The Child With Fluid and Electrolyte Imbalance

Water is retained in the body in a relatively constant amount and, with few exceptions, is freely exchangeable among all body fluid compartments. The proximity of the extravascular compartment to the cells allows for continuous change in volume and distribution of fluids, largely determined by solutes (especially sodium) and physical forces (Fig. 23.1). Transport mechanisms are the basis for all activity within the cells, and because the cells have limited ability to store materials, movement in and out of cells must be rapid. Internal control mechanisms are responsible for distribution and maintenance of fluid balance (Box 23.1).

Changes in Fluid Volume Related to Growth

The percentage of TBW varies among individuals, and in adults and older children, it is related primarily to the amount of body fat. Consequently, females, who have more body fat than males, and obese persons tend to have less water content in relation to weight.

The fetus is composed primarily of water, with little tissue substance. As the organism grows and develops, a progressive decrease occurs in TBW, with the fastest rate of decline taking place during fetal life. The changes in water content and distribution that occur with age reflect the changes that take place in the relative amounts of bone, muscle, and fat making up the body. At maturity the percentage of TBW is somewhat higher in the male than in the female and is probably a result of the differences in body composition, particularly fat and muscle content (Fig. 23.2).

Another important aspect of growth change as it corresponds to water distribution is related to the ICF and ECF compartments. In the fetus and prematurely born infant, the largest proportion of body water is contained in the ECF compartment. As growth and development proceed, the proportion within the ECF compartment decreases as the ICF and cell solids increase. The ECF diminishes rapidly from approximately 40% of body weight at birth to less than 30% at 1 year of age. The different effects on males and females become apparent at puberty.

Water Balance in Infants

Because of several characteristics, infants and young children have a greater need for water and are more vulnerable to alterations in fluid and electrolyte balance. Compared with older children and adults, they have a greater fluid intake and output relative to size. Water and electrolyte disturbances occur more frequently and more rapidly, and children adjust less promptly to these alterations.

The fluid compartments in the infant vary significantly from those in the adult, primarily because of an expanded extracellular compartment. The ECF compartment constitutes more than half of the TBW at birth and has a greater relative content of extracellular sodium and chloride. The infant loses a considerable amount of fluid in the first few days after birth and still maintains a larger amount of ECF than the adult until about 2 to 3 years of age. This contributes to greater and more rapid water loss during this age period.

Fluid losses create compartment deficits that reflect the duration of dehydration. In general, approximately 60% of fluid is lost from the ECF, and the remaining 40% comes from the ICF. The amount of fluid lost from the ECF increases with acute illness and decreases with chronic loss.

Fluid losses may be divided into insensible, urinary, and fecal losses and vary with the patient's age. Approximately two-thirds of insensible water losses occurs through the skin, and the remaining one-third is lost through the respiratory tract. Environmental heat and humidity, skin integrity, body temperature, and respiratory rate all influence insensible fluid loss. Infants and children have a much greater tendency to become highly febrile than do adults. Fever increases insensible water loss by approximately 7 ml/kg/24 hr for each 1°F rise in temperature above 37.2°C (99°F). Fever and increased surface area relative to volume both contribute to greater insensible fluid losses in young patients.

Types of Dehydration

Because sodium is the primary osmotic force that controls fluid movement between the major fluid compartments, dehydration is often described according to plasma sodium concentrations (e.g., isonatremic, hyponatremic, or hypernatremic). Other osmotic forces, however, such as glucose in diabetic ketoacidosis and protein in nephrotic syndrome, may also play a dominant role. Consequently, dehydration is conventionally classified as isotonic, hypotonic, or hypertonic.

Isotonic (isosmotic or isonatremic) dehydration occurs in conditions in which electrolyte and water deficits are present in approximately balanced proportions. This is the primary form of dehydration occurring in children. The observable fluid losses are not necessarily isotonic, but losses from other avenues make adjustments so that the sum of all losses, or the net loss, is isotonic. Because no osmotic force is present to cause a redistribution of water between the ICF and ECF, the major loss is sustained from the ECF compartment. This significantly reduces the plasma volume and thus the circulating blood volume, with its effect on the skin, muscles, and kidneys. Shock is the greatest threat to life in isotonic dehydration, and the child with isotonic dehydration displays symptoms characteristic of hypovolemic shock. Plasma sodium remains within normal limits, between 130 and 150 mEq/L (Friedman, Beck DeGroot, et al., 2015; Huether, 2014).

Hypotonic (hyposmotic or hyponatremic) dehydration occurs when the electrolyte deficit exceeds the water deficit. Because ICF is more concentrated than ECF in hypotonic dehydration, water transfers from the ECF to the ICF to establish osmotic equilibrium. This movement further increases the ECF volume loss, and shock is a frequent result. Because there is a greater proportional loss of ECF in hypotonic dehydration, the physical signs tend to be more severe with smaller fluid losses than in isotonic or hypertonic dehydration. Plasma sodium concentrations are typically less than 130 mEq/L (Huether, 2014).

