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Dehydration is caused by a negative fluid balance resulting from decreased intake, increased output (renal, GI, or insensible losses from the skin or respiratory tract), or from systemic responses to specific disease states such as burns or sepsis. Dehydration causes total body water and electrolyte losses in the intracellular fluid (ICF) and extracellular fluid (ECF) compartments. Dehydration can present with a spectrum of signs and may be asymptomatic, mild, moderate, or severe with or without hypovolemic shock. Common causes of dehydration are listed in Table 142-1.

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Table 142-1 Common Causes of Dehydration
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Infants and children are particularly susceptible to dehydration for a number of developmental and physiologic reasons. They are dependent on caretakers to provide oral fluids and therefore cannot regulate their intake. In addition, both physiologic and pathophysiologic differences between children and adults lead to increased fluid requirements and increased fluid losses in young children. Rapidly growing infants have an extremely high basal metabolic rate and require a relatively high number of calories relative to their body weight; metabolism requires a correspondingly large amount of water. The daily turnover of free water in infants is up to three to four times that in adults. This includes increased insensible losses from the skin and respiratory tract, which are usually electrolyte free. Water loss through urination accounts for approximately 50% of daily fluid requirements and is the predominant cause of sensible losses; because infants have a decreased ability to concentrate urine, they lose a relatively large amount of free water through their kidneys in order to excrete solutes. In addition, a larger percentage of young infants’ total body water is contained in the extravascular space in comparison with older children and adults. This puts them at greater risk for cardiovascular compromise when confronted with sudden fluid losses.1,2

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Hypermetabolic states increase the need for free water. The most common of these are fever (which increases the free water requirement by approximately 12% per degree centigrade of elevation in temperature above normal), and increased sweating.

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Dehydration is often described in relation to serum sodium concentrations as isonatremic dehydration (sodium level of 130 to 150 mEq/L), hyponatremic dehydration (sodium level of <130 mEq/L), or hypernatremic dehydration (sodium level of >150 mEq/L). Isonatremic dehydration is the most common form of dehydration. Isonatremic (isotonic) dehydration occurs when the fluid sodium losses are proportionate in the ...

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