Physiology of Infants & Children

RESOURCES

OBJECTIVES & QUESTIONS

  • FUNDAMENTALS
  • PATHOLOGY & AETIOLOGY
  • CLINICAL FEATURES
  • DIAGNOSIS & INVESTIGATIONS
  • MANAGEMENT

Expand All
Close All

Overview & Vital Signs

How are paediatric patients classified by age?

Neonate
Infant
Child
Adolescent
Within 44 weeks of age from the date of conception
Up to 12 months of age
Child 1 to 12 years
13 to 16 years

What are the normal values for vital signs in children & infants?

Normal vital sign values vary significantly with age:

Age

Newborn

1 - 3 mo

3 - 6 mo

6 - 12 mo

1 - 2 yrs

2 - 4 yrs

4 - 6 yrs

6 - 8 yrs

8 - 10 yrs

10 - 12 yrs

12 - 14 yrs

>14 yrs

APLS Normal Values
Respiratory Rate
40-60
30-50
30-45
25 -35
20-30
20-28
20-26
18-24
18-22
16-20
16-20
16-20
Heart Rate
90-180
110-180
110-180
110-180
80-160
80-140
80-120
75-115
70-110
70-110
60-110
60-110
Systolic BP
60-90
70-104
70-104
72-110
72-110
74-110
78-112
82-115
86-118
90-121
90-126
92-130
ACC Threshold Values
(Used in Shock)
Heart Rate
120–180
120–180
120–180
120–180
120–160
100–140
100–140
100–140
100–140
100–140
90–140
90–140
Mean BP
≥ 55
≥ 60
≥ 60
≥ 60
≥ 65
≥ 65
≥ 65
≥ 65
≥ 65
≥ 65
≥ 65
≥ 65
A: Airway

What are the features of the paediatric airway that differ from adults and what implications do these have?

Features
  • Large head, short neck and a prominent occiput.
  • Epiglottis is long, stiff and U-shaped. It flops posteriorly
  • Tongue is relatively large
  • 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.
  • Airway is funnel shaped and narrowest at the level of the cricoid cartilage
  • Trachea is shorter in length than adult's
  • Trachea is smaller in diameter than adult's
  • Epithelium is loosely bound to the underlying tissue
  • Neonates preferentially breathe through their nose
Implications
  • ‘Sniffing the morning air’ position will not help bag mask ventilation or visualisation of the glottis
  • Head needs to be in a neutral position.
  • A straight blade laryngoscope may be advantageous to place underneath and lift up the epiglottis
  • Inadequate displacement of the tongue may impede visualization of the glottis during laryngoscopy
  • Typically does not affect laryngoscopic view but can make insertion of the endotracheal tube more challenging or traumatic
  • Allows uncuffed tube to form an acceptable seal potentially reducing risk of mucosal damage associated with an inflated cuff
  • 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
  • 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
  • May be blocked easily by secretions
  • Careful suctioning of the nose is important
B: Respiratory System

What are the features of the paediatric respiratory system that differ from adults and what implications do these have?

Features
  • Limited respiratory reserve due to fatigable muscles, differing respiratory mechanics, increased dead space and reduced functional residual capacity (FRC)
  • Ventilation is primarily diaphragmatic
  • Diaphragm is relatively horizontal rather than dome shaped
  • Bulky abdominal organ which can easily splint the diaphragm
  • Horizontal ribs prevent the ‘bucket handle’ action seen in adults
  • Limits ability to increase tidal volume
  • Chest wall is significantly more compliant than that of an adult
  • FRC is relatively low
  • Closing volume is larger than FRC until 6-8 years
  • Work of respiration may be up to 15% of oxygen consumption
  • Muscles of respiration contain low levels of type 1 muscle fibres when immature
  • Physiological dead space is around 30% and may be significantly increased by anaesthetic equipment
  • Respiratory centre is relatively immature
  • Neonates and young infants have irregular respirations and are at a greater risk of apnoea
Implications
  • ‘Sniffing the morning air’ position will not help bag mask ventilation or visualisation of the glottis
  • Head needs to be in a neutral position.
  • FRC decreases with apnoea and anaesthesia
  • Stomach insufflation due to poor bag-mask ventilation can splint the diaphragm - nasogastric tube decompression can significantly improve ventilation
  • Contributes to limited respiratory reserve
  • Important to vary ventilatory rate according to age of the child
  • Contributes to limited respiratory reserve
  • Reduced time to desaturation during apneoa
  • Prone to atelectasis - PEEP increases increases lung volumes and reduces the risk of atelectasis during anaesthesia
  • Contributes to limited respiratory reserve
  • Easily prone to fatigue
  • Contributes to limited respiratory reserve
  • CPAP improves oxygenation and reduces work of breathing during spontaneous ventilation
  • Prone to rebreathing during invasive ventilation - apparatus dead space and resistance should be kept to a minimum
  • Apnoeas may be a manifestation of significant systemic disease in neonates and young infants
  • Apnoeas are common postoperatively in premature infants - vigilant monitoring required
C: Cardiovascular System

