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Dehydration versus volume depletion
The terms dehydration and volume depletion are commonly used interchangeably but they refer to different physiologic conditions resulting from different types of fluid loss. Volume depletion denotes reduction of effective circulating volume in the intravascular space, whereas dehydration denotes loss of free water in greater proportion than the loss of sodium. The distinction is important because volume depletion and dehydration can exist independently or concurrently and the treatment for each is different. However, much of clinical literature does not differentiate between the 2 conditions; this article will therefore follow this convention and use the terms dehydration, hypovolemia, and volume depletion interchangeably to refer to intravascular fluid deficits here.
Body fluid distribution
The body contains 2 major fluid compartments: the intracellular fluid (ICF) and the extracellular fluid (ECF). The ICF comprises of two thirds of the total body water (TBW), while the ECF accounts for the remaining third. The ECF is further divided into the interstitial fluid (75%) and plasma (25%). The TBW comprises approximately 70% of body weight in infants, 65% in children, and 60% in adults.
Infants' and children’s higher body water content, along with their higher metabolic rates and increased body surface area to mass index, contribute to their higher turnover of fluids and solute. Therefore, infants and children require proportionally greater volumes of water than adults to maintain their fluid equilibrium and are more susceptible to volume depletion. Significant fluid losses may occur rapidly, leading to depletion of the intravascular volume.
Volume depletion can be concurrent with hyponatremia. This is characterized by plasma volume contraction with free water excess. An example is a child with diarrhea who has been given water to replace diarrheal losses. Free water is replenished, but sodium and other solutes are not.
In hyponatremic volume depletion, the patient may appear more ill clinically than actual fluid losses would otherwise indicate. The degree of volume depletion may be clinically overestimated. Serum sodium levels less than 120 mEq/L may result in seizures—the risk of seizure is much higher in the setting of acute onset of hyponatremia, as opposed to gradual onset. If intravascular free water excess is not corrected during volume replenishment, the shift of free water to the intracellular fluid compartment may cause cerebral edema, especially in children.
In hypernatremic volume depletion, plasma volume contracts with a disproportionately larger loss of free water. An example is the child with diarrhea whose fluid losses have been replenished with hypertonic soup, boiled milk, water and baking soda, or improperly diluted infant formula. Volume has been restored, but free water has not. The degree of volume depletion may be underestimated and the patient may appear less ill clinically than fluid losses indicate. Usually, at least a 10% volume deficit exists with hypernatremic volume depletion.
As in hyponatremia, hypernatremic volume depletion may result in serious central nervous system (CNS) effects as a result of structural changes in central neurons. However, cerebral shrinkage occurs instead of cerebral edema. This may result in intracerebral hemorrhage, seizures, coma, and death. Overly rapid correction of hypernatremia, however, may result in cerebral edema. For this reason, volume restoration should be performed gradually over 24 hours or more. Gradual restoration prevents a rapid shift of fluid across the blood-brain barrier and into the intracellular fluid compartment.
Potassium shifts between intracellular and extracellular fluid compartments occur more slowly than free water shifts. Serum potassium levels may not reflect intracellular potassium levels. Although a potassium deficit is present in all patients with volume depletion, it is not usually clinically significant. However, failure to correct for a potassium deficit during volume replacement may result in clinically significant hypokalemia. Potassium should not be added to replacement fluids until adequate urine output is obtained.
Acid and base problems
The most common acid-base derangement that occurs with volume depletion, especially in infants, is metabolic acidosis. Mechanisms include bicarbonate loss in stool, ketone production from starvation, and lactic acid production from decreased tissue perfusion in hypovolemia. Decreased renal perfusion also causes decreased glomerular filtration rate, which, in turn, leads to decreased hydrogen (H+) ion excretion. These factors can combine to produce a metabolic acidosis.
In most patients, acidosis is mild and easily corrected with volume restoration; increased renal perfusion permits excretion of excess H+ ions in the urine. Administration of glucose-containing fluids further decreases ketone production.
The mechanisms of dehydration may be broadly divided into 3 categories: (1) decreased intake due to diseases such as stomatitis, (2) increased output from diarrhea or osmotic diuresis from uncontrolled diabetes mellitus, and (3) increased insensible losses such as with fever.