Hypertonic (hyperosmotic or hypernatremic) dehydration results from water loss in excess of electrolyte loss and is usually caused by a proportionately larger loss of water or a larger intake of electrolytes. This type of dehydration is the most dangerous and requires much more specific fluid therapy. This sometimes occurs in infants with diarrhea who are given fluids by mouth that contain large amounts of solute or in children receiving high-protein nasogastric tube feedings that place an excessive solute load on the kidneys. In hypertonic dehydration, fluid shifts from the lesser concentration of the ICF to the ECF. Plasma sodium concentration is greater than 150 mEq/L (Huether, 2014).

Because the ECF volume is proportionately larger, hypertonic dehydration consists of a greater degree of water loss for the same intensity of physical signs. Shock is less apparent in hypotonic dehydration. However, neurologic disturbances, such as seizures, are more likely to occur. Cerebral changes are serious and may result in permanent damage. These include disturbance of consciousness, poor ability to focus attention, lethargy, increased muscle tone with hyperreflexia, and hyperirritability to stimuli (e.g., tactile, auditory, bright lights).

Degree of Dehydration

A determination of the type and degree of dehydration is necessary to develop an effective plan of therapy. The degree of dehydration has been described as a percentage of body weight dehydrated: mild—less than 3% in older children or less than 5% in infants; moderate—5% to 10% in infants and 3% to 6% in older children; and severe—more than 10% in infants and more than 6% in older children (Carson, Mudd, & Madati, 2016; Greenbaum, 2015). Water constitutes only 60% to 70% of the infant's weight. However, adipose tissue contains little water and is highly variable in individual infants and children. A more accurate means of describing dehydration is to reflect acute fluid loss (time frame of ≥48 hours) in milliliters per kilogram of body weight. For example, a loss of 50 ml/kg is considered to be a mild fluid loss, whereas a loss of 100 ml/kg produces severe dehydration.

Weight is the most important determinant of the percent of total body fluid loss in infants and younger children. However, often the preillness weight is unknown. Other predictors of fluid loss include a changing level of consciousness (irritability to lethargy), altered response to stimuli, decreased skin elasticity and turgor, prolonged capillary refill (>2 seconds), increased heart rate, and sunken eyes and fontanels.

Clinical signs provide clues to the extent of dehydration (Table 23.3). The earliest detectable sign is usually tachycardia, followed by dry skin and mucous membranes, sunken fontanels, signs of circulatory failure (coolness and mottling of extremities), loss of skin elasticity, and prolonged capillary filling time (see Table 23.4 for clinical manifestations of dehydration and Fig. 23.3 for signs of dehydration). There is evidence that the clinical signs of abnormal capillary refill, abnormal skin turgor, and abnormal respiratory pattern are the most useful in predicting dehydration of 5% or more in children (Carson, Mudd, & Madati, 2017).

Compensatory mechanisms attempt to maintain fluid volume by adjusting to these losses. Interstitial fluid moves into the vascular compartment to maintain the blood volume in response to hemoconcentration and hypovolemia, and vasoconstriction of peripheral arterioles helps maintain pumping pressure. When fluid losses exceed the body's ability to sustain blood volume and blood pressure, circulation is seriously compromised and the blood pressure falls. This results in tissue hypoxia with accumulation of lactic acid, pyruvate, and other acid metabolites, which contribute to the development of metabolic acidosis.

Renal compensation is impaired by reduced blood flow through the kidneys, and little urine is formed. Increased serum osmolality stimulates the secretion of antidiuretic hormone (ADH) to conserve fluid and initiates the renin-angiotensin mechanisms in the kidney, causing further vasoconstriction. Aldosterone is released to promote sodium retention and conserve water in the kidneys. If dehydration increases in severity, urine formation is greatly diminished and metabolites and hydrogen ions that are normally excreted by this route are retained.

Shock, a common manifestation of severe depletion of ECF volume, is preceded by tachycardia and signs of poor perfusion and tissue oxygenation (by pulse oximeter readings). Peripheral circulation is poor as a result of reduced blood volume; therefore the skin is cool and mottled with decreased capillary filling. Impaired kidney circulation often leads to oliguria and azotemia. Although low blood pressure may accompany other symptoms of shock, in infants and young children it is usually a late sign and may herald the onset of cardiovascular collapse (see p. 967, Congestive Heart Failure).

Diagnostic Evaluation

To initiate a therapeutic plan, several factors must be determined:

• • The degree of dehydration based on physical assessment
• • The type of dehydration based on the pathophysiology of the specific illness responsible for the dehydrated state
• • Specific physical signs other than general signs
• • Initial plasma sodium concentrations
• • Serum bicarbonate concentration
• • Any associated electrolyte (especially serum potassium) and acid-base imbalances (as indicated)

Initial and regular, ongoing evaluations assess the patient's progress toward equilibrium and the effectiveness of therapy.

