What are the features of the paediatric renal system and fluid handling that differ from adults and what implications do these have?
Features
Implications
- Renal blood flow and glomerular filtration rate are low in the neonate and young infant due to increased vascular resistance
- Reach adult levels between 1 and 2 years age
- Alters the pharmacokinetics of drugs, particularly those renally excreted, requiring age related dose adjustment
- Less able to handle and excrete exogenous fluid and sodium loads – low sodium fluids often used in neonates
- Loop of Henle is short in newborns and tubular function is immature during first year
- Less able to reabsorb fluid and sodium resulting in production of dilute urine
- Hydrogen ion excretion, acid secretion & bicarbonate levels are lower
- Neonates and infants prone to dehydration as unable to produce concentrated urine to withstand fluid deprivation
- Prone to develop severe metabolic acidosis due to difficulties handling acid –base balance
- Total amount of water is 75-80 % of body weight in childhood, 60 % in adults
- There is a larger proportion of extra cellular fluid in children (40% body weight as compared to 20% in the adult)
- Alters the pharmacokinetics of drugs due to differences in the volume of distribution
- In the infant the bladder lies entirely in the abdomen descending into the pelvis by puberty
- Kidneys have less protective perinephric fact in children
- Bladder and kidneys more vulnerable to injury, particularly in blunt trauma
How does the normal blood volume change with age?
Age
Blood Volume
Premature Infant
90-100 ml/kg
Term Infant
80–90 ml/kg
Infant <1 year
75-80 ml/kg
Children
70-75 ml/kg
Adult
65-70 ml/kg
What are the features of the paediatric airway that differ from adults and what implications do these have?
Features
Implications
- Large head, short neck and a prominent occiput.
- ‘Sniffing the morning air’ position will not help bag mask ventilation or visualisation of the glottis
- Head needs to be in a neutral position.
- Epiglottis is long, stiff and U-shaped. It flops posteriorly
- A straight blade laryngoscope may be advantageous to place underneath and lift up the epiglottis
- Tongue is relatively large
- Inadequate displacement of the tongue may impede visualization of the glottis during laryngoscopy
- Larynx is high, anterior (at the level of C3 - C4) and more acutely angled
- Vocal cords are angled more anteriorly rather than a right angle (90°) to the trachea.
- Typically does not affect laryngoscopic view but can make insertion of the endotracheal tube more challenging or traumatic
- Airway is funnel shaped and narrowest at the level of the cricoid cartilage
- Allows uncuffed tube to form an acceptable seal potentially reducing risk of mucosal damage associated with an inflated cuff
- Trachea is shorter in length than adult's
- Tubes must be carefully inserted to the correct length to sit at least 1cm above the carina
- Tubes should be taped securely to prevent tube dislodgement or endobronchial intubation with head movement
- Trachea is smaller in diameter than adult's
- Epithelium is loosely bound to the underlying tissue
- Trauma to the airway easily results in oedema:
- One millimetre of oedema can narrow a baby’s airway by 60% (resistance ∝ 1/radius)
- A small leak should be present around an uncuffed tube to prevent development of subglottic oedema
- Neonates preferentially breathe through their nose
- May be blocked easily by secretions
- Careful suctioning of the nose is important
What are the features of the paediatric central nervous system and psychology that differ from adults and what implications do these have?
Features
Implications
- Cerebral metabolic requirement for oxygen (CMRO2) and cerebral blood flow are higher in young children
- Brain forms higher % of body weight (10–15% of body weight at birth, but only 2% of body weight by the age of 8 years)
- More susceptible to periods of hypoglycaemia, hypoxia and decreased cerebral blood flow
- Blood brain barrier is poorly formed
- Drugs such as barbiturates, opioids, antibiotics and bilirubin cross the blood brain barrier easily causing a prolonged and variable duration of action
- Entry of ammonia into the brain can leads to hyperammonaemic encephalopathy
- Head is large and heavy relative to the size of the body
- Balanced on a neck poorly supported by weak muscles and ligaments
- Both head and cervical spine are easily injured
- Cerebral vessels in the preterm infant and neonate are thin walled and fragile
- Prone to developing intraventricular haemorrhages
- Anxiety related to separation, unfamiliar people and environments becomes marked over the age of 6 months
- Parental anxiety readily perceived and reacted by children
- Anxiety and associated behavioural change can lead to a significant impact on outcomes such as pain and length of stay
- Reducing child and parent anxiety important in optimising outcomes from anaesthesia and intensive care