What are the features of the paediatric cardiovascular system that differ from adults and what implications do these have?

Features
  • Myocardium less contractile
  • Ventricle is less compliant and less able to generate tension during contraction
  • Limits size of and changes in stroke volume
  • Vagal parasympathetic tone is most dominant
  • Sinus arrhythmia is common
  • Resting and maximal heart rate values decrease spontaneously with age
  • Cardiac output is 300-400 ml/kg/min at birth and decreases to 200 ml/kg/min within a few months
  • Ductus arteriosus contracts in the first few days of life and fibroses within 2-4 weeks
  • Closure is pressure dependent with neonatal pulmonary vasculature reacting to the rise in PaO2 and pH and the fall in PaCO2 at birth
Implications
  • Cardiac output becomes rate dependent
  • Bradycardia associated with reduced cardiac output and in the neonate external compression should be started if the rate is <60 bpm
  • More prone to developing low cardiac output ('cold') shock during sepsis
  • Prone to bradycardia with hypoxia, which should should be treated with oxygen and ventilation initially
  • Age dependent parameters should be targeted during resuscitation
  • Alterations in pressure and in response to hypoxia and acidosis can lead to reversion to the transitional circulation in the first few weeks after birthv
D: Central Nervous System

What are the features of the paediatric central nervous system and psychology that differ from adults and what implications do these have?

Features
  • 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)
  • Blood brain barrier is poorly formed
  • Head is large and heavy relative to the size of the body
  • Balanced on a neck poorly supported by weak muscles and ligaments
  • Cerebral vessels in the preterm infant and neonate are thin walled and fragile
  • Anxiety related to separation, unfamiliar people and environments becomes marked over the age of 6 months
  • Parental anxiety readily perceived and reacted by children
Implications
  • More susceptible to periods of hypoglycaemia, hypoxia and decreased cerebral blood flow
  • 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
  • Both head and cervical spine are easily injured
  • Prone to developing intraventricular haemorrhages
  • 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

E: Endocrine System & Metabolism

What are the features of the paediatric endocrine system & metabolism that differ from adults and what implications do these have?

Features
  • Young children have an undeveloped hypothalamus
  • Large surface area to weight ratio
  • Increased thermal conductance
  • Subcutaneous tissue is 20-30% thinner than adults
  • Poorly developed shivering, sweating and vasoconstriction mechanisms
  • In neonates and young infants, heat production occurs through metabolism of brown fat (located in small amounts around the scapulae, the mediastinum, the kidneys and adrenal glands) - known as non-shivering thermogenesis
  • Neonates and young infants have limited glycogen stores and rely heavily on gluconeogenesis for maintenance of glucose levels
  • Fat tissue as a proportion of body weight is higher in infants, peaking between 3–6 months of age
Implications
  • Ability to regulate temperature is impaired
  • Prone to excessive heat loss and thermoregulation easily overwhelmed by environmental conditions
  • Requires higher optimal ambient temperature in neonates and young infants
  • Brown fat metabolism occurs at the expense of increased oxygen consumption
  • Volatile agents inhibit brown fat metabolism further affecting temperature regulation during general anaesthesia
  • Hypoglycaemia is common in the stressed neonate and young infant
  • Glucose levels should be monitored regularly in the neonatal period
  • Infusions of 10% glucose may be required to prevent hypoglycaemia and resultant neurological damage
  • Altered body composition may affect volume of distribution of certain drugs
F: Renal System & Fluid Handling

What are the features of the paediatric renal system and fluid handling that differ from adults and what implications do these have?

