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Dehydration occurs when there is fluid loss in excess of intake. Although fluid losses can occur for different reasons, gastrointestinal losses are the most common. Dehydration is one of the most frequent reasons for hospitalization in children. Dehydration can be classified as mild, moderate, or severe. Mild dehydration represents a less than 5% loss of body weight; moderate, 5% to 10%; and severe, greater than 10%. Isonatremic dehydration occurs when the serum sodium level is between 130 and 150 mEq/L. This type of dehydration, which is the most common, is the focus of this chapter (electrolyte abnormalities are discussed in Chapter 74).
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Dehydration is a general state in which there is a total-body fluid deficit. Under normal physiologic conditions, water constitutes 70% of lean body mass. In infants, the proportion is approximately 75%. Two-thirds of the fluid is intracellular, and one-third is extracellular. Of the extracellular fluid, 75% is interstitial and 25% is intravascular. Fluid that is lost from the body often has an electrolyte composition similar to that of plasma. Most of the fluid deficit during the early stages of dehydration is from the extracellular space, but over time, the fluid losses equilibrate, and fluid leaves the intracellular space. During the recovery phase, fluid administered to the patient is located in the extracellular space and needs time to equilibrate with the intracellular space.
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DIFFERENTIAL DIAGNOSIS
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Dehydration is not a disease but rather a symptom or consequence of another process. It can be thought of as a common final pathway. The clinician must search for the cause of dehydration, which can be due to decreased intake, increased losses, or a combination of the two (Table 73-1). Among the myriad causes of dehydration, a few stand out. The most common cause that brings patients to medical attention is acute gastroenteritis. Decreased intake secondary to stomatitis, pharyngitis, or viral respiratory disease is also fairly common.
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INITIAL STABILIZATION
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Careful attention to the ABCs (airway, breathing, circulation) is required for appropriate stabilization in any emergent situation. However, with dehydration, circulatory derangements are most common. In particular, it is important to identify signs of shock, which include poor peripheral perfusion, obtundation, severe tachycardia, and blood pressure changes—either narrowed pulse pressure or frank hypotension. Any dehydrated patient in shock ...