Isotonic dehydration occurs when there is a proportionately equal loss of sodium and water; the serum sodium thus remains within normal range of 130 to 145 meq/L. It most often results from diarrheal illness and is the most common fluid and electrolyte problem encountered in pediatrics. Fluid is initially lost from the extracellular space. Intracellular fluid then shifts into the vascular tree, which protects circulating blood volume at the expense of intracellular dehydration.
The clinical manifestations of isotonic dehydration depend on the absolute volume deficit, the rate at which fluid is lost, and the age of the patient. When fluid is lost over a relatively long period, a large deficit may be well tolerated and clinical manifestations can be rather subtle, even though up to 40 percent of intracellular fluid may be lost. This most commonly occurs in patients with protracted diarrheal illnesses. In contrast, sudden massive loss of fluid such as occurs in cholera-associated or rotavirus diarrhea can be fatal if not treated aggressively, because most of the volume is lost from the extracellular space and there is insufficient time for intracellular fluid to shift into the vascular tree. This is especially true in young infants, because a relatively large percentage of their total body water is contained in the extracellular space. Rapid fluid loss can result in cardiovascular collapse, whereas older children will remain well compensated. EVALUATION
The most accurate way to estimate the degree of dehydration is calculating weight loss, which in acute situations amounts to free water deficit. However, this information is rarely available. In practice, estimating the degree of dehydration depends on integrating multiple factors. Patients usually have a history of vomiting and diarrhea. It is useful to quantitate the approximate number of stools and to determine whether the patient is able to tolerate any oral feedings without vomiting. Parents are asked what liquids the child has been given, since excess free water can cause hyponatremia, and "homemade" remedies may contain excess sodium. A history of decreased urine output implies significant fluid loss.
Physical examination has been demonstrated to provide a reliable estimation of the degree of dehydration 2 (Table 128-2). Hypotension indicates hypovolemic shock.
The patient's mental status is important: normal mental status usually implies mild dehydration, whereas irritability signifies at least moderate fluid loss. Lethargy implies severe volume loss and/or an electrolyte abnormality, especially hypernatremia. Decreased skin turgor and sunken eyes and fontanelle imply moderate to severe fluid loss and usually occur when intracellular fluid has had time to diffuse into the intravascular space. In these patients, vital signs may be only slightly abnormal and not indicative of the degree of dehydration.
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