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).

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.

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.

INITIAL STABILIZATION

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 should receive large amounts of isotonic fluid administered via large-bore intravenous lines in 20 mL/kg boluses. Constant reassessment of the patient’s vital signs, urine output, and physical findings can assist in determining the appropriate duration of therapy. Even in initially stable-appearing patients, paying careful attention to early indicators of shock can avoid a precipitous deterioration in their condition. A substantial percentage of moderately and severely dehydrated patients are also hypoglycemic, making it important to consider checking the bedside glucose level in dehydrated patients.

HISTORY AND PHYSICAL EXAMINATION

Because treatment may depend on an estimation of degree of dehydration, it is important to be as accurate as possible in the history and physical examination. In the setting of acute illness (<5 days of symptoms), acute weight loss is the best indicator of the fluid deficit; each kilogram of weight loss indicates a deficit of 1 L. Unfortunately, a recent “well” weight is rarely known in children being treated for dehydration; thus an estimate of the deficit must be made on the basis of the clinical evaluation.

The history is perhaps most helpful in determining the cause of dehydration, particularly the symptoms, their duration, and what therapies have been implemented (Table 73-2). Of specific interest are the presence, quality, and quantity of both vomiting and diarrhea. Determining whether there has been a reduction in urine output is helpful; although decreased urination has a low positive predictive value for dehydration, a history of normal urine output is reassuring.

It is medically accurate to reserve the diagnosis of gastroenteritis for those patients who have diarrhea with or without vomiting. Numerous diseases can be confused with gastroenteritis if the cause of the emesis is not elucidated. Common diseases occasionally misdiagnosed as gastroenteritis include urinary tract infection, pyelonephritis, lower lobe pneumonia, and appendicitis.