FLUID AND ELECTROLYTE THERAPY IN CHILDREN
Fred A. McCurdy, M.D., Ph.D., M.B.A., F.A.A.P., General Pediatrician
Learning Objectives: Upon completion of the clerkship, the student should, for a patient of any size with
normal renal function, be able to:
1.
Write maintenance fluid orders.
2.
Estimate deficits of volume, sodium and potassium and write orders for IV therapy of
uncomplicated isonatremic dehydration.
3.
Estimate deficits of volume, sodium and potassium and write IV therapy orders for
patients with hyponatremic or hypernatremic dehydration.
4.
Know the applicable principles included in the handout and supplemental reading and
apply them to the management of acutely contracted blood volume.
5.
Demonstrate a thorough understanding of oral rehydration therapy (ORT) by designing a
treatment plan to manage, at home, a child with moderate dehydration due to an acute
diarrheal illness.
Learning Activities:
1.
Example case studies - 1 hour
2.
Patient care activities
3.
Independent study:
a.
Essential reading
(1)
Handout
(2)
Bernstein D and Shelov SP (ed): Pediatrics, 1st ed., 1996, pp. 78-90
(Parenteral Fluid and Electrolyte Therapy).
b.
Supplemental reading
(1)
Segar WE, Parenteral fluid therapy, Curr Probs Peds, vol 3, 1973
(2)
Santosham M and Greenbough WB, Oral rehydration therapy: a global
perspective, J Peds, 1991(suppl); 118:S44-S51
(3)
Any recent edition of The Harriet Lane Handbook
MAINTENANCE FLUID AND ELECTROLYTES
-Based on the caloric expenditure model, each calorie expended requires provision of water in the
ratio of 1 ml/cal metabolized/day at rest.
-Also, according to the caloric expenditure model maintenance sodium and potassium ranges/100
ml of maintenance fluid/day have been determined. Upper limit has been chosen for sodium and
lower limit has been chosen for potassium to be placed in the “Segar box” (see summary on page
4).
-Common names for commercially available saline solutions and their sodium concentrations are:
Normal saline (0.9% NaCl/L)
154 mEq Na+/L
One-half normal saline (0.45% NaCl/L)
77 mEq Na+/L
One-third normal saline (0.33% NaCl/L)
57 mEq Na+/L
One-quarter normal saline (0.2% NaCl/L)
34 mEq Na+/L
Ringer’s lactate
130 mEq Na+/L
+
(Contains 4 mEq K , 109 mEq Cl , 28 mEq bicarb equivalent all/Liter, and 3
mg/dl of Ca++)
-Addition of glucose to each of the above at a minimum of 5 gm/100 ml (5% dextrose) minimizes
tissue catabolism to the point that protein stores are somewhat “spared” from providing substrate
for gluconeogenesis. Ketosis from fat metabolism is also prevented.
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Copyright © Fred McCurdy, 2001
Fluid and Electrolytes In Children
DEFICIT FLUID AND ELECTROLYTES
-Water deficit can be determined by subtracting the patient’s current weight from his/her weight
just before he/she became dehydrated. However, it is usually determined by clinical criteria and is
a percentage estimate of the total amount of body weight lost as water.
-Can be calculated by determining the pre-illness weight using the following:
Pre-illness wt (kg) =
Current wt (kg)
100
100 - % estimated dehydration
and subtracting the current weight from the pre-illness weight. The difference is
considered the amount of water lost in kg or liters.
-Can also be calculated by multiplying the current weight (kg) by the estimated %
dehydration yielding a figure that represents the estimated water loss in kg or liters.
“Segar box” requires that the water deficit be expressed in milliliters so you should
multiply the body weight (gms) by the % of estimated dehydration.
-Estimated % dehydration is really a “best guess” based upon physical examination findings. By
convention, dehydration is divided into mild, moderate and severe. Infants, those less than one
year (one source says <20 Kg), are mildly dehydrated at a 5% loss, moderate at 10% and
severe at 15%. Children, those greater than 1 year (or by the other source, >20 kg), are mildly
dehydrated at a 3% loss, moderate at 6% and severe at 9%. Reasons behind this are that
infants are “mostly water”, a higher percentage of total body water is in the extracellular fluid
space in infants when compared to child and adult values, and all physical characteristics used to
describe differing degrees of dehydration are really measures of the integrity or relative degree of
expansion of the extracellular fluid space.
-Sodium deficit is calculated by first determining that volume (in liters) of deficit water that has
been lost from the extracelluar fluid space (ECF) (assuming that 60% of total water loss has been
from the ECF) and multiplying this figure times the mean concentration of sodium in the
“idealized” ECF (value of 140 mEq/L is chosen because it is the mean concentration of sodium in
the normal ECF). In this calculation it is further assumed that electrolyte loss is “isotonic” to the
fluid space from which the fluid and electrolyte have been lost and thus my reference to the
“isotonic Na+ deficit”.
