Estimation of Parenteral Fluid Requirements

Fluid and Electrolyte Therapy

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Estimation of Parenteral Fluid Requirements

Frank G. Boineau, MD,* and John E. Lewy, MDt

The estimation of parenteral fluid requirements is based on the usual losses of water and electrolytes that occur under normal conditions. This article will review those physiologic losses of water and electrolytes from the body, the source of the loss, and the increased body loss of water associated with fever. Replacement of usual water and electrolyte losses from the body may be estimated by three different methods based on: (1) metabolic requirements of the body (caloric expenditure), (2) body weight, and (3) body surface area. Physiologically, insensible water loss from the skin and respiratory track are directly related to the metabolic rate (caloric expenditure). The metabolic rate, in turn, depends on age and body size, which is usually represented by body weight. Urine volume is related to metabolic rate, osmolar load presented to the kidney for excretion, and kidney-concentrating ability. In this article, physiologic losses of both water and electrolytes will be estimated from metabolic rate and body weight.

CHANGES IN BODY WATER COMPARTMENTS WITH AGE Changes in body water compartments occur as the child grows. There is a rapid decrease in total body water as a percentage of body weight during the first year of life-from 75 per cent at birth to approximately 70 per cent at 6 months and 60 per cent at 1 year of age. 3 Thereafter, the percentage of total body water gradually decreases until the child reaches puberty, when, in boys, the percentage is greater because of the differences in body fat content between boys and girls (Table 1). Intracellular water plateaus at about 35 per cent of body weight after the child reaches 3 years of age. After puberty a gender difference occurs. Extracellular water From the School of Medicine, Tulane University, New Orleans, Louisiana

*Associate Professor of Pediatrics,

Department of Pediatrics tProfessor and Chairman, Department of Pediatrics

Pediatric Clinics of North America-Vol. 37, No.2, April 1990

257

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Table 1. Changes in Total Body Water (TBW) and Body Compartments During Development AGE

TBW

EXTRACELLULAR FLUID

INTRACELLULAR FLUID

(% body weight)

(% body weight)

(% body weight)

75-80 70-75 60

45 27

35 40-45

60 55

20 18

40-45 40

Premature Newborn 1 year Adolescence Males Females

Adapted from Friis-Hansen B: Body water components in children: Changes during growth and related changes in body composition. Pediatrics 28:107, 1963; Kooh SW, Metcoff J: Physiological considerations in fluid and electrolyte therapy with particular reference to diarrheal dehydration in children. J Pediatr 62: 107, 1963.

decreases from 45 per cent of body weight at birth to 35 per cent at 6 months and 27 per cent at 1 year. Figure 1 schematically depicts the relative proportions of total body water, and extracellular and intracellular fluid in 1 kg of body mass with increasing age. The intracellular water is represented by the difference between the lines representing total body water and extracellular water volume. A rapid reduction in total body water and extracellular water occurs during the first year of life; the decline is sharpest during the neonatal period. With increasing age the quantities of water and solutes in the body have been shown to accumulate at a constant rate relative to the growth of the body as a whole. 5

MAINTENANCE FLUID NEEDS FOR PARENTERAL THERAPY Insensible Water Losses Maintenance fluid is the amount the body needs for replacement of usual daily losses from the normal functions of the respiratory system, skin, and urinary and gastrointestinal tracts. Insensible water loss is the term given to losses through the skin and pulmonary 100 90 80 70

1000 900 800 700 600 500

60 50

TOTAL BODY WATER

40

400

(/) (/)

c(

.... == z> wC

0 30 0 a: 1D

..J

==

Wei Q.~

20

a: W

Q.

1 00 '----'---'---'----'--'---'---'-............... 1 0 -1

-3

-5

2

AGE, YEAR

3

4

5

ADULT

Figure 1. Changes in the volume of body water with increasing age. The curves were approximated from data in the literature. The intracellular water is represented by the difference between the total body water and the extracellular water. (From Kooh SW, Metcoff J: J Pediatr 62:107, 1963; with permission.)