Pediatric dehydration is frequently the result of increased output from gastroenteritis, characterized by vomiting and diarrhea. However, vomiting and diarrhea may be caused by other processes as summarized below.
CNS causes of vomiting include the following:
Increased intracranial pressure
Psychogenic vomiting is not seen in infants and is rare in children
GI causes of vomiting include the following:
Drug toxicity (ingestion, overdose, drug effects)
Endocrine causes of vomiting include the following:
Renal causes of vomiting include the following:
Renal tubular acidosis
GI causes of diarrhea include the following:
Malabsorption (eg, milk intolerance, excessive fruit juice)
Irritable bowel syndrome
Inflammatory bowel disease
Short gut syndrome
Endocrine causes of diarrhea include the following:
Congenital adrenal hypoplasia
Volume depletion from increased output not caused by vomiting or diarrhea may be divided into renal or extrarenal causes. Renal causes of volume depletion include the following:
Use of diuretics
Renal tubular acidosis
Renal failure (eg, renal trauma, urinary tract obstruction, salt-wasting nephritis)
Diabetes insipidus, hypothyroidism, and adrenal insufficiency
Extrarenal causes of volume depletion include the following:
Third-space extravasation of intravascular fluid (eg, pancreatitis, peritonitis, sepsis, heart failure)
Other causes of volume depletion as mentioned above include poor oral intake and insensible losses from fever, sweating, burns, or pulmonary processes.
Dehydration is a common complication of illness observed in pediatric patients presenting to the emergency department (ED). Early recognition and early intervention are important to reduce risk of progression to hypovolemic shock and end-organ failure.
In most cases, volume depletion in children is caused by fluid losses from vomiting or diarrhea. On physical examination, combinations of findings can be used to determine the degree of dehydration. Laboratory studies are of limited utility in cases of mild dehydration, but they may be considered under certain conditions and are recommended in patients with more severe dehydration.
Mild or moderate volume depletion should be treated with oral rehydration when possible. Intravenous fluid therapy is necessary when oral therapy fails or volume depletion is severe.
Dehydration, particularly from gastroenteritis, is a common pediatric complaint in the ED. Approximately 30 million children are affected annually, with 1.5 million presenting to outpatient care, 200,000 requiring hospitalizations, and 300 dying in the United States.
Worldwide, according to the Centers for Disease Control and Prevention (CDC), for children younger than 5 years, the annual incidence of diarrheal illness is approximately 1.5 billion, while deaths are estimated between 1.5 and 2.5 million per year. Though these numbers are staggering, they actually represent an improvement from the early 1980s, when the death rate was approximately 5 million per year.
Infants and younger children are more susceptible to volume depletion than older children. In general, however, pediatric patients with volume depletion have an excellent prognosis if they are appropriately treated.
Morbidity varies with the degree of volume depletion and the underlying cause. The severely volume-depleted infant or child is at risk for death from cardiovascular collapse. Hyponatremia resulting from replacement of free water alone may cause seizures. Improper management of volume repletion may cause iatrogenic morbidity or mortality.
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|Symptom||Degree of Dehydration|
|Mild (< 3% body weight lost)||Moderate (3-9% body weight lost)||Severe (>9% body weight lost)|
|Mental status||Normal, alert||Restless or fatigued, irritable||Apathetic, lethargic, unconscious|
|Heart rate||Normal||Normal to increased||Tachycardia or bradycardia|
|Quality of pulse||Normal||Normal to decreased||Weak, thready, impalpable|
|Breathing||Normal||Normal to increased||Tachypnea and hyperpnea|
|Eyes||Normal||Slightly sunken||Deeply sunken|
|Fontanelles||Normal||Slightly sunken||Deeply sunken|
|Tears||Normal||Normal to decreased||Absent|
|Skin turgor||Instant recoil||Recoil < 2 seconds||Recoil >2 seconds|
|Capillary refill||< 2 seconds||Prolonged||Minimal|
|Adapted from King CK, Glass R, Bresee JS, et al. Managing acute gastroenteritis among children: oral rehydration, maintenance, and nutritional therapy. MMWR Recomm Rep. Nov 21 2003;52(RR-16):1-16.|