Fluid therapy preserves the normal volume and composition of body fluids and, if needed, corrects any existing abnormalities. In children, the most common clinical abnormality requiring fluid therapy is hypovolemia, primarily due to vomiting and diarrhea from gastroenteritis.
Thus, it is clinically useful to divide fluid therapy into two potential components: provision of volume for homeostatic needs (maintenance therapy), and provision of fluid for deficit requirements (repletion therapy).
Maintenance therapy replaces the ongoing losses of water and electrolytes occurring via normal physiologic processes.Repletion therapy replaces the water and electrolyte deficits that have accrued via some perturbation in normal processes. Repletion returns the patient to a normal volume and electrolyte status. Thus, in a patient who is hypovolemic, fluid therapy will include both repletion and maintenance therapy.
MAINTENANCE WATER NEEDS
Daily water needs are based upon insensible losses from the respiratory tract and skin, and sensible losses from urine and stool. Water requirements are estimated in direct relation to caloric energy expenditures, with approximately 100 mL of exogenous water needed for every 100 kcal/kg of energy expended.
The most widely utilized method for calculation of water needs is based upon the estimated caloric expenditures of hospitalized children at bed rest. Caloric expenditure varies directly with body weight, with the rate changing over three broad weight ranges.
- Weight less than 10 kg — 100 kcal/kg.
- Weight from 10 to 20 kg — 1000 kcal for first 10 kg of body weight plus 50 kcal/kg for any increment of weight above 10 kg.
- Weight from 20 to 80 kg — 1500 kcal for first 20 kg of body weight plus 20 kcal/kg for any increment of weight above 20 kg.
Calculation
The above caloric groupings are directly applied to determine water maintenance requirements. Two methods of calculation are currently popular, one based upon a volume calculated for needs over 24 hours, and the other upon a volume that would be delivered on an hourly basis.
Both methods assume that urinary losses are isosmotic to plasma. Since the kidney can both concentrate and dilute the urine, normal children generally tolerate fluid intakes below or above these calculated values, but these calculations serve as a starting point for a presumptive maintenance fluid volume.
At body weights above 80 kg, the water requirement based upon total body weight falls, so these methods would significantly overestimate the fluid requirements in individuals whose body weight exceeds 80 kg. In these individuals, total maintenance needs are generally capped at 2.4 liters daily.
- Method 1 — Maintenance fluid volume for a 24-hour period:
- Weight less than 10 kg — 100 mL/kg.
- Weight >10 kg to 20 kg — 1000 mL for first 10 kg of body weight plus 50 mL/kg for any increment of weight over 10 kg.
- Weight >20 kg to 80 kg — 1500 mL for first 20 kg of body weight plus 20 mL/kg for any increment of weight over 20 kg, up to a maximum of 2400 mL daily
- Method 2 — Maintenance fluid needed on an hourly basis:
- Weight less than 10 kg — 4 mL/kg per hour.
- Weight >10 kg to 20 kg — 40 mL/hour for first 10 kg of body weight plus 2 mL/kg per hour for any increment of weight over 10 kg.
- Weight >20 kg to 80 kg — 60 mL/hour for first 20 kg of body weight plus 1 mL/kg per hour for any increment of weight over 20 kg, to a maximum of 100 mL/hour, up to a maximum of 2400 mL daily
Comparison between methods
The total daily volume of fluid prescribed by the hourly format is a bit lower than the daily format, but the difference is almost always of no clinical significance. For example, the maintenance needs for a 12-kg child are calculated using both methods:
- Utilizing the 24-hour method, the maintenance needs would be 1100 mL for 24 hours (1000 mL for the first 10 kg, plus 100 mL for the next 2 kg [50 mL/kg per day for each kg of body weight between 10 and 20 kg]).
- Utilizing the hourly method, the maintenance needs would be slightly lower at 44 mL per hour or 1056 mL for 24 hours (40 mL/hour for the first 10 kg of body weight, plus 4 mL/hour for the next 2 kg [2 mL/kg per hour for each kg of body weight between 10 and 20 kg]).
Sensible and insensible water loss
As mentioned above, daily water needs are based upon insensible losses from the respiratory tract and from the skin, and sensible water losses in urine and stool output.
Under normal physiological conditions, insensible losses account for approximately 45 mL per 100 kcal of energy expended. In patients greater than 10 kg, the insensible needs are calculated based upon body surface area at a rate of about 300 to 400 mL/M2.
- Skin losses, due to evaporation from convection and conduction, account for two-thirds of the insensible losses (30 mL per 100 kcal), and increase with higher core body temperature.
