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Dehydration in Children
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 remains a major cause of morbidity and mortality in infants and young children worldwide. Dehydration is a symptom or sign of another disorder, most commonly diarrhea. 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.
Dehydration results from
The most common source of increased fluid loss is the GI tract—from vomiting, diarrhea, or both (eg, gastroenteritis—see Overview of 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 or ileus).
Decreased fluid intake is common during mild illnesses such as pharyngitis or during serious illnesses of any kind. Decreased fluid intake is particularly problematic when the child is vomiting or when fever, tachypnea, or both increase insensible losses. It may also be a sign of neglect.
All types of lost fluid contain electrolytes in varying concentrations, so fluid loss is always accompanied by some degree of electrolyte loss. The exact amount and type of electrolyte loss varies depending on the cause (eg, significant amounts of HCO 3 - may be lost with diarrhea but not with vomiting). However, fluid lost always contains a lower concentration of Na than the plasma. Thus, in the absence of any fluid replacement, serum Na rises (hypernatremia). Hypernatremia causes water to shift from the intracellular and interstitial space into the intravascular space, helping, at least temporarily, to maintain vascular volume. With hypotonic fluid replacement (eg, with plain water), serum Na may normalize but can also decrease (hyponatremia). Hyponatremia results in some fluid shifting out of the intravascular space into the interstitium at the expense of vascular volume.
Symptoms and signs vary according to degree of deficit (see Table: Clinical Correlates of Dehydration) and by the serum Na level. Because of the fluid shift out of the interstitium into the vascular space, children with hypernatremia appear more ill (eg, with very dry mucous membranes, a doughy appearance to the skin) for a given degree of water loss than do children with hyponatremia. However, children with hypernatremia have better hemodynamics (eg, less tachycardia and better urine output) than do children with hyponatremia, in whom fluid has shifted out of the vascular space. Dehydrated children with hyponatremia may appear only mildly dehydrated until closer to cardiovascular collapse and hypotension.
Clinical Correlates of Dehydration
In general, dehydration is defined as follows:
However, using a combination of symptoms and signs to assess dehydration is a more accurate method than using only one sign. Another way to assess the degree of dehydration in children 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 preillness 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.
Laboratory testing is usually reserved for moderately or severely ill children, in whom electrolyte disturbances (eg, hypernatremia, hypokalemia, metabolic acidosis or alkalosis) are more common, and for children who need IV fluid therapy. Other laboratory abnormalities in dehydration include relative polycythemia resulting from hemoconcentration, elevated BUN, and increased urine specific gravity.
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, weight, and sometimes serum electrolyte levels.
The American Academy of Pediatrics and the WHO both recommend oral replacement therapy for mild and moderate dehydration. Children with severe dehydration (eg, evidence of circulatory compromise) should receive fluids IV. Children who are unable or unwilling to drink or who have repetitive vomiting can receive fluid replacement orally through frequently repeated small amounts, through an IV, or through an NGT (see Practical Example : Solutions).
Patients with signs of hypoperfusion should receive fluid resuscitation with boluses of isotonic fluid (eg, 0.9% saline or lactated Ringer solution). 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, sometimes 3 boluses of 20 mL/kg will likely be required. The end point of the fluid resuscitation phase is reached when peripheral perfusion and BP are restored and the heart rate is returned to normal (in an afebrile child).
Total deficit volume is estimated clinically as described previously. Na deficits are usually about 60 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, particularly in children with diarrhea because the electrolyte content of diarrhea is typically 50 to 100 mEq/L (see Table: Estimated Electrolyte Deficits by Cause). K replacement (usually by adding 20 to 40 mEq K per liter of replacement fluid) should not begin until adequate urine output is established.
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: 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.
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) account for about one third of total maintenance water (slightly more in infants and less in adolescents and adults).
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 is the Holliday-Segar formula, which uses patient weight to calculate metabolic expenditure in kcal/24 h, which approximates fluid needs in mL/24 h (see Holliday-Segar Formula for Maintenance Fluid Requirements by Weight). The Holliday-Segar formula uses 3 weight classes because metabolic expenditure changes are based on weight. More complex calculations (eg, those using body surface area) are rarely required. Maintenance 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.
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. According to the Holliday-Segar formula, 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). (N ote : 2 to 3 mEq/100 mL/24 h is equivalent to a solution that is 20 to 30 mEq/L.) This need is met by using 0.2% to 0.3% saline with 20 mEq/L of K in a 5% dextrose solution. However, recent literature suggests that hospitalized dehydrated children receiving 0.2% saline for maintenance fluid sometimes develop hyponatremia, perhaps because they release significant amounts of antidiuretic hormone because of stimuli such as stress, vomiting, dehydration, and hypoglycemia, causing increased free water retention. Due to this possibility of iatrogenic hyponatremia, many centers are now using a more isotonic fluid such as 0.45% or 0.9% saline for maintenance in dehydrated children and reserving 0.2% saline for routine maintenance in nondehydrated children, eg, those who require IV fluids but can have nothing by mouth before a test or procedure. Iatrogenic hyponatremia may be a greater problem for more seriously ill children and those who are hospitalized after surgery. Although the appropriate fluid remains controversial, all clinicians agree the important point is to closely monitor dehydrated patients receiving IV fluids. 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.
Holliday-Segar Formula for Maintenance Fluid Requirements by Weight
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 every 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 HCO 3 , 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 resulting from diarrhea in a eunatremic patient (see Table: Estimated Electrolyte Deficits by Cause) are an estimated 80 mEq of Na and 80 mEq of K.
Residual fluid deficit is 800 mL (1000 initial − 200 mL resuscitation), and Na deficit is 54 mEq (80 − 26 mEq). This residual amount is given over the next 24 h. Typically, half (400 mL) is given over the first 8 h (400 ÷ 8 = 50 mL/h) and the other half is given over the next 16 h (25 mL/h). The fluid used is 5% dextrose/0.45% saline. This amount 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).
Five percent dextrose/0.2% or 0.45% 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).
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