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Dehydration is significant depletion of body water and, to varying degrees, electrolytes. Symptoms and signs include thirst, lethargy, dry mucosa, decreased urine output, and, as the degree of dehydration progresses, tachycardia, hypotension, and shock. Diagnosis is based on history and physical examination. Treatment is with oral or IV replacement of fluid and electrolytes.

Dehydration, usually caused by diarrhea, remains a major cause of morbidity and mortality in infants and young children worldwide. Infants are particularly susceptible to the ill effects of dehydration because of their greater baseline fluid requirements (due to a higher metabolic rate), higher evaporative losses (due to a higher ratio of surface area to volume), and inability to communicate thirst or seek fluid.

Etiology and Pathophysiology

Dehydration results from increased fluid loss, decreased fluid intake, or both.

The most common source of increased fluid loss is the GI tract from vomiting, diarrhea, or both (eg, gastroenteritis). Other sources are renal (eg, diabetic ketoacidosis), cutaneous (eg, excessive sweating, burns), and 3rd-space losses (eg, into the intestinal lumen in bowel obstruction). All types of lost fluid contain electrolytes in varying concentrations, so fluid loss is always accompanied by electrolyte loss.

Decreased fluid intake is common during serious illness of any kind and is particularly problematic when the child is vomiting and during hot weather. It may also be a sign of neglect.

Symptoms, Signs, and Diagnosis

Symptoms and signs vary according to degree of deficit (see Table 1: Dehydration and Fluid Therapy: Clinical Correlates of Dehydration) and are affected by serum Na concentration: hemodynamic findings are exaggerated by hyponatremia and reduced by hypernatremia. In general, dehydration without hemodynamic changes is considered mild (about 5% body wt in infants and 3% in adolescents); tachycardia defines moderate dehydration (about 10% body wt in infants and 6% in adolescents); and hypotension with impaired perfusion defines severe dehydration (about 15% body wt in infants and 9% in adolescents). A more accurate method with acute dehydration is change in body weight; all short-term weight loss > 1%/day is presumed to represent fluid deficit. However, this method depends on knowing a precise, recent pre-illness weight. Parental estimates are usually inadequate; a 1-kg error in a 10-kg child causes a 10% error in the calculated percentage of dehydration—the difference between mild and severe dehydration.

Table 1
Clinical Correlates of Dehydration Severity Fluid Deficit in mL/kg (percent body wt)* Signs Infants Adolescents Mild 50 (5%) 30 (3%) Slightly dry buccal mucous membranes, increased thirst, slightly decreased urine output Moderate 100 (10%) 50–60 (5–6%) Dry buccal mucous membranes, tachycardia, little or no urine output, lethargy, sunken eyes and fontanelles, loss of skin turgor Severe 150 (15%) 70–90 (7–9%) Same as moderate plus a rapid, thready pulse; no tears, cyanosis; rapid breathing; delayed capillary refill; hypotension; mottled skin; coma *Standard estimates for children between infancy and adolescence have not been established. For children between these age ranges, clinicians must estimate values between those for infants and those for adolescents based on clinical judgment.

Laboratory testing is usually reserved for moderately or severely ill children, in whom electrolyte disturbances (eg, hypernatremia, hypokalemia, metabolic acidosis) are more common. Other laboratory abnormalities include relative polycythemia from hemoconcentration, elevated BUN, and increased urine specific gravity.

Treatment

Treatment is best approached by considering separately the fluid resuscitation requirements, current deficit, ongoing losses, and maintenance requirements. The volume (eg, amount of fluid), composition, and rate of replacement differ for each. Formulas and estimates used to determine treatment parameters provide a starting place, but treatment requires ongoing monitoring of vital signs, clinical appearance, urine output and specific gravity, weight, and sometimes serum electrolyte levels. Children with severe dehydration (eg, evidence of circulatory compromise) should receive fluids IV. Those unable or unwilling to drink or who have repetitive vomiting can receive fluid replacement IV, through an NGT, or sometimes orally through frequently repeated small amounts (see Dehydration and Fluid Therapy: Solutions).

