Approach to the Child Who is Dry

Background

The problem of dehydration is a common one in the pediatric population. This is, in part, due to children’s greater susceptibility to fluid losses. Some contributing factors to keep in mind are:

• Reduced ability of developing kidneys to concentrate urine
• High surface to volume ratio (leads to insensible losses)
• Susceptibility to diarrhea illnesses

Review of Basic Fluid Needs

Total Body Water (TBW), the absolute amount of water existing within a person, can be thought of as contained within two main compartments: the intracellular compartment and the extracellular compartment.  The extracellular compartment can be further subdivided into an interstitial component and a plasma component. Fluid can be lost from, or shift between, any of these compartments.

It is crucial to keep these compartments in mind as excesses or deficits in any one compartment can lead to pathology. For example, intravascular depletion can lead to a lack of perfusion resulting in end organ damage, whereas fluid excess can translate into cerebral edema.

In pediatrics, TBW is not static as it varies with age when considered as a percentage of body weight. At term, TBW is approximately 75% of body weight. This will be even higher in premature neonates. TBW decreases to about 60% of body weight near age 1 and remains here until puberty. At this time, the fat content of females increases more than males who, in turn, acquire more muscle mass than girls. Remember that fat repels water, therefore TBW of boys remains at 60% of weight while TBW of girls decreases to about 50% by the end of puberty.

Clinical Note

From the same logic, overweight or obese children will have a lower TBW as percentage of body weight. Something to keep in mind with the growing preponderance of obesity in the North American pediatric population.

It is important that we have these fluid compartments in mind, but how do we decide the amount of fluid each pediatric patient requires to stay euvolemic? Clinically, in a normal healthy child, fluid needs are estimated from the caloric expenditures of the child in an approximately 1:1 ratio (100ml for every 100kcal/kg). Since caloric requirements vary directly with weight, fluid requirements can be estimated as shown in Table 1.

Table 1. Calculating maintenance fluid needs from body weight

Thus, from this we can determine the minimum fluid needs of a normal child.

How To Assess Fluid Status

History/ Questions to ask

• number of diapers (expect 6 or more wet diapers per day)
• Amount taken in
• Sources of excess output? (diarrhea, vomiting, pathologic polyuria eg diabetes)
• Causes for increased fluid needs? (burns, fever, tachypnea)

It is important to understand the source and magnitudes of NORMAL water loss. There are three main sources for normal loss of water from a child: Urine, stool, and insensible water losses. Urine is the most important contributor, comprising 60% of losses. Insensible losses, which are made up of evaporative depletions from the lungs and skin, comprise 35% while fluid lost with the stool is normally only a minor component (5%). It should be noted that tears and sweat should be considered an additional source of water loss.

Percent Components of Fluid Loss:

It is important to first assess the degree of dehydration (often expressed as the percent of body weight lost) in the child as this will determine the seriousness of the situation and the treatment. Table 2 provides a list of clinical signs to be sought during your examination and groups them into clinical pictures of severity.

TABLE 2. CLINICAL EVALUATION OF DEHYDRATION

Approach to Fluid Therapy

Definitions

There are three main types of fluid therapy:

Maintenance Therapy

Replaces any ongoing losses of water and electrolytes under normal physiologic conditions and maintains a normal volume and electrolyte status.

Replacement Therapy

Used to replenish excessive fluid losses (ex. vomitting or polyuria) such that may occur on top of normal maintenance fluids.

Deficit Therapy

Used when dehydration has occurred, almost always requires concurrent replenishment of lost sodium and potassium.

Concepts and Strategies

Maintenance Therapy

The maintenance fluid requirements can generally be calculated from a patient’s weight as discussed above in TABLE 1 with the patient’s normally functioning kidney supplying precise adjustments to maintain euvolemia. However, it is important to keep in mind that this calculation gives 24 hr requirements but that orders are given as a per hour basis. Table 3 provides a clear scheme of such orders.

Note: Na+ and K+ are often added to maintenance fluids so as to not stress homeostatic mechanisms.

Table 3. MAINTENANCE WATER RATE  (note: max. fluid rate is usually 100mL/hr)

Replacement Therapy

A basic and accurate way to calculate the replacement fluid requirements is to measure the volume (and sometimes the electrolyte composition) of the fluid lost be it stool, urine or gastric fluids..

Deficit Therapy

As noted earlier, when working with a dehydrated child the initial step is to determine the severity and type of dehydration. The type of dehydration refers to the present serum sodium concentrations (hyponatremic, isonatremic, or hypernatremic) of the child and is determined by the concentration of the fluid lost as well as the attempts that have already been made to correct the deficit. For example, a child who has been given large amounts of free water to replace deficits will probably be hyponatremic.

Most cases of pediatric dehydration will be isonatremic, but attention to the type of dehydration is important as hyponatremic or hypernatremic dehydration can lead to fluid shifting to unwanted compartments leading to complications and even death.

When giving deficit therapy, it must always be remembered that rapid rehydration can cause neurological damage. During dehydration, neural tissues produce osmotically active particles that allow for prevention of intracellular fluid loss. Thus, an increase in supply of fluid to these tissues can result in an excessive influx of free water, causing cerebral edema. If in doubt, go slow with replacement of deficit fluid.

References

Kliegman, RM, HB Jenson, KJ Marcdante, & RE Behrman.  2006.  Nelson Essentials of Pediatrics.  Elsevier Saunders: Philadelphia.

Endom, Erin E, Michael J Somers. 2007. Treatment of hypovolemia (dehydration) in children. UpToDate.

Crocetti, Michael, M. A. Barone. Oski’s Essential Pediatrics, 2nd Edition. Philadelphia: Lippincott William & Wilkins.

Acknowledgements

Written by: Matt Bouchard

Edited by: Jeff Bishop