In most cases of diarrheal dehydration, electrolyte concentrations remain
near normal. Loss of volume without electrolyte disturbance constitutes
the simplest type of dehydration: isotonic. In this case, the treatment
consists of replacing the volume deficit over 24 hours. Just as there
are several ways to calculate maintenance fluids (the Holliday-Segar
method, caloric method, and the body
surface area method), there are a couple of ways to calculate the fluid
and electrolyte deficit. Both methods start with a clinical determination
of the degree of dehydration.
Method I: Caloric
In this method, the degree of dehydration is related to body weight.
A 10% loss in body weight is considered "10% dry". Fluid will
be replaced on a weight-for-weight basis. That is, if a person is
down 1500 grams, their deficit is 1500 mL. The only difficult item
to remember is the amount of electrolytes to add to the replacement fluid.
Follow this rule:
|For every 100 mL of replacement fluid, add 8 mEq of Na+,
6 mEq of K+ and 6 mEq of Cl-
Please note that in this method, the maintenance component of therapy
is based on 100 kcal metabolized per day, whereas the deficit requirements
are based on body weight.
A 12 kg, 94 cm tall child is assessed as being 10% dehydrated (he was
about 13 kg a few days ago when he was feeling well). His serum chemistry
shows: Na=143, K=4.5, Cl=104, HCO3=18. He is now in stable condition
and requires treatment for his isotonic dehydration.
Using the caloric method (or the Holiday-Segar method which is equivalent
in this case), his energy requirement is 1100 kcal/day which translates
to 1100 mL/day.
This fluid can be given at a constant rate over 24 hours (96 cc per hour);
however, the usual method is to replete half the amount over the first
eight hours (1150 mL/8 hours = 144 mL/hour), and the remainder over the
next 16 hours (1150 mL/16 hours = 72 mL/hour). A reasonable IV fluid
choice would be D5½NS at the above rates. After the patient's
first void, 20 mEq/KCl per liter would be added. Although a greater
deficit of K+ is indicated, it is unusual to infuse higher concentrations
of K+. If there were concern about the patient retaining sodium,
one-third normal saline would provide just slightly less than the calculated
sodium requirement above (51 mEq/L), so it could also be used.
|Total per Liter
Method II: Body Surface Area
In the surface area method, the degree of dehydration is related to the
deficit in total body water rather than body weight. It is assumed
that in acute dehydration, the loss of weight is actually a loss of water.
The TBW as a percentage of body weight varies with age. The surface
area method further assumes that electrolytes are lost evenly from the
extracellular and intracelluar fluid. It also assumes that half the
potassium loss can be replaced in one day.
First, figure out the amount of fluid that must be given to make up
the deficit. To determine how much electrolytes to add to this fluid,
divide the fluid volume into two equal portions. The first half is
targeted to replace fluid from the extracellular compartment, so the electrolytes
should be make proportional to expected serum values for Na, K, and Cl,
namely a sodium of 140 mEq/L, potassium of 4 mEq/L and Cl of 100 mEq/L.
The other half is considered to be replacement for the intracellular compartment
where potassium is normally about 160 mEq/L. The other electrolytes
in this compartment are negligible.
The same child as above comes into the ER, but this time, he would like
his iv fluid therapy to be calculated on a body surface area basis.
Being an obliging pediatrician, you comply.
He is older than 6 months, so his total body water is about 60% of
body weight. Since he is about 10% dehydrated on a weight basis,
he is about 16.6% depleted in terms of total body water (=10%/0.6).
Since his normal body water would be 7.2 liters (=60% of 12 kg), he must
be depleted by about 1200 mL (=16.6% of 7.2 liters). Maintenance
is calculated by the body surface area method. In this case, his
surface area is calculated as 0.56 m2.
As in the caloric method, his fluid requirements are a combination of
his daily maintenance requirements plus whatever he needs to replace the
Again, this could be given over a 24 hour period at a constant rate of
85 cc/hour or more commonly, divided as above repleting half the deficit
in the first 8 hours at a rate of 128 mL/hour, and the remainder over the
next 16 hours at a rate of 64 mL/hour. An appropriate choice of repletion
fluid would again be D5½NS, with 20 mEq/L potassium added after
|Total per Liter
The two methods for calculating replacement fluids agree well in terms
of concentration of electrolytes per liter of replacement fluid.
However, the caloric method predicts a total fluid requirement about 250
mL greater than the surface area method. These differences would
lessen if the upper end of the body surface area range (1800 mL/m2) were
used to calculate maintenance or if the lower range of the caloric formula
were used (80 mL/kcal). These differences are minor compared to the
total fluid volume, so this discrepancy does not cause problems.
In cases where the patient has received a bolus of fluid for immediate
volume repletion, this bolus must be subtracted from the above calculations.
Here is another example, using the caloric method of calculation:
A 40 kg girl presents with 12% dehydration (she weighed 45 kg last week).
In the ER, she received a two 20 mg/kg boluses of normal saline.
Calculate fluid replacement therapy using the caloric method:
In this case, the optimal IV rehydration solution would be D5¼NS,
which has a sodium of 38.5 mEq/L. As usual, the solution should contain
20mEq KCl per liter after the first void. The fluid can be administered
over a 24 hour period at a constant rate of about 210 mL/hour. Considering
the two large boluses which this patient has received over a short time
period, it would be better to replete at this constant rate rather than
trying to give half the deficit in the first 8 hours which would require
an iv rate of about 400 mL/hour. Even if this rate were feasible
given iv access, there would be a risk of "flooding" the patient too rapidly.
|Total per Liter
The above strategies of fluid repletion must be tempered with clinical
correlation. If the patient is taking fluids orally, less iv fluids
are required. If the patient has any sign of fluid overload, congestive
heart failure, pulmonary edema, or other contraindicatory sign, the fluid
rate should be cut back. A serum chemistry obtained the next day
is helpful in determining the success of the fluid repletion, and can be
used to adjust any further iv fluids.
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Last modification: April 30, 1998