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Approach to Parenteral Therapy in Dehydrated Infants or Children

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Phases of fl uid therapy in children are often described and provide a useful framework for organizing fl uid
therapy and (importantly) following the results of the therapy.

Phase 1
Th e aim of the fi rst phase of therapy is restoration of the circulating intravascular volume in children with
severe dehydration and peripheral circulatory failure (shock). If an infant or child has a history of volume
defi cit and is severely dehydrated or has signs of shock, rapid intravenous infusion of a volume-expanding
agent at 20 mL/kg body weight is appropriate. (Usually this agent is isotonic saline, although lactated Ringer or
5% albumin may be used.) Continuous monitoring during this therapy is necessary, and intensive nursing and
medical support are required. After the infusion, repeat evaluation of vital signs and physical examination are
essential, and it is necessary to repeat the “bolus” of isotonic saline until cardiovascular stability (i.e., improved
capillary refi ll, lower heart rate, higher blood pressure, improved mental status) ensues. Typically, between one and three such infusions are necessary.

Children in hypovolemic shock should not receive inotropic agents unless restoration of the intravascular volume fails to improve cardiac output. Although commonly practiced in the ICU setting, there is no
clinical benefi t from using low-dose dopamine to maintain urine output. Assessment of serum electrolytes,
urea nitrogen, and creatinine should occur prior to fl uid therapy because these help guide the subsequent
phases of fl uid therapy. Children with dehydration but no evidence of shock do not require “bolus” fl uid
therapy, and phase 1 can be eliminated. Many physicians recommend subtracting the initial fl uid boluses
given to the infant from the maintenance and defi cit fl uids calculated for the initial 24 hours; this is optional
and depends on the individual patient.

Phase 2
Phase 2 in fl uid replacement therapy requires attention to several components of fl uid therapy as discussed previously (see Methods for Estimating Maintenance Fluid Needs). All children need maintenance fl uid, and for
children with isonatremic or hyponatremic dehydration, maintenance fl uids should be calculated as described
previously. Maintenance therapy for children with hypertonic dehydration is described in the following section.
In addition to maintenance fl uids, replacement of losses from ongoing diarrhea, vomitus, or nasogastric secretions is essential. Accurate replacement involves measurement of the electrolyte content of these body fl uids
in the clinical laboratory, volume measurement at the bedside, and administration of appropriate replacement
fl uids. Table 4-9 gives typical values for these ongoing losses, which can be used as guidelines. If these ongoing
losses are signifi cant, frequent replacement (every 1 to 2 hours) is necessary; for less severe losses, less frequent
replacement may be appropriate.
Defi cit therapy (replacement of previously lost fl uids) can be calculated in several ways for children with
isonatremic or hyponatremic dehydration. If accurate and recent preillness body weight is known, then the

amount of fl uid needed to be replaced (as defi cit) is simply the preillness weight minus the current weight (in
kilograms). A body weight that decreases by 1 kg from diarrheal fl uid loss requires 1 L of fl uid replacement
(1 L of water weighs 1 kg). It is essential to give this and other defi cit replacements in addition to “regular”
maintenance fl uids and replacement of ongoing losses.

Th e amount of fl uid lost in the acute dehydration episode that should be replaced is then estimated as
x (preillness weight) minus the current weight, which equals the volume of fl uid to be replaced. Alternatively, the amount of fl uid defi cit can be estimated in infants based on the percent dehydration

Fluid Calculations in Isonatremic Dehydration

CASE 4-3
A male infant is seen in the emergency department because of diarrhea, decreased oral fl uid intake, and irritability. The boy’s weight at present is 8.1 kg; his mucous membranes are dry, and no tears are present when he cries. Parents report that the last urine output was about 12 hours prior to the emergency department visit. pressure is 86/45 mm Hg, heart rate is 152 beats/min, capillary refi ll is 3 seconds, and skin elasticity is slow to retract. Based on these fi ndings, you estimate the child to be 10% dehydrated and in need of urgent
fl uid resuscitation.

Question 
What are the fi rst steps in this child’s rehydration therapy?

Answer 
The fi rst step involves obtaining vascular access and serum electrolytes. The second step involves giving 20 mL/
kg body weight of isotonic saline and reassessing the child. The amount of isotonic saline is 20 mL/kg multiplied by 8.1 kg, or 162 mL; it should be given as quickly as possible (usually over 10–20 minutes).
The child responds to the fl uid challenge with a decrease in heart rate to 115 beats/min and improved
capillary refi ll of 2 seconds; and urine output, although scant, is obtained and is concentrated, with a specifi c gravity of greater than 1.035. The child’s serum sodium is 135 mEq/L.

Fluid Calculations in Hyponatremic Dehydration
As mentioned previously, children with hyponatremia often appear more ill than isonatremic children with
similar volume defi cits. Th e procedure for calculating the amount of fl uid to be given is the same as described
previously (see Methods for Estimating Maintenance Fluid Needs); in fact, the only diff erence is that more
sodium is given. It is necessary to approach children as if they have an isonatremic dehydration requiring replacement of the fl uid defi cit. Maintenance fl uid and sodium requirements are outlined in Tables 4-6 and 4-12.
Th e sodium defi cit may be estimated using the following equation:
(Desired [Na]  observed [Na])  weight (kg)  0.6 (volume of distribution of Na)
It is recommended not to raise the serum sodium concentration more than 10 to 15 mEq/day to avoid neurologic complications such as seizures and pontine demyelination. Hyponatremia presenting with neurologic
manifestations requires rapid correction with 3% sodium chloride not to exceed 1 to 2 mEq/L/h.

 

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