In the examination of an infant or younger child, one of the most important determinants of the extent of dehydration is body weight because this can assist in determining the percentage of total body fluid lost; however, because the preillness weight is often unknown, clinical manifestations must be evaluated (see Research Focus box). Important clinical manifestations include changing sensorium (irritability to lethargy); decreased response to stimuli; integumentary changes (decreased elasticity and turgor); prolonged capillary refill; increased heart rate; sunken eyes; and, in infants, sunken fontanels. Using multiple predictors increases the sensitivity of assessing the fluid deficit, and studies have shown a reasonably high degree of agreement between experienced observers in assessment of the level of dehydration. Objective signs of dehydration are present at a fluid deficit of less than 5%.

Therapeutic Management

Medical management is directed at correcting the fluid imbalance and treating the underlying cause. When the child is alert, awake, and not in danger, correction of dehydration may be attempted with oral fluid administration. Most cases of dehydration are mild and can be managed at home by this method. Several commercial rehydration fluids are available for use (see Table 25.3). Oral rehydration management consists of replacement of fluid loss over 4 to 6 hours, replacement of continuing losses, and provision for maintenance fluid requirements. In general, the mildly dehydrated child may be given 50 ml/kg of oral rehydration solution (ORS), whereas the child with moderate dehydration may be given 100 ml/kg of ORS. The child with fluid losses from diarrhea may be given an additional 10 ml/kg for each stool (Greenbaum, 2015). Amounts and rates are determined from body weight and severity of dehydration and are increased if rehydration is incomplete or if excess losses continue, until the child is well hydrated and the basic problem is under control.

The child may not be thirsty even though dehydrated and may refuse oral fluids initially for fear of continued emesis (if occurring) or because of decreased strength, oral stomatitis, or thrush. In such children rehydration may proceed by administering 2 to 5 ml of ORS by a syringe or small medication cup every 2 to 3 minutes until the child is able to tolerate larger amounts; if the child has emesis, administering small amounts (5 ml) of ORS every 5 minutes or so may help overcome fluid deficit, and the emesis will often lessen over time (Hendrickson, Zaremba, Wey, et al., 2017). Evidence indicates that oral administration of ondansetron (Zofran) to children with acute gastroenteritis and vomiting reduces emesis and increases time to oral rehydration, thus preventing intravenous (IV) therapy (Carter & Fedorowicz, 2012; Hendrickson, Zaremba, Wey, et al., 2017). Oral rehydration therapy (ORT) is effective for treating mild or moderate dehydration in children, is less expensive, and involves fewer complications than therapy (Carson, Mudd, Madati, 2016; Kleinman & Greer, 2014). ORSs enhance and promote the reabsorption of sodium and water. These solutions greatly reduce vomiting and the need for IV infusions (Hendrickson, Zaremba, Wey, et al., 2017). ORSs, including lower-osmolarity ORS (224 mmol/L), are available in the United States as commercially prepared solutions and are successful in treating the majority of infants with dehydration. See the Quality Patient Outcomes box. (See Diarrhea, Chapter 25, for a complete discussion of fluid replacement therapy for dehydration.)

Mechanisms of Edema Formation

A defect of any of the homeostatic mechanisms maintaining fluid balance can cause accumulation of interstitial fluid. Disequilibrium results from anything that (1) alters the retention of sodium, such as renal disease or hormonal influences; (2) affects the formation or destruction of plasma proteins, such as starvation or liver disease; or (3) alters membrane permeability, such as minimal change nephrotic syndrome or trauma.

Edema may be localized to a small or large area, such as that occurring in urticaria, infection, and pulmonary congestion, or it can be generalized, as in the hypoproteinemia of the nephrotic syndrome and starvation. A severe, generalized accumulation of great amounts of fluid in all body tissues is termed anasarca.

Assessment

Generalized edema resulting from any of the previously listed types is manifested by swelling in the extremities, face, perineum, and torso. Loss of normal skin creases may be assessed. Daily weights are more sensitive indicators of water gain or loss and should be obtained. Abdominal girth measurement changes may also be an indicator of edema in children. Pitting edema may occur and can be assessed by pressing the fingertip against a bony prominence for 5 seconds. If the tissue rebounds immediately on removing the finger, the patient does not have pitting edema. A quick way to determine the severity is to measure the degree of pitting edema (Fig. 23.4).

Therapeutic Management

The primary goal in the management of edema is treatment of the underlying disease process, which is discussed elsewhere in relation to the specific disorder. However, an essential aspect in the management of any fluid overload is early recognition, in which nurses play a vital role. The management of edema is discussed throughout the text with specific conditions. See the Quality Patient Outcomes box.