-A similar calculation is made for potassium deficit, but it is assumed that 40% of the total water
deficit has been lost from the intracellular fluid space (ICF) and that the mean concentration of
potassium in this space is 150 mEq/L. In this calculation it is further assumed that electrolyte
loss is “isotonic” to the fluid space from which the fluid and electrolyte have been lost and thus my
reference to the “isotonic K+ deficit”.
HYPERNATREMIC DEHYDRATION
-“Pure free water” deficit is calculated when dealing with a patient who has hypernatremic
dehydration. The formula given in the “Segar box” yields 2 the amount of water necessary to
correct the patient’s serum sodium into the normal range. Slow correction is necessary to prevent
brain swelling which occurs when the serum sodium is decreased rapidly. Footnote #7 to the
“Segar box” gives the maximum rate of change for serum sodium.
-Patients with hypernatremic dehydration frequently appear clinically less dehydrated than they
actually are due to their well-maintained intravascular volume.
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Copyright © Fred McCurdy, 2001
HYPONATREMIC DEHYDRATION
-Additional sodium needed to raise a hyponatremic patient=s serum sodium into the normal range
is calculated by the formula given in the “Segar box”. A rapid rise in serum sodium will lead, in
some instances, to central pontine demyelinolysis (CPM) or the osmotic demyelinization
syndrome. Slow correction of hyponatremia should help avoid this problem. Footnote #7 to the
“Segar box” gives the maximum rate of change for serum sodium.
-Patients with hyponatremic dehydration frequently appear more clinically dehydrated than they
actually are due to a poorly maintained intravascular volume.
FLUID RESUSCITATION
-Acute volume loss will frequently lead to shock or a shock-like state. This occurs more readily in
children. Therapy for this is aimed at rapid expansion of the ECF and is best accomplished by
giving an rapid infusion of either normal saline or Ringer’s Lactate in a volume of 20 ml/kg
(without glucose) over 30-60 minutes. This volume should be repeated as often as necessary to
reduce the patient’s capillary refill to <2 seconds.
ORAL REHYDRATION THERAPY
-A simplified method for determining how much fluid to give would be:
-Mild dehydration
50 ml/kg given over the first 4 hours followed by 100 ml/kg/day until the diarrhea
subsides
-Moderate dehydration
100 ml/kg given over the first 4 hours followed by 100 ml/kg/day until the
diarrhea subsides
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Copyright © Fred McCurdy, 2001
SUMMARY OF FLUID AND SOLUTE CALCULATIONS (“SEGAR BOX”) FOR 1 DAY
H2O
Na+
K+
Maintenance (Daily
fluid and electrolyte
requirement)
100 ml/kg-first 10 kg
1000 ml + 50 ml/kg 10-20 kg
1500 ml + 20 ml/kg >20 kg
3 mEq/100 ml of
maintenance fluids/day1
2 mEq/100 ml of
maintenance fluids/day1
+(plus) Total Deficits
Total deficit (ml) =
weight (gm) X %
estimated dehydration
ECF deficit (in liters) X
140 mEq/L =
“isotonic” Na+ deficit
from the ECF
ICF deficit (in liters) X
150 mEq/L =
“isotonic” K+ deficit
from the ICF
BLANK
BLANK
135 - Sodium6 X 0.6 X
wt (kg) = amount of
sodium (mEq) needed
to raise the serum
sodium to normal
( MAX = 12 mEg/L
rise/day)7
BLANK
-(minus) “pure free
water” deficit if
hypernatremic2
+(plus) “Pure free
water” deficit (for
hypernatremia only)5
(ECF loss = 60% of
H2O deficit)
(ICF loss = 40% of
H2O deficit)
Sodium3-145 X 4 ml4
2
kg
X wt (kg) = “pure free
water” deficit (ml)
(MAX = 12 mEq/L
decline/day)7
+(plus) Excess solute
losses (for
hyponatremia only)
BLANK
TOTALS
1
Acceptable ranges for maintenance sodium and potassium/100 ml of maintenance fluids/day: Sodium = 2.5 - 3.0
mEq; Potassium = 2.0 - 3.0 mEq
2
Subtract the “pure free water” deficit from the total deficit before proceeding to calculate the “isotonic” Na+ and
“isotonic” K+ deficits. (See the next line for how to calculate “pure free water”.)
3
Currently measured serum sodium above the normal range
4
Amount of water needed to theoretically drop the serum sodium by 1mEq/L
5
REMEMBER that the “pure free water” deficit is a portion of the total deficit and that the reason for this
calculation is to determine how much “pure free water” deficit is needed to be subtracted from the total deficit prior
to calculating the “isotonic deficits” for sodium and potassium. Once you have used the “pure free water” deficit,
remove the “pure free water” deficit value from the Segar box and add only the maintenance and the total deficit.
(That is why there is a notation to “- (minus) ‘pure free water’ deficit” in the “Total Deficit” square and a “+ (plus)
‘pure free water’ deficit” in the square directly below “Total Deficit”.)
6
Currently measured serum sodium below the normal range.
7
Research and clinical experience provide us with data that substantiates a maximum safe rate for sodium change
(either from high to normal or from low to normal). That rate of change is 0.5 mEq/L/hour or 12 mEq/L/day.
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Copyright © Fred McCurdy, 2001