Features
  • 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
  • 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
  • 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)
  • In the infant the bladder lies entirely in the abdomen descending into the pelvis by puberty
  • Kidneys have less protective perinephric fact in children
Implications
  • 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
  • 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
  • Alters the pharmacokinetics of drugs due to differences in the volume of distribution
  • Bladder and kidneys more vulnerable to injury, particularly in blunt trauma
G: Gastrointestinal & Hepatic System

What are the features of the paediatric gastrointestinal and hepatic system that differ from adults and what implications do these have?

Features
  • Liver function is initially immature, with decreased hepatic enzyme function
  • Liver and spleen ride comparatively low within the abdomen and have minimal protection from ribs
  • Intestines, especially the sigmoid and right colon, are not fully attached in the peritoneal cavity
Implications
  • Drug metabolism may be slower leading to a longer duration of action (e.g. barbiturates and opioids)
  • Abdominal viscera more prone to injury, particularly in blunt trauma
H: Haematological System

What are the features of the pediatric haematologcial system that differ from adults and what implications do these have?

Features
  • Foetal haemoglobin (HbF) forms, 70-90% of the haemoglobin molecules at birthDrops to around 5% HbF by 3 months with HbA predominating
  • Foetal haemoglobin contains less 2,3-DPG and combines more readily with oxygen
  • Haemoglobin levels change rapidly with age:
    • Newborn born with levels 180-200 g/L
    • Levels drop over 3-6 months to 90-120 g/L
    • Stabilises thereafter to at approximately 130 g/L
  • Development of the coagulation system not complete until about 6 months of age
  • Vitamin K dependent factors are 70% of adult values at birth
  • Total blood volume smaller in children than infants
  • Relative blood volume per unit mass larger in the newborn and decreases with age
Implications
  • Allows the foetus is able to retrieve oxygen from maternal blood
  • Haemoglobin levels should be interpreted in an age dependent manner
  • Vitamin K is given at birth to prevent haemorrhagic disease of the newborn
  • Coagulation screening tests are prolonged in normal infants up to the age of 6 months, which is reflected in values for the normal ranges
  • Lower total blood volume leads to reduced tolerance for blood loss during surgery or trauma
  • Transfusion is generally recommended when 15% of the circulating blood volume has been lost
  • Higher relative blood volume required to support higher basal metabolic rate
  • Affects pharmacokinetics of certain drugs

How does the normal blood volume change with age?

Age
Premature Infant
Term Infant
Infant <1 year
Children
Adult
Blood Volume
90-100 ml/kg
80–90 ml/kg
75-80 ml/kg
70-75 ml/kg
65-70 ml/kg
I: Immune System

What are the features of the paediatric immune system that differ from adults and what implications do these have?

Features
  • Innate immune system is muted at birth and matures with increasing age
  • The adaptive immune system is immature, providing increased protection over time with accumulation of immunological memory
  • Neonates and young infants receive IgG antibody transfer from the mother transplacentally and in breast milk
Implications
  • Newborns and young infants relatively susceptible to bacterial and viral infections
  • Allows the foetus to tolerate non-shared maternal antigens
  • Passive IgG antibodies provide some protection against infection
  • Young children become more vulnerable to infections when passive IgG transfer stops until immune memory matures

Drug Handling

What are the differences in the way children handle drugs in comparison to adults? What implications do these have?

Features
  • Liver function is initially immature, with decreased hepatic enzyme function
  • 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
  • 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
  • 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)
  • Blood brain barrier is poorly formed
Implications
  • Drug metabolism may be slower leading to a longer duration of action (e.g. barbiturates and opioids)
  • Alters the pharmacokinetics of drugs, particularly 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
  • 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
  • Alters the pharmacokinetics of drugs
  • Drugs such as barbiturates, opioids, antibiotics and bilirubin cross the blood brain barrier easily causing a prolonged and variable duration of action

Author

The Guidewire
Trainee in ICM & Anaesthesia

Reviewer

The Guidewire
Trainee in ICM & Anaesthesia