259

PARENTERAL FLUID REQUIREMENTS

Table 2. Maintenance Water Loss Components Based on Body Weight* COMPONENT

NB-6m

6m-5y

5-1Oy

ADOLESCENCE

40 60 20 120

30 60 10 100

20 50

10 40

70

50

Insensible Urinary Fecal Total

ABBREVIATIONS: NB = full-term newborn in open crib; m = months; and y = years. *Average daily water needs, ml per kg body weight per 24 hours, in different age groups.

system. Such losses will of course depend on the age of the child, assuming normal growth. About two thirds of an infant's daily insensible water loss is through the skin, with one third through the pulmonary system. 7 Insensible water loss is affected by several factors including ambient humidity, body clothing, body temperature, respiratory rate, and depth and ambient temperature. Insensible water loss in the neonate needs special attention, for here these factors are especially important. The factors that influence insensible water loss in the neonate include gestational age, method used for controlling body temperature, and use of phototherapy. Fever also increases insensible water loss by 7 ml per kg body weight per 24 hours for each degree of temperature above 9goF.8 One must remember that the correction for fever is based on a constant temperature elevation. A transient rise in temperature will elevate insensible water loss only as long as the fever persists. Table 2 lists maintenance water loss components including insensible water losses in infants, children, and adolescents. The volume of insensible water losses in milliliters can be related to energy expended in calories with 1 ml of water being lost for each calorie metabolized. 4 Calories expended, in tum, can be related to the body weight at any particular age. Evaporative water loss through the skin and pulmonary system constitute insensible losses of electrolyte-free water. Under normal conditions insensible water loss is approximately 30 ml per 100 calories expended through the skin and 15 ml per 100 calories expended through the pulmonary system. Skin water losses increase with calorie expenditure as from muscle activity or shivering, as does the electrolyte content of sweat. For each degree centigrade elevation in body temperature above normal, a 10 per cent increase in calorie expenditure occurs. 1 Insensible water losses based on calorie expenditure are presented in Table 3. Urinary Water Losses The kidneys are the final pathway regulating fluid and electrolyte balance of the body. Urine volume is determined by the solute load presented to the kidneys requiring excretion and the urine osmolality. The solute load is determined by the rate of metabolism of the body reflected by the apposition of solute for growth and Table 3. Maintenance Water and Electrolyte Requirements Based on Caloric Expenditure BODY WEIGHT

CALORIES EXPENDED

WATER REQUIREMENTS

ELECTROLYTE REQUIREMENTS

(kg)

(CaUKg body wt./day)

(mU100 calories/day)

(mEq/100 calories/day)

3-10 10-20 >20

100

1000 calories + 50/kg for each kg > 10 1500 calories + 20/kg for each kg >20

Insensible Skin = 30 Lungs = 15 Renal = 50 Stool = 5

Sodium = 2.5-3.0 Potassium = 2.0-2.5 Chloride = 4.5-5.5

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in the production of urea and by the solute intake. The final urine osmolality reflects the renal solute load of osmotically active particles as well as the kidneys' absorption or excretion of salt and water. Average osmolar excretion in newborn infants receiving a typical infant formula is 16 to 20 mOsm per kg body weight per day. Urine solute excretion during parenteral fluid therapy is approximately 20 mOsm per kg body weight per day. If the solute load is higher, the water allowance for excretion should be higher. An average estimate for water excretion is 50 to 60 ml per kg body weight per day. The relationship between solute excretion, urinary osmolality, and the urinary excretion rate is shown in Figure 2. If the kidneys do not elaborate a urine more concentrated than 100 mOsm per liter, excretion of 20 mOsm per kg body weight per day would obligate a loss of 200 ml of urine per kg body weight per day. If the urine osmolality were increased to 300 mOsm per liter, approximately 66 ml per kg body weight per day of urine would be required to excrete the same osmolar load with conservation of 134 ml of body water per kg per day. Further concentration of the urine to 1000 mOsm per liter would reduce the volume of urine loss to 20 ml per kg of body weight per day, yielding an additional conservation of 50 ml of water per kg per day. It is obvious that the greatest conservation of body water is accomplished by changing from elaboration of a dilute urine to an isosmotic one. The young infant can concentrate the urine to about 800 mOsm per liter. When giving parenteral fluid it is best not to tax the kidneys to concentrate maximally but rather to provide enough fluid to achieve a urine osmolality between 300 to 400 mOsm per liter. The urinary loss of water approximates 50 ml per 100 calories expended provided urine osmolality is isosmotic (equal to that of the extracellular fluid). Fluid therapy based on the aforementioned requirements will place minimal osmotic work on the kidneys. An additional 5 ml per 100 calories is obligated by stool water. Thus a total of 100 ml per 100 calories expended accounts for the maintenance fluid losses under baseline conditions. When calculating normal maintenance water 500 ----- RANGE --AVERAGE

,I

, :,, ,

I

400

I

I

I I I

,, ,,

I

300

, ,

I I

I

I

200

I I I

LOAD REQUIRING EXCRETION mOsm/kg/day

\ \ \ \

\

100

I

, I

\ 1',33 I

"

120 .............. 'I

.......

