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The most common disorder of fluid balance in children requiring emergency care is dehydration. Dehydration is the result of a negative fluid balance that can result from decreased fluid intake (mouth or throat disorders, systemic illness, neurologic illness, and other causes); increased output (vomiting, diarrhea, fever, environmental heat, respiratory illness, renal losses and other causes); or fluid shifts in conditions such as burns and sepsis.

The clinical appearance of patients with dehydration and fluid and electrolyte disturbances depends on the degree of dehydration, the rate at which the fluid was lost, and the age of the patient. Older children may tolerate a slow total body water loss as great as 40%, while rapid and large volume loss (eg, rotavirus or cholera) can cause rapid deterioration and cardiovascular collapse in young infants.

Though the gold standard for assessing dehydration is comparison of pre-illness weight with weight on presentation to the ED, a reliable and recent pre-illness weight is rarely available in the emergency department. Physical examination can provide an estimation of the degree of dehydration, which is typically classified as mild, moderate, or severe. Clinical signs and symptoms of dehydration are listed in Table 81-1 (also consider Table 73-1 if dehydration is due to GI losses). An important exception to the reliability of signs and symptoms to predict degree of dehydration occurs in hypernatremic dehydration, when fluid loss occurs primarily from the interstitial and intracellular spaces and clinical signs of intravascular volume depletion may be minimal. In this setting, however, the skin may have a characteristic doughy feel.

Table 81-1 Clinical Guidelines for Assessing Dehydration in Children

If available, the absolute and relative fluid deficit can be calculated from a pre-illness weight: 1 kilogram of weight loss is equivalent to 1 L of fluid deficit. In the absence of a reliable pre-illness comparison weight, the diagnosis of dehydration is based primarily on historical data and physical examination findings (Table 81-1).

Laboratory tests are not needed in mild to moderate cases of dehydration but may be helpful in some cases where results of a basic metabolic panel may help classify the type ...

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