How does the total body fluid compare in children to adults?
It is higher in children
What % of the body is made up of water at birth?
80%
What % of the body is made up of water by adulthood?
55-60%
Where is water distributed throughout the body?
- Intracellular space
- Extracellular space
What proportion of water is distributed in the intracellular space?
2/3
What proportion of water is distributed in the extracellular space?
1/3
Where is water distributed within the extracellular space?
- Interstitial
- Intravascular
What proportion of extracellular water is interstitial?
75%
What proportion of extracellular water is extracellular?
25%
What does the distribution of water between the intracellular and extracellular spaces depend on?
The pressure and osmotic gradients between them
What is dehydration?
Loss of water and electrolytes
What might cause children to become dehydrated?
- Reduced oral fluid intake
- Additional fluid losses
- Increased insensible losses
- Loss of normal fluid retaining mechanisms
What might cause reduced oral fluid intake?
- Reduced appetite due to illness
- Vomiting
- Sore throat
What might cause additional fluid losses?
- Fever
- Diarrhoea
What might cause increased insensible losses?
- Increased sweating
- Tachypnoea
What might cause a loss of the normal fluid-retaining mechanisms?
- Capillary leak
- Burns
- Permeable skin of premature infants
- Increased urinary losses secondary to renal disease
Why are infants and young children more prone to dehydration than older children and adults?
- Body made up of more water
- High surface area in relation to their height or weight
- Relatively high evaporative water losses
- Higher metabolic rate, so higher turnover of water and electrolytes
- Rely on others to give them fluids
Does dehydration itself cause death?
No
How can dehydration lead to death?
It can cause shock, which can lead to death
When does shock occur in dehydration?
When there is rapid loss of at least 25% of intravascular volume that is not replaced at a similar rate from the interstitial space
Can shock occur without dehydration?
Yes
What does the treatment of shock require?
Rapid administration of intravascular volume of fluid
What should be true of fluid administered to treat shock?
It should approximate in electrolyte content to plasma
How should dehydration without shock be treated?
Gradual replacement of fluids
What should the electrolyte content of the fluid used to treat dehydration resemble?
The electrolyte content of the fluid that is lost, or to the total body electrolyte content
What can be used to objectively measure the total body fluid changes?
Weight
How can percentage dehydration be calculated?
(weight before - weight after ) / weight before = % dehydration
What is the limitation of calculating percentage dehydration?
Pre-illness weight is rarely available in emergency situations
What can be used to assess dehydration when pre-illness weight is not available?
Clinical symptoms and signs
Why are ‘red flag’ symptoms of dehydration important?
They help identify children with severe dehydration at increased risk of shock
At what level of fluid losses does clinical dehydration become apparent?
> 25-50ml/kg (2.5-5%)
At what level of fluid losses is shock seen in dehydration?
Over 10%
What are the types of dehydration?
- Isotonic/isonatraemic
- Hypotonic/hyponatraemic
- Hypertonic/hypernatraemic
What is hypotonic dehydration defined as?
Dehydration in association with plasma sodium concentration <135mmol/L
What is considered to be extreme hyponatraemia?
<125mmol/L
What is extreme hyponatraemia, or a rapid fall in serum sodium, associated with?
- Cognitive impairment
- Seizures
- Brainstem herniation
- Death due to cerebral oedema
What fluids are no longer used in paediatrics due to the risk of hyponatraemia?
Hypotonic crystalloid fluids, e.g. NaCl 0.18% with glucose 4%
What is hypertonic dehydration?
Dehydration with sodium >145mmol/L
What causes hypertonic dehydration?
Excessive free water loss, or rarely, the administration of excess sodium
What protects against hypernatraemia in healthy individuals?
Thirst and the stimulation of ADH
When does sustained hypernatraemia predominantly occur?
When thirst or access to water is impaired
Who is at the highest risk of hypernatraemic dehydration?
Infants, especially breastfed infants establishing lactation and infants with gastroenteritis
What fluid shifts occur in hypernatraemia?
From intracellular compartment to extracellular spaces
What does the shift of fluid from intracellular compartments to extracellular spaces in hypernatraemia lead to?
Cellular dehydration
What cells are particularly vulnerable to complications arising from cellular dehydration?
Brain cells
What is severe hypernatraemic dehydration associated with?
- Cerebral haemorrhage
- Seizures
- Paralysis
- Encephalopathy
What can rapid rehydration with hypertonic fluids cause?
- Cerebral oedema
- Central pontine myelinolysis
What can central pontine myelinolysis lead to?
- Coma
- Convulsions
- Death
What is done as a result of the potential for rapid rehydration with hypertonic fluids to cause serious complications?
If IV therapy is required to treat dehydration, first check plasma electrolytes so rehydration fluid choice and replacement rate is appropriate
What is meant by maintenance fluid therapy?
The provision of fluid and electrolytes to replace anticipated losses from breathing, sweating, and urine output
What assumptions is maintenance fluid calculations based on?
- 100kcal/kg/day of caloric requirement
- 3ml/kg/day of urine output
- Normal stool output
What is the name of the formula used to calculate maintenance fluid requirements?re ma
Holliday-Segar formula
How is maintenance fluid requirement calculated using the Holliday-Segar formula?