Resuscitation: Patients with signs of hypoperfusion should receive fluid resuscitation with boluses of isotonic fluid (eg, 0.9% saline or Ringer's lactate). The goal is to restore adequate circulating volume to restore BP and perfusion. The resuscitation phase should reduce moderate or severe dehydration to a deficit of about 8% body wt. If dehydration is moderate, 20 mL/kg (2% body wt) is given IV over 20 to 30 min, reducing a 10% deficit to 8%. If dehydration is severe, 3 boluses of 20 mL/kg (2% body wt) will likely be required. The end point of the fluid resuscitation phase is restoring peripheral perfusion and BP and returning increased heart rate toward normal.

Deficit replacement: Total deficit volume is estimated clinically as described previously. Na deficits are usually about 80 mEq/L of fluid deficit, and K deficits are usually about 30 mEq/L of fluid deficit. The resuscitation phase should have reduced moderate or severe dehydration to a deficit of about 8% body wt; this remaining deficit can be replaced by providing 10 mL/kg (1% body wt)/h for 8 h. Because 0.45% saline has 77 mEq Na per liter, it is usually an appropriate fluid choice. K replacement (usually by adding 20 to 40 mEq K per liter of replacement fluid) should not begin until adequate urine output is established.

Dehydration with significant hypernatremia (eg, serum Na > 160 mEq/L) or hyponatremia (eg, serum Na < 120 mEq/L) requires special consideration to avoid complications (see Metabolic, Electrolyte, and Toxic Disorders in Neonates: Treatment).

Ongoing losses: Volume of ongoing losses should be measured directly (eg, NGT, catheter, stool measurements) or estimated (eg, 10 mL/kg per diarrheal stool). Replacement should be milliliter for milliliter in time intervals appropriate for the rapidity and extent of the loss. Ongoing electrolyte losses can be estimated by source or cause (see Table 2: Dehydration and Fluid Therapy: Estimated Electrolyte Deficits by Cause). Urinary electrolyte losses vary with intake and disease process but can be measured if deficits fail to respond to replacement therapy.

Table 2
Estimated Electrolyte Deficits by Cause Cause Sodium (mEq/L) Potassium (mEq/L) Diarrhea     Isotonic dehydration 80 80 Hypotonic dehydration 100 80 Hypertonic dehydration 20 10 Pyloric stenosis 80 100 Diabetic ketoacidosis 80 50

Maintenance requirements: Fluid and electrolyte needs from basal metabolism must also be accounted for. Maintenance requirements are related to metabolic rate and affected by body temperature. Insensible losses (evaporative free water losses from the skin and respiratory tract in a ratio of 2:1) account for about ½ of maintenance needs.

Volume rarely must be exactly determined but generally should aim to provide an amount of water that does not require the kidney to significantly concentrate or dilute the urine. The most common estimate uses patient weight to calculate metabolic expenditure in kcal/24 h, which approximates fluid needs in mL/24 h (see Table 3: Dehydration and Fluid Therapy: Standard Basal Metabolic Rates Used for Calculating Maintenance Fluid Requirements*). A simpler calculation (the Holliday-Segar formula) uses 3 weight classes (see Table 4: Dehydration and Fluid Therapy: Holliday-Segar Formula for Maintenance Fluid Requirements by Weight). Body surface area derived from a nomogram (see Fig. 1: Dehydration and Fluid Therapy: Nomogram for calculating the body surface area of children.) also can be used, allowing 1500 to 2000 mL/m²/24 h. More complex calculations are rarely required. These fluid volumes can be given as a separate simultaneous infusion, so that the infusion rate for replacing deficits and ongoing losses can be set and adjusted independently of the maintenance infusion rate.

Table 3
Standard Basal Metabolic Rates Used for Calculating Maintenance Fluid Requirements* Wt (kg) kcal/24 h†‡ Male Both Sexes Female 3   140   5   270   7   400   9   500   11   600   13   650   15   710   17   780   19   830   21   880   25 1020   960 29 1120   1040 33 1210   1120 37 1300   1190 41 1350   1260 45 1410   1320 49 1470   1380 53 1530   1440 57 1590   1500 61 1640   1560 *Metabolic expenditure in kcal/24 h approximates maintenance fluid needs in mL/24 h. †Add or subtract 12% to basal metabolic rate (general metabolic rate in kcal/24 h) for each degree C above or below rectal temperature of 37.8° C. ‡Add 0–30% to basal metabolic rate for usual activity; hypometabolic or hypermetabolic states require greater adjustments.

