Table of Contents
Physiology of Fluid Replacement
The total body water in a fetus constitutes almost 80 % of its body weight and this gradually decreases as the fetus matures until the total body water in an adult constitutes only 55—60 % of his/ her total body weight.
In addition, babies have 45 % of total body weight as extracellular water and 35% as intracellular water. In contrast to this, adults have only 20 % body weight as extracellular water and 40 % as intracellular water. More importantly children have a higher body surface area to volume ratio compared to adults thus, they need more aggressive fluid resuscitation protocols.
These factors are important to consider during fluid therapy and concomitant drug therapy in children.
Types of Fluid Replacement
Fluid therapy can be classified as maintenance, deficit and replacement therapy based on the child’s requirement.
Maintenance therapy helps to replenish the loss of fluids and electrolytes through sensible and insensible losses such as urine, respiration, sweating, and stools. This can vary depending on the reason for the child’s hospitalization and other factors like excessive anti-diuretic hormone production. Maintenance therapy can be given either orally or intravenously. Children require higher amounts of maintenance fluids as:
- Children have a higher metabolic rate associated with higher caloric usage and consequently, higher fluid requirements.
- Children have a relatively higher body surface area to body weight ratio compared to adults.
- Higher respiratory rate in children means that their insensible loss through respiration is higher than in adults
- Factors such as fever, asthma, congenital abdominal wall defects (associated with higher fluid loss through evaporation), burns and pain may all require larger amounts of maintenance fluids.
The Holliday-Segar method is used to calculate the requirement of maintenance fluids in children. It estimates that for every 100 kilocalories utilized during metabolism, approximately 100 ml of fluid will be needed to replace the losses. Other more complicated methods of calculating fluids losses and requirement of maintenance fluid can also be used.
Fluids which are lost prior to the child presenting to the physician are termed as deficit fluids. The common causes of deficits are vomiting, and diarrhea. Deficit fluid requirement can be calculated based on a child’s clinical condition. Clinical examination of the child helps determine the requirement. Clinical signs of dehydration in children include: weight loss, excessive thirst, dry mucous membranes, sunken eyes, and diminished urine output.
Once dehydration is confirmed clinically, the severity is determined by calculating the pre-illness weight and the weight measured during current illness; then dividing this figure by the pre-illness weight and multiplying by 100.
The degrees of dehydration are:
Mild: 5% body weight loss (Fluid deficit in ml/kg) and 3% in adolescents without any hemodynamic changes
Moderate: 10% body weight in infants and 6% in adolescents with tachycardia
Severe: 15% body weight in infants and 9% in adolescents with hypotension and evidence of tissue/ organ perfusion abnormalities.
If not adequately treated, dehydration worsens to develop into shock which is symbolized by weak or absent pulses, cold extremities, prolonged capillary refill (>3sec), and altered mental status.
Laboratory tests to assess electrolyte levels are also required to determine disturbances like hypo/hypernatremia, hypokalemia, and metabolic acidosis/ alkalosis.
Neonates with dehydration and hypernatremia (Na >160mEq/L) or severe hyponatremia (<120mEq/L) have to be managed carefully to prevent complications.
In older children, based on the clinical estimate of total volume deficit, sodium deficits are estimated to be approximately 60mEq/L and potassium deficits at 30mEq/L of fluid deficit. Oral therapy is recommended by the American Academy of Pediatrics and the WHO for mild to moderate dehydration. Intravenous or nasogastric therapy is advised for children with severe dehydration or those who have excessive vomiting or are unwilling to take oral therapy. Oral therapy is preferred as it is associated with a negligible incidence of infection compared to intravenous therapy.
Replacement therapy is used to correct the fluid-electrolyte imbalance occurring as a result of ongoing illness (compared to “prior illness” in deficit therapy) and helps the patient to attain normal plasma volume and electrolyte levels. Replacement therapy is required in children who suffer from unremitting diarrhea or vomiting or have cerebrospinal fluid shunts or chest tubes in situ. These ongoing losses have to be replaced and cannot be corrected with maintenance fluid therapy alone.
Ongoing losses can be estimated or measured and the replacement therapy is calculated depending on the extent and severity of loss. Electrolyte losses can be estimated based on the etiology and the source of the loss.
The electrolyte imbalance can be corrected based on the renal function of the child. In presence of renal dysfunction, serial electrolyte measurements are required. A child with anuria does not require electrolytes as they are recycled. Intravenous fluids usually include sodium, potassium, and chloride.
5 % dextrose with 0.2 % sodium chloride compensates for the maintenance requirement of sodium in most children.
Fluid Electrolyte Therapy Monitoring
Administration of fluid and electrolyte requires close monitoring of vital signs, urine output, weight, clinical appearance of the child and serial measurements of serum electrolytes. Skin turgor is important for clinical monitoring of a child’s level of dehydration and recovery following fluid therapy. In infants and young children, body weight is also an important criterion as they present with significant weight loss if dehydrated. The specific gravity of urine can also be helpful in monitoring the level of hydration. Clinical parameters are usually the best indicators and only severe cases may require serum electrolyte monitoring.
Another important consideration in fluid therapy is the amount of fluid used with the child’s medications. Over-hydration can be prevented by watching the amount of fluid used to reconstitute medicines. In addition, it is necessary to observe which fluid (e.g. dextrose etc) was used as too much dextrose can result in hyponatremia secondary to over-hydration.