"'-~!~----- ------------URINE CONCENTRATION mOsmll

Figure 2. The rel~tion between urine volume and concentration based upon the solute load requiring excretion during parenteral fluid therapy. (From Kooh SW, Metcoff J: J Pediatr 62:107, 1963; with permission.)

261

PARENTERAL FLUID REQUIREMENTS

requirements, allowance should be made for endogenously generated water, especially in the presence of renal failure or congestive heart failure. One source of endogenous body water is oxidation of carbohydrate and fat, which yields carbon dioxide and water. The water of oxidation averages 12 to 17 ml per 100 calories metabolized. Another source, called preformed water, approximates 3 ml per 100 calories and is derived from tissue catabolism during disease states. If sweating is minimal, then sweat and stool losses are roughly balanced by these endogenous sources. The urinary volume as a function of urinary concentration for various solute loads and caloric expenditure is presented in Figure 3. The average infant or child has an osmolar excretion of 20 mOsm per 100 calories metabolized. Thus the usual allowance for urinary water losses is 55 ml per 100 calories metabolized. The best guide to the amount of fluid needed to replace urinary losses in the infant with renal dysfunction is a record of the urinary output and the urine osmolality or a specific gravity. The anuric infant woulq be rapidly and markedly overloaded with fluids if maintenance urine replacement was continued after anuria or severe oliguria had been confirmed. Thus, as with insensible water loss, average urinary loss is a mythical figure, and each patient must be considered individually. Lastly, stool water losses are generally small and in the absence of diarrhea are about 10 ml per kg of body weight per day in infants and toddlers. Stool water loss in older children is small and can be ignored. 5 MAINTENANCE ELECTROLYTE NEEDS FOR PARENTERAL THERAPY Urine is the major source of electrolyte losses in healthy children. Essentially no loss occurs during ventilation. Approximately 0.5 mEq of sodium and potassium per kg body weight are lost through the skin every 24 hours in insensible fluid loss. With sweating, considerable amounts of sodium and potassium may be lost. Urinary losses of sodium average 2 to 3 mEq per kg body weight per day. Urinary excretion of potassium averages 1 to 2 mEq per kg body weight per day. Urinary anions are usually in the form of chloride salts and organic anions. Stool 300

250

200

150

100

Figure 3. Urinary volume as a function of urinary concentration for various solute loads, varying from high (40 mOsm/100 Cal) to low (10 mOsmllOO cal). (From Winters RW: Principles of Pediatric Fluid Therapy. Boston, Little, Brown & Co., 1982, p 70; with permission. )

50

200

400

600

800

1000

URINE CONCENTRATION

1200

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Table 4. Maintenance Urinary Electrolyte Losses* 2-3 1-2 3-5

Sodium Potassium Chloride

*Average daily needs, mEq per kg body weight per 24 hours. loss of electrolytes are minimal and can be ignored in the absence of diarrhea. Usually supplying 3 mEq per kg per day of sodium and 2 mEq per kg per day of potassium with 5 mEq per day of chloride is sufficient. If renal disease is suspected or similarly electrolytes do not change in the anticipated direction, urinary electrolytes should be measured. 5 Maintenance urine electrolyte losses are presented in Table 4. Maintenance requirements for electrolytes are part of the parenteral therapy needed to maintain body fluid homeostasis. The electrolyte intake of normal infants can be expressed in terms of calorie intake or expenditure. Human milk provides 1.0 to 1.5 mEq per 100 calories of sodium and potassium, whereas cow's milk contains two to three times the quantity.2 A generous estimation for electrolyte maintenance therapy is 2.5 mEq of sodium and potassium per 100 calories expended, using the chloride salt. 6 Exceptions include unusual ongoing loss of electrolytes by sweating, vomiting and diarrhea, surgical drainage tubes, burns, diuretic therapy, and renal electrolyte losing disorders. Determination of electrolyte composition of abnormal fluid losses is required to arrive at appropriate parenteral therapy. One can estimate the daily maintenance requirements for water, electrolytes, and nutrition by calculating the calorie expenditure. This is illustrated in Table 3, assuming basal metabolic expenditures that are increased only slightly during bed rest. Metabolic activity, and therefore calorie requirements, change with stress so that these factors (e.g., fever, surgery, exercise, burns, and diabetes insipidus) as well as factors that increase gastrointestinal or sweat losses need to be considered and water requirements adjusted accordingly.

