Maintaining fluid and electrolyte balance is crucial for the well-being of children, and subtle imbalances can have significant consequences. Children are susceptible to dehydration and volume depletion and have different fluid requirements from adults.
The improper use of intravenous fluids in children can have harmful consequences, and it is vital that clinicians working with children have a robust understanding of the principles of paediatric fluid prescription. For this reason, it is also a common topic in both undergraduate and postgraduate examinations.
Compartments and fluid spaces
There are two major fluid compartments in the human body:
- Intracellular fluid (ICF) – the water within the cells. This is the matrix in which intracellular organelles are suspended and chemical reactions take place.
- Extracellular fluid (ECF) – the water outside the cells.
These compartments are separated by the plasma membrane of the cells and differ markedly in terms of the concentrations of the ions dissolved in them.
The extracellular fluid is further divided into three other compartments:
- Interstitial fluid (ISF): This is the tissue fluid found in the spaces between the cells and accounts for approximately 65% of the ECF.
- Intravascular fluid: This is the plasma, which is the liquid component of the blood and accounts for approximately 25% of the ECF.
- Transcellular fluid: This is the final 10% of fluid in the ECF and comprises intraocular fluid, cerebrospinal fluid, urine in the bladder, and fluid within the lumen of the bowel.
The total body water varies with age. At birth, approximately 80% of a child’s body weight is water. This falls gradually during childhood and is roughly 70% in infants, 65% in children, and 60% in adults.
Fluid and electrolyte balance
Fluid and electrolyte balance is fundamental to various physiological processes in the human body, including maintaining blood pressure, supporting organ function, and regulating cellular activity. In children, this balance is particularly delicate due to their unique physiology and developmental stages.
The physiology of fluid and electrolyte balance in children is a complex and is regulated by a system of entwined physiological functions. Children, especially infants and young toddlers, have distinct physiological characteristics that influence their fluid and electrolyte needs.
The kidneys play a central role in regulating fluid and electrolyte balance. In children, the immature renal system affects their ability to concentrate urine and conserve or excrete electrolytes efficiently. This makes children more susceptible to dehydration and electrolyte imbalances, especially during periods of illness, fever, or increased metabolic demand..
Children often experience higher rates of gastrointestinal losses due to factors such as vomiting and diarrhoea. These losses can lead to rapid dehydration and electrolyte disturbances, emphasising the importance of early recognition and intervention in paediatric patients.
Insensible water loss
Children have a higher ratio of body surface area to body weight compared to adults, resulting in increased insensible water losses through the skin and respiratory tract. This makes them more susceptible to dehydration, particularly in conditions associated with increased respiratory rates or fever.
The high metabolic rate in children, especially infants, contributes to increased fluid and electrolyte needs. Rapid growth and development demand adequate hydration and electrolyte support for cellular functions, energy production, and overall physiological processes.
Sodium, potassium, chloride, calcium, phosphate, and magnesium are essential electrolytes that play vital roles in neuromuscular function, cellular integrity, and acid-base balance. Children have specific electrolyte requirements that vary with age, and maintaining these balances is critical for proper growth and development.
Hormones such as antidiuretic hormone (ADH) and aldosterone play key roles in regulating fluid balance and electrolyte levels. The immature endocrine system in children may affect the responsiveness of these hormonal mechanisms, influencing their ability to adapt to changes in hydration status.
The thirst mechanism is an essential aspect of fluid balance. Children may not always effectively communicate their thirst, and caregivers need to be attentive to signs of dehydration, especially in younger children and infants.
In summary, the physiology of fluid and electrolyte balance in children is characterised by unique developmental factors that impact their vulnerability to dehydration and electrolyte imbalances. Understanding these physiological nuances is crucial for healthcare professionals in assessing, managing, and maintaining optimal fluid and electrolyte status in paediatric patients.
Next: Paediatric Intravenous Fluid Prescribing – Routine Maintenance Fluids
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