Isotonic Dehydration

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.

Example 1.

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.

	H2O (mL)	Na⁺ (mEq)	K⁺ (mEq)	Cl^- (mEq)
Maintenance	1100	33	22	22
Deficit	1200	96	72	72
Total	2300	129	94	72
Total per Liter	1L	56	40	31

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.

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.

Example 2.
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 deficit:

H₂O (mL) Na⁺ (mEq) K⁺ (mEq) Cl^- (mEq)

Maintenance 840 29 18 18

Deficit (Extracellular) 600 84 2.5 60

Deficit (Intracellular) 600 0 48 0

Total 2040 113 69 78

Total per Liter 1L 55 34 38

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 first void.

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:

Example 3.
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:

H₂O (mL) Na⁺ (mEq) K⁺ (mEq) Cl^- (mEq)

Maintenance 1900 57 38 38

Deficit 4800 384 288 288

Bolus -1600 -246 0 -246

Total Fluids 5100 195 326 80

Total per Liter 1L 38 64 16

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.

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