Dehydration And Fluid Management In Children

Introduction
Total body fluid is higher in children than adults. At birth, the body is made up of 80% water. By adulthood it is only 55–60%. Water is distributed two thirds in the intracellular space and one third in the extracellular space (divided 75% interstitial and 25% intravascular). The distribution of water between these compartments depends on the pressure and osmotic gradients between them.

Dehydration
Dehydration is loss of water and electrolytes.

Causes Of Dehydration in Children: Children may become dehydrated from:
• Reduced oral fluid intake: reduced appetite due to illness, vomiting, sore throat
• Additional fluid losses: fever, diarrhea
• Increased insensible losses: increased sweating, tachypnoea
• Loss of the normal fluid-retaining mechanisms: capillary leak, burns, the permeable skin of premature infants, increased urinary losses secondary to renal disease.

Infants and young children are more prone to dehydration than older children and adults because:
• Their body is made up of more water
• They have a high surface area in relation to their height or weight (high surface area : volume
ratio)
• They have relatively high evaporative water losses
• They have a higher metabolic rate and so higher turnover of water and electrolytes
• They rely on others to give them fluids.


Dehydration itself does not cause death, but shock does. Shock occurs when there is rapid loss of at least 25% of the intravascular volume that is not replaced at a similar rate from the interstitial space .
Shock can occur in the absence or presence of dehydration, depending on the rate of fluid loss and fluid shifts between compartments.

Pathophysiology: The treatment of shock requires rapid administration of intravascular volume of fluid that approximates in electrolyte content to plasma.

In contrast, the treatment of dehydration requires gradual replacement of fluids with an electrolyte
content that relates to the electrolyte losses, or to the total body electrolyte content.

Types of dehydration
There are three types of dehydration based on plasma sodium levels:
• Isotonic or isonatraemic – equal loss of water and electrolytes
• Hypotonic or hyponatraemic – primarily a loss of electrolytes and in particular sodium. In this type the plasma sodium concentration is <135 mmol/L. Extreme hyponatraemia (<125 mmol/L) or a rapid fall in serum sodium concentration is associated with cognitive impairment, seizures, brain stem herniation and death due to cerebral edema.
• Hypertonic or hypernatraemic – primarily a loss of water. In this type the serum sodium concentration is  >145 mmol/L. Severe hypernatraemic dehydration is associated with cerebral haemorrhage, seizures, paralysis, and encephalopathy.

Therefore, if intravenous fluid therapy is required to treat dehydration, it is important to first check plasma electrolytes so that rehydration fluid choice and replacement rate is appropriate.

Maintenance fluid therapy
Maintenance therapy is the provision of fluid and electrolytes to replace anticipated losses from breathing, sweating and urine output.
The maintenance fluid calculation is based on several assumptions: 100 kcal/kg/day of caloric requirement, 3 mL/kg/day of urine output and normal stool output. Fluid requirements are greatest for the first 10 kg of body weight, with a marked reduction of fluid required per kg after the first 20 kg of body weight.

However, in critical illness or injury, losses may be profoundly disturbed. For example, in syndrome of inappropriate antidiuretic hormone secretion (SIADH) secondary to acute respiratory or neurological pathology, reduced renal fluid loss may reduce fluid requirements to as low as
30 mL/kg/day, whereas in severe diarrhoeal illness, fluid requirements may increase up to fourfold. It is therefore important to make individual regular assessments of all patients requiring intravenous fluid therapy.

Enteral feeds, by mouth or nasogastric tube, are in general used as maintenance fluid if tolerated. If intravenous maintenance fluid is required, isotonic crystalloid fluids are used, for example, sodium chloride 0.9% with dextrose 5%. In neonates, glucose 10% with added sodium chloride is used as neonates have a higher glucose requirement.

Rehydration fluid therapy
Rehydration therapy aims to replace fluid losses and correct any electrolyte deficits. In clinical practice, the severity of dehydration is often determined and categorized by clinical features.

In children with no clinically detectable dehydration, regular oral fluid intake, including breastfeeding and other milk feeds, should be encouraged. If a child is at risk of dehydration, oral rehydration solution (ORS) can be offered as supplemental fluid to prevent dehydration occurring.

In clinical dehydration, where there are no red flag symptoms or shock, the aim of management is to
rehydrate via the oral route using a low osmolarity ORS (240–250 mOsm/L). A volume of 50 mL/kg of ORS is recommended for fluid deficit replacement over 4 hours in addition to a child’s maintenance fluid volume. The ORS can be given orally or via a nasogastric tube.

Rehydration with intravenous fluids is only indicated if there are red flag symptoms, persistent vomiting despite nasogastric tube administration of ORS, or suspected/confirmed shock.

In cases of shock, a rapid intravenous fluid bolus of 20 mL/kg of 0.9% sodium chloride should be
given. If features of shock persist and repeated fluid boluses are needed, then other causes of shock other than dehydration need to be considered .
Of note, the isotonic crystalloid 0.9% sodium chloride is currently the recommended first line fluid for resuscitation of all seriously ill children, though in practice other isotonic fluids such as Ringer’s lactate are often used. There is currently insufficient evidence in the literature to support preferential use of one type of fluid over another.