- Respiratory losses account for one-third of insensible losses (15 mL per 100 kcal), and result from the warming and humidification of inspired air
Since water loss from stool is negligible in healthy children, sensible water losses are primarily due to the daily urine output (55 mL per 100 kcal). An obligate urine water loss is required to excrete the daily solute load that ensues from dietary intake and cellular metabolism. The daily water maintenance requirements, as calculated above, assume a normal and age-appropriate dietary solute load, and a urine that is isosmotic to plasma (approximately 290 mosmol/L). The minimal obligate urine volume is 25 mL for every 100 kcal of energy expended and requires maximal stimulation of antidiuretic hormone (ADH) release, with a urine osmolality that can reach 1200 to 1400 mosmol/L.
Thus, individuals who do not receive their calculated maintenance water therapy, or who have increased losses, can still maintain water balance by limiting urinary water loss via increased release of ADH, and increased fluid intake because of osmoreceptor stimulation. In these settings, ADH release is stimulated by initial negative water balance, which raises the plasma osmolality and, thereby, triggers osmoreceptors (figure 1). Persistent water deprivation may also trigger baroreceptors that sense changes in intravascular or atrial stretch as effective intravascular volume falls.
Variability in maintenance water needs
Insensible or sensible water losses can vary with a number of clinical scenarios, often as a function of fluid intake, physical exertion, and complicating disease states. As examples:
- Premature infants have increased insensible water losses from the skin due to an increased surface area for mass and a thinner dermis. Water losses are accentuated if the infant is cared for in an open radiant heater or is receiving phototherapy.
- Patients with a colostomy or ileostomy will have increased stool water losses due to their inability to reabsorb intestinal fluid that is usually presented to more distal regions of the digestive tract.
- Patients with oliguric renal failure will have decreased urinary water losses and are at risk for volume overload if maintenance fluid is not modified.
- Patients on ventilators with prehumidified air will have decreased water losses that normally occur with respiration.
- Patients with burns will have increased insensible water and electrolyte losses.
- Patients with fever will also have increased insensible water and electrolyte losses.
In such patients, the daily fluid intake must be appropriately adjusted to maintain balance. Unreplaced water losses will lead to both hypernatremia and volume depletion, while water intake in excess of excretory capacity will lead to both hyponatremia and volume expansion.
When prescribing fluid therapy, the clinician always should consider net volume balance as a dynamic interplay between input and output (ie, an accounting ledger of gains and losses). Often, clinicians may focus on specific parameters, such as urine flow, with a common misperception that urine output exceeding 0.5 to 1 mL/kg per hour corresponds to good renal output. However, the appropriateness of any urine volume over any unit of time directly corresponds to the patient's volume and solute balance, and the concomitant exogenous provision or loss of fluid or solute. Thus, a large volume of fluid in a normovolemic patient should normally lead to a large volume diuresis. In contrast, the hypovolemic patient given a similar large volume of fluid should have limited output until the volume depletion is corrected.
MAINTENANCE ELECTROLYTE NEEDS
Maintenance electrolyte requirements, like water requirements, are estimated based upon caloric energy expenditure. In children, the daily requirements are:
- Sodium and chloride — 2 to 3 meq/100 mL of water per day.
- Potassium — 1 to 2 meq/100 mL of water per day.
Urinary electrolyte losses account for the majority of maintenance electrolyte needs, with fewer electrolyte losses normally coming from losses in sweat and stool. As is the case with water balance, the maintenance electrolyte intake must be considered within the specific clinical context, especially when an abnormal physiologic condition is ongoing. For example, sodium and potassium intake may need to be reduced in patients with oliguric renal failure to prevent volume expansion and hyperkalemia; conversely, their intake may need to be increased in patients with diarrhea or burns to prevent volume depletion and hypokalemia.
MAINTENANCE PARENTERAL THERAPY
Based on the above estimations, the standard commercially available parenteral solution of one-quarter isotonic or normal saline with 20 meq/L potassium will meet the usual electrolyte maintenance needs of a healthy, normovolemic child when a maintenance volume of fluid is administered. The above solution is often given along with 5 percent dextrose, which provides approximately 20 percent of daily caloric needs assuming a maintenance fluid rate.
In a 15-kg child, for example, maintenance water intake is approximately 1250 mL/day (100 mL/kg for the first 10 kg [1000 mL] plus 50 mL/kg for the remaining 5 kg [250 mL]). This volume of one-quarter isotonic saline with 20 meq/L of potassium will provide:
- Sodium intake of 47 meq/day (3.1 meq/kg).
- Potassium intake of 25 meq/day (1.67 meq/kg).
References:UTD