CALORIC NEEDS FOR SHORT-TERM PARENTERAL THERAPY The nutritional component of maintenance therapy should provide substrate for metabolism. Optimal nutritional therapy provides an equal number of calories for those expended. For short-term maintenance therapy in a previously wellnourished infant or child, however, a combination of some parenteral nutrition and the patient's own fat stores is adequate to prevent severe ketosis and tissue catabolism. This is accomplished by administering sufficient glucose equal to approximate 20 per cent of the total caloric expenditure, or 5 gm of glucose per 100 calories expended. In a more chronic condition, especially in the undernourished child, nutritional therapy should provide the required calories by parenteral or enteral administration of carbohydrate, fat, amino acids, minerals, and vitamins. An appropriate solution for average maintenance therapy contains 30 to 35 Table 5. Case 1: Estimates of Fluid and Electrolyte Needs for 24 Hours MAINTENANCE NEEDS

Insensible losses Urinary losses Stool losses TOTAL

WATER

(ml)

SODIUM

(mEq)

POTASSIUM

(mEq)

CHLORIDE

300 600

30

20

50

900

30

20

50

(mEq)

263

PARENTERAL FLUID REQUIREMENTS

Table 6. Case 2: Estimates of Fluid and Electrolyte Needs for 24 Hours MAINTENANCE NEEDS

Insensible losses Urinary losses Stool losses TOTAL

WATER

(ml)

240 360 120 720

SODIUM

(mEq)

POTASSIUM

(mEq)

CHLORIDE

18

12

30

18

12

30

(mEq)

mEq per liter of sodium and 20 mEq per liter of potassium as chloride salts in 5 per cent dextrose in water. The solution can be modified to meet unusual requirements for water and electrolytes as needed.

MAINTENANCE FLUID AND ELECTROLYTE PROBLEMS Two problems are discussed subsequently to illustrate fluid and electrolyte calculations of maintenance needs. Case I This 10-kg 12-month-old infant needed maintenance fluid therapy after a minor surgical procedure. The infant was normal before the operation and loss minimal fluids during surgery. After surgery the infant required 24 hours of maintenance fluid and electrolytes. The child was afebrile, and renal function was normal before surgery. Estimates of both fluid and electrolyte needs for 24 hours are in Table 5. The proper solution to administer would be 5 per cent dextrose in water containing 33 mEq per liter sodium chloride and 22 mEq per liter potassium chloride. Table 5 calculated fluid and electrolyte needs based on body weight and age. Potassium should never be given intravenously unless renal function has been determined to be adequate to excrete potassium. Case 2 This 6-kg 5-month-old infant needed intravenous fluid and electrolytes for 24 hours and also replacement of gastric fluid owing to nasogastric suction for 24 hours. Estimates of fluid and electrolyte needs for 24 hours are in Table 6. The maintenance fluid should be given in 5 per cent dextrose and contain 25 mEq per liter sodium chloride and 17 mEq per liter potassium chloride. The volume of nasogastric fluid should be measured every 4 to 6 hours and replaced with an equal volume of 5 per cent dextrose in water containing 75 mEq per liter sodium chloride and 15 mEq per liter potassium chloride. Gastric fllJid usually contains 50 mEq per liter sodium, 5 to 15 mEq per liter potassium, and 110 mEq per liter chloride. 9 The chloride losses need to be replaced with additional sodium as the cation to prevent a body deficit of chloride that can lead to metabolic alkalosis.

REFERENCES 1. Dubois EF: The basal metabolism in fever. JAMA 77:352, 1921 2. Fomon SJ (ed): Infant Nutrition, ed 2. Philadelphia, WB Saunders, 1984 3. Friis-Hansen B: Body water compartments in children: Changes during growth and related changes in body composition. Pediatrics 28:107, 1963 4. Holliday MA, Segar WE: Maintenance need for water in parenteral fluid therapy. Pediatrics 19:823, 1957 5. Kooh SW, Metcolf J: Physiological considerations in fluid and electrolyte therapy with particular reference to diarrheal dehydration in children. J Pediatr 62:107, 1963

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6. Lattanzi WE, Siegel NJ: A practical guide to fluid and electrolyte therapy. Curr Probl Pediatr 16: 1, 1986 7. Levine SW, Wilson JR: Respiratory metabolism in infancy and in childhood: VII. Elimination of water through the skin and respiratory passages of infants. Am J Dis Child 35:54, 1928 8. Mirkin G: Insensible weight loss in infants with fever. Pediatrics 30:279, 1962 9. Winters RW: Principles of Pediatric Fluid Therapy, ed 2. Boston, Little, Brown, 1982, p 70 Address reprint requests to: School of Medicine Tulane University 1430 Tulane Avenue New Orleans, LA 70112