolyte loss.

Does the Physical Examination Detect Dehydration?

Acute water loss in a dehydrating illness results in an acute weight loss, but the amount of water lost is not exactly the same as the weight loss. For purposes of calculation, the three categories of dehydration are assigned percent values to estimate water loss. For infants less than 20 kg, dehydration is classified as mild (5%), moderate (10%), or severe (15%). An estimate of the severity of dehydration is based on physical findings (Table 26-3). In mild dehydration, physical evidence is usually lacking (or at most minimal). As dehydration worsens, physical findings become more obvious. Capillary refill is the most reliable and reproducible marker of hydration status.

Table 26-3 Physical Examination Findings for Dehydration

How Do I Know If Emergency (“Bolus”) Fluids Are Needed?

Hypovolemic shock is the most common reason for emergency intravenous fluid. The presence of lethargy, prominent tachycardia, mottled and cool skin, delayed capillary refill, and prominent tenting indicate that an infant or child is in shock. A rapid infusion of isotonic fluid supports the circulation and reverses signs of shock. Occasionally, a rapid infusion of isotonic fluid for a vomiting child who has only mild-moderate dehydration will reduce nausea and allow consumption of oral fluids without further vomiting.

Are Laboratory Tests Needed to Evaluate Dehydration?

If dehydration is severe, and especially if vomiting is prominent, serum sodium is particularly important to monitor. Other laboratory tests include serum electrolytes, bicarbonate or total CO₂, and creatinine. Arterial oxygen should be monitored in hyponatremia. Urinalysis is useful to assess specific gravity and presence of ketones. Additional tests such as blood glucose are ordered based on the illness history. Tests are usually not needed for mild-moderate dehydration.

What Are the Steps to Treat Severe Dehydration?

This section emphasizes isonatremic dehydration. Calculations for hyponatremia, hypernatremia, and diabetic ketoacidosis are more complicated (see Bibliography).