Table 4
Holliday-Segar Formula for Maintenance Fluid Requirements by Weight Wt (kg) Water Electrolytes (mEq/L H2O) mL/day mL/h 0–10 kg 100/kg 4/kg Na 30, K 20 11–20 kg 1000 + 50/kg for each kg >10 40 + 2/kg for each kg > 10 Na 30, K 20 > 20 kg 1500 + 20/kg for each kg > 20 60 + 1/kg for each kg > 20 Na 30, K 20

Fig. 1
Nomogram for calculating the body surface area of children. (Adapted from Geigy Scientific Tables, ed. 8, vol. 1, edited by C Lentner. Basle, Switzerland, Ciba-Geigy Ltd., 1981, pp. 226–227; used with permission.)

Baseline estimates are affected by fever (increasing by 12% for each degree > 37.8° C), hypothermia, and activity (eg, increased for hyperthyroidism or status epilepticus, decreased for coma).

Composition differs from solutions used to replace deficits and ongoing losses. Patients require Na 3 mEq/100 kcal/24 h (3 mEq/100 mL/24 h) and K 2 mEq/100 kcal/24 h (2 mEq/100 mL/24 h). This need is met by using 0.2% to 0.3% saline with 20 mEq/L of K in a 5% dextrose solution. Other electrolytes (eg, Mg, Ca) are not routinely added. It is inappropriate to replace deficits and ongoing losses solely by increasing the amount or rate of maintenance fluids.

Practical Example

A 7-mo-old infant has diarrhea for 3 days with weight loss from 10 kg to 9 kg. The infant is currently producing 1 diarrheal stool q 3 h and refusing to drink. Clinical findings of dry mucous membranes, poor skin turgor, markedly decreased urine output, and tachycardia with normal BP and capillary refill suggest 10% fluid deficit. Rectal temperature is 37° C; serum Na, 136 mEq/L; K, 4 mEq/L; Cl, 104 mEq/L; and HCO₃, 20 mEq/L.

Fluid volume is estimated by deficits, ongoing losses, and maintenance requirements.

The total fluid deficit given 1 kg wt loss = 1 L.

Ongoing diarrheal losses are measured as they occur by weighing the infant's diaper before application and after the diarrheal stool.

Baseline maintenance requirements by the weight-based Holliday-Segar method are 100 mL/kg × 10 kg = 1000 mL/day = 1000/24 or 40 mL/h.

Electrolyte losses from diarrhea (see Table 2: Dehydration and Fluid Therapy: Estimated Electrolyte Deficits by Cause) are an estimated 80 mEq of Na and 80 mEq of K.

Procedure

Resuscitation: The patient is given an initial bolus of Ringer's lactate 200 mL (20 mL/kg × 10 kg) over 30 min. This replaces 26 mEq of the estimated 80 mEq Na deficit.

Deficits: Residual fluid deficit is 800 mL (1000 initial − 200 mL resuscitation), and Na deficit is 54 mEq (80 − 26 mEq). This is given over 8 h as 5% dextrose/0.45% saline at 100 mL/h. This replaces the Na deficit (0.8 L × 77 mEq Na/L = 62 mEq Na). When urine output is established, K is added at a concentration of 20 mEq/L (for safety reasons, no attempt is made to replace complete K deficit acutely).

Ongoing losses: Five percent dextrose/0.45% saline also is used to replace ongoing losses; volume and rate are determined by the amount of diarrhea.

Maintenance fluid: Five percent dextrose/0.2% saline is given at 40 mL/h with 20 mEq/L of K added when urine output is established. Alternatively, the deficit could be replaced during the initial 8 h followed by the entire day's maintenance fluid in the next 16 h (ie, 60 mL/h); 24 h of maintenance fluid given in 16 h reduces mathematically to a rate of 1.5 times the usual maintenance rate and obviates the need for simultaneous infusions (which may require 2 rate-controlling pumps).

Last full review/revision May 2007 by Kenneth B. Roberts, MD

Content last modified May 2007