- 100ml/kg/day for the first 10kg
- 50ml/kg/day for the second 10kg
- 20ml/kg/day for subsequent kilograms
What is the limitation of using the Holliday-Segar formula to calculate maintenance fluid requirements in critical illness or injury?
Losses may be profoundly disturbed
What is the limitation of using the Holliday-Segar formula to calculate maintenance fluid requirements in critical illness or injiry?
Give an example of when maintenance fluid requirements might decrease in critical illness or injury?
In SIADH, secondary to acute respiratory or neurological pathology
Why might maintenance fluid requirements be reduced in SIADH secondary to critical illness?
Due to reduced renal fluid loss
How low might maintenance fluid requirements be in the context of SIADH in critical illness/injury?
30ml/kg/day
Give an example of when maintenance fluid requirements might increase in critical illness or injury?
Acute diarrhoeal illness
By how much might maintenance fluid requirements increase in acute diarrhoeal illness?
4x
What is the first choice route for providing maintenance fluids?
Enteral feeds (mouth or NG tube)
What fluid is the first choice if IV maintenance fluid is required?
Isotonic crystalloid fluids, e.g. NaCl 0.9% with dextrose 5%
What is the first choice IV maintenance fluid in neonates?
Glucose 10% with added NaCl
Why is glucose 10% with added NaCl the first choice IV maintenance fluid in neonates?
Because they have a higher glucose requirement
What is the aim of rehydration fluid therapy?
To replace fluid losses and correct any electrolyte deficits
How can the deficit volume to be replaced in rehydration fluid therapy be calculated?
% dehydration x weight (kg) x 10
What is the limitation of the formula to calculate the deficit volume to be replaced in rehydration fluid therapy?
It requires you to know the percentage dehydration accurately, which is often not the case in clinical practice
How is dehydration graded clinically?
- No clinically detectable dehydration
- Clinical dehydration
- Clinical shock
How should children with no clinically detectable dehydration be managed?
Regular oral intake, including breastfeeding and other milk feeds, should be encouraged
What can be used if a child is at risk of developing dehydration?
Oral rehydration solution as a supplemental fluid
How is clinical dehydration with no red flag symptoms or shock managed?
Rehydration via the oral route using a low osmolarity ORS
What osmolarity of ORS is used to manage clinical dehydration?
240-250mOsm/L
What volume of ORS is recommended for fluid deficit replacement in clinical dehydration?
50ml/kg over 4 hours
What does fluid deficit replacement with ORS need to be given in addition to in clinical dehydration?
A child’s maintenance fluid
How can ORS be given?
Orally or via NG tube
When is rehydration with IV fluids indicated?
- When there are red flag symptoms
- Persistent vomiting despite NG tube administration of ORS
- Suspected/confirmed shock
How should cases of shock caused by dehydration be managed?
Rapid IV fluid bolus
What volume of IV fluid bolus is given to treat shock caused by dehydration?
20ml/kg
What IV fluid is given to treat shock caused by dehydration?
0.9% NaCl
What should be done if repeated rapid fluid boluses are needed to treat shock caused by dehydration?
Other causes of shock need to be considered
What should be done once the shock caused by dehydration has been resolved?
Rehydration can start with IV fluids
What does the choice of fluid and rate of deficit correct depend on once resolved shock caused by dehydration depend on?
The serum electrolyte levels, in particular serum sodium levels
What is used for fluid deficit replacement and maintenance in isonatraemic or hyponatraemic dehydration?
An isotonic solution such as 0.9% sodium chloride +/- 5% glucose
How much fluid should be given for fluid deficit replacement in dehydrated children who initially require rapid intravenous fluid boluses for suspected or confirmed shock?
100mg/kg/day (added to maintenance fluid requirements)
How much fluid should be given for fluid deficit replacement in dehydrated children who are not shocked at presentation?
50mg/kg/day (in addition to maintenance fluids)
Over what time period should rehydration with IV fluid therapy be undertaken in isonatraemic or hyponatraemic dehydration?
3-6 hours
Why is slow rehydration (i.e. over 24 hours) no longer recommended in isonatraemic and hyponatraemic dehydration?
- Results in children remaining dehydrated for longer
- Delays restarting oral fluids
- Associated with longer hospital stays
What fluid is used for fluid deficit replacement and maintenance in hypernatraemic dehydration?
Isotonic solutions such as 0.9% sodium chloride +/- 5% glucose
What is the difference between fluid replacement in hypernatraemic dehydration compared to iso/hyponatraemic?
Fluid deficit must be replaced slowly, and typically over 48 hours
Why should the fluid deficit be replaced more slowly in hypernatraemic dehydration?
It is important the serum sodium levels are reduced slowly, as rapid correction is associated with cerebral oedema
What is the maximum rate of reduction of serum sodium levels?
0.5mmol/L per hour
Why is it important to consider electrolyte disturbances when managing DKA?
Children presenting with DKA are often severely dehydrated, with abnormalities in glucose and sodium levels
What is the risk of over hydration and/or rapid correction of blood sodium and glucose levels in DKA?
Risk of cerebral oedema
How is shock in DKA managed?
10ml/kg aliquots of 0.9% sodium chloride, limiting the total fluid deficit replacement to 8% dehydration or 30ml/kg and rehydrating over 48 hours
