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 gastrointestinal 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 or ileus).
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 bicarbonate may be lost with diarrhea but not with vomiting). However, fluid lost always contains a lower concentration of sodium than the plasma. Thus, in the absence of any fluid replacement, serum sodium 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 sodium 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 of dehydration vary according to degree of deficit (see Table: Clinical Correlates of Dehydration) and by the serum sodium 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 but are actually closer to hypotension and cardiovascular collapse than are equally dehydrated children with elevated or normal sodium levels.
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 metabolic alkalosis) are more common, and for children who need IV fluid therapy. Other laboratory abnormalities in dehydration include relative polycythemia resulting from hemoconcentration, elevated blood urea nitrogen (BUN), and increased urine specific gravity.
Treatment of dehydration is best approached by considering the following separately:
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 a nasogastric tube (see Oral Rehydration : Solutions).
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 blood pressure and perfusion. The resuscitation phase should reduce moderate or severe dehydration to a deficit of about 8% body weight. If dehydration is moderate, 20 mL/kg (2% body weight) is given IV over 20 to 30 minutes, reducing a 10% deficit to 8%. If dehydration is severe, 3 boluses of 20 mL/kg (6% body weight) may be required. The end point of the fluid resuscitation phase is reached when peripheral perfusion and blood pressure are restored and the heart rate is returned to normal (in an afebrile child).
Total deficit volume is estimated clinically as described previously. Sodium deficits are usually about 60 mEq/L (60 mmol/L) of fluid deficit, and potassium deficits are usually about 30 mEq/L (30 mmol/L) of fluid deficit. The resuscitation phase should have reduced moderate or severe dehydration to a deficit of about 8% body weight; this remaining deficit can be replaced by providing 10 mL/kg/hour (1% body weight/hour) for 8 hours. Because 0.45% saline has 77 mEq sodium per liter (77 mmol/L), 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 (50 to 100 mmol/L—see Table: Estimated Electrolyte Deficits by Cause); 0.9% saline may be used as well. Potassium replacement (usually by adding 20 to 40 mEq potassium per liter [20 to 40 mmol/L] of replacement fluid) should not begin until adequate urine output is established.
Volume of ongoing losses should be measured directly (eg, nasogastric tube, 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 electrolyte abnormalities fail to respond to replacement therapy.
(See also the American Academy of Pediatrics' clinical practice guideline for maintenance IV fluids in children.)
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 hours, which approximates fluid needs in mL/24 hours (see Holliday-Segar Formula for Maintenance Fluid Requirements by 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).
The traditional approach to calculating the composition of maintenance fluids was also based on the Holliday-Segar formula. According to that formula, patients require
(NOTE: 2 to 3 mEq/100 mL is equivalent to 20 to 30 mEq/L [20 to 30 mmol/L].)
This calculation indicates that maintenance fluid should consist of 0.2% to 0.3% saline with 20 mEq/L (20 mmol/L) of potassium in a 5% dextrose solution. Other electrolytes (eg, magnesium, calcium) are not routinely added. Normally, serum osmolarity controls moment-to-moment ADH release. Antidiuretic hormone (ADH) release can also occur in response to vascular volume and not osmolarity (nonosmotic ADH release). Recent literature suggests that hospitalized dehydrated children receiving 0.2% saline for maintenance fluid sometimes develop hyponatremia. This development is likely due to volume-related ADH release as well as to significant amounts of stimuli-related ADH release (eg, from stress, vomiting, dehydration, hypoglycemia). The ADH causes increased free water retention. Iatrogenic hyponatremia may be a greater problem for more seriously ill children and those who are hospitalized after surgery where stress plays a bigger role.
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. The most recent American Academy of Pediatrics' clinical practice guideline (2018) recommends all patients 28 days to 18 years of age receive isotonic solutions with appropriate potassium chloride and dextrose as maintenance IV fluids. This change also has the benefit of allowing use of the same fluid to replace ongoing losses and supply maintenance needs, which simplifies management. Although practice variation still exists in choosing appropriate maintenance IV fluids, all clinicians agree the important point is to closely monitor dehydrated patients receiving IV fluids, which includes monitoring of serum electrolyte levels.
Holliday-Segar Formula for Maintenance Fluid Requirements by Weight
A 7-month-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 hours and refusing to drink. Clinical findings of dry mucous membranes, poor skin turgor, markedly decreased urine output, and tachycardia with normal blood pressure and capillary refill suggest 10% fluid deficit. Rectal temperature is 37° C. Serum measurements are sodium, 136 mEq/L (136 mmol/L); potassium, 4 mEq/L (4 mmol/L); chloride, 104 mEq/L (104 mmol/L); and bicarbonate, 20 mEq/L (20 mmol/L).
Fluid volume is estimated by deficits, ongoing losses, and maintenance requirements.
The total fluid deficit given 1 kg weight 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 mL/24 hours or 40 mL/hour.
Electrolyte losses resulting from diarrhea in a eunatremic patient (see Table: Estimated Electrolyte Deficits by Cause) are an estimated 80 mEq of sodium and 80 mEq of potassium.
Traditional rehydration calculations aim to precisely estimate electrolyte losses and select replacement fluids that provide that specific amount. Although this process aids understanding of the pathophysiology of fluid balance, in practice, many pediatric centers no longer calculate precise electrolyte requirements. Instead, they simply use isotonic fluid for resuscitation and then a single fluid, either 0.9% or 0.45% saline in 5% dextrose, for deficits, ongoing losses, and maintenance. This simpler approach minimizes the chance of an arithmetic error, allows use of a single IV pump, and appears to result in similar clinical outcomes.
Residual fluid deficit is 800 mL (1000 initial − 200 mL resuscitation). This residual amount is given over the next 24 hours. Typically, half (400 mL) is given over the first 8 hours (400 ÷ 8 = 50 mL/hour) and the other half is given over the next 16 hours (25 mL/hour).
The estimated residual sodium deficit is 54 mEq (80 − 26 mEq). The fluid used is 5% dextrose/0.45% saline or 5% dextrose/0.9% saline. This amount replaces the sodium deficit (when using 0.45% saline, 0.8 L × 77 mEq sodium/L [77 mmol/L] = 62 mEq sodium); the additional 62 mEq of sodium given by using 0.9% saline is not clinically significant as long as renal function is intact.
When urine output is established, potassium is added at a concentration of 20 mEq/L (20 mmol/L; for safety reasons, no attempt is made to replace complete potassium deficit acutely).
Five percent dextrose/0.9% saline is given at 40 mL/hour with 20 mEq/L (20 mmol/L) of potassium added when urine output is established. Alternatively, the deficit could be replaced during the initial 8 hours followed by the entire day’s maintenance fluid in the next 16 hours (ie, 60 mL/hour); 24 hours of maintenance fluid given in 16 hours 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).
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Academy of Pediatrics: Clinical practice guideline for maintenance intravenous fluids in children