Developmental - Paediatric physiology

Question 1

What is the difference between a neonate, infant, child and adolescent?

Answer 1

Neonate: A baby < 44 weeks post-conception (usually first 28 days of life)

Infant: 28 days –> 1 year

Child: 1 year –> 12 years

Adolescent: 13 –> 17 years

Question 2

Summarise the unique airway attributes in childhood relevant to anaesthesia

Answer 2

1. Big HEAD (neutral > sniffing)
2. Big TONGUE
3. Big EPIGLOTTIS
4. Short neck
5. Short trachea
6. Narrowest at cricoid ring (not cords)
7. Cephalad larynx (C3 neonate –> C4 infant –> C5/C6 adult)

Question 3

Why do excessive secretions/NGTs affect neonatal/infant breathing more than adults

Answer 3

Before 4 months of age. Neonates and infants are obligate nasal breathers

Question 4

Why is the shorter trachea problematic for the anaesthetist

Answer 4

Increase risk of endobronchial intubation

Question 5

What is an appropriate position for ETT in paeds

Answer 5

Tip 1 cam superior to the carina

Question 6

Why is it relevant that the narrowest point of the airway is the carina and not the vocal cords in children

Answer 6

The pseudostratified columnar epithelium is very prone to developing oedema following airway trauma. Caution with ETT cuff pressures and often an ETT cuff is not necesary

Below age 10 –> better to use uncuffed tube
There should be a small air leak during positive pressure ventilation

Question 7

What is the formula for ETT size

Answer 7

(Age/4) + 4

Question 8

Summarise the unique respiratory physiology in children relevant to anaesthesia

Answer 8

1. High VO2 (High BMR) 6ml/kg in kids (3 ml/kg adults)
2. High Va (achieves normal PaCO2 from high BMR)
3. High RR to achieve increase Va
4. Normal Vt (6 - 8 ml/kg)
5. Diaphragm dominant (minimal bucket handle & intercostal recession in RDS)
6. Low FRC (Increased chest wall compliance)
7. CC > FRC (neonates) –> V:Q mismatch
8. Easy fatigued muscles of respiration

Question 9

How is PaCO2 different in children? Discuss the physiological mechanism for this.

Answer 9

As children have an increased BMR and double the oxygen consumption of adults, it would be expected that PaCO2 should be higher in children. This is not the case as the paediatric physiology adjusts to this increased production of CO2.

Alveolar ventilation is increased to eliminate the additional CO2. Vt is not altered vs adult (6 - 8 ml/kg) so the alveolar ventilation is increased via increase in respiratory rate.

Question 10

Which muscle of respiration is responsible for the majority of the breathing mechanics in paediatric physiology. Explain the mechanism and implications of this

Answer 10

Diaphragm
1. Ribs soft and aligned horizontally –> absent ‘bucket-handle’ mechanism
2. Increased chest wall compliance –> chest wall recession with more negative intra-pleural pressure
Therefore, inspiration is dependent on diaphragm.

Acute abdomen/gas insufflation –> splints the diaphragm with more physiological disruption than in adults

Question 11

What is the mechanism for reduced FRC in children? What are the implications of this?

Answer 11

1. Soft ribs –> higher chest wall compliance.
   The point at which inward elastic recoil forces of the lung match reduced outward chest wall forces is reduced –> reduced FRC.

FRC is a reservoir for O2. Low FRC leads to:

1. Rapid desaturation during apnoea
2. CC can exceed FRC (esp. in neonates) –> V:Q mismatch.

Question 12

What are the factors that determine FRC

Answer 12

1. Thoracic wall compliance i.e. force of outward elastic recoil of chest wall versus the inward elastic recoil of the lungs. The point at which these forces are balanced determines the FRC
2. intrinsic PEEP –> partial adduction of vocal cords during expiration
3. Inspiratory muscle tone

Question 13

Why are the muscles of respiration easily fatigued in paediatrics

Answer 13

1. Lower lung volumes and FRC

The above factors position the breathing mechanics on a less favourable part of the pressure-volume curve. This results in reduced lung compliance requiring increased work of breathing. This means that fatigue is sooner

2. Increased chest wall compliance + Inadequate becket handle mechanism = reduced breathing efficiency
3. Lack of type 1 muscle fibres –> easy fatiguability.

Question 14

Summarise the unique aspects of cardiovascular physiology in children relevant to anaesthesia

Answer 14

The following cardiovascular differences are most pronounced in neonates and become more adult-like with age

1. Cardiac Index (CO/BSA) increased 30 - 60%
2. Limited Frank-Starling response (CO is HR dependent)
3. HR decreases with age (HR 120 –> 70)
4. Sinus arrhythmia
5. BP increases with age (SBP 70 –>120)
6. PSNS mature. SNS immature in neonates –> Bradycardia response to hypoxia (Rx with ventilation rather than atropine)

Question 15

Why is the Frank-Starling response limited in neonates and infants?

Answer 15

Neonatal myocardium has a lower proportion of contractile proteins.

Ventricles cannot increase wall tension in response to increased preload.

This leads to a fixed SV and CO is HR dependent.

HR < 60 in neonate is an indication for CPR

Question 16

What is sinus arrhythmia

Answer 16

Variation of heart rate with breathing.
Inhibition of vagal tone during inspiration –> HR up
Stimulation of vagal tone during expiration –> HR down

ECG –> sinusoidal variation in R-R interval

Not pathological

Usually seen in children below teenage years

May be seen in athletes who have high vagal tone

Question 17

Why does bradycardia occur during hypoxia in children r(especially infants and neonates) rather than tachycardia as is usually observed in adults

Answer 17

PSNS is mature at term
SNS remains relatively immature

Neonatal response to stress is therefore predominantly parasympathetic –> bradycardia during hypoxia

Question 18

Summarise the unique aspects of the CNS in paediatrics relevant to anaesthesia

Answer 18

1. ICP –> anterior fonatelle (compensation/palpation
2. BBB –> Immature and incomplete in neonates (Bilirubin and drugs - increased sensitivity to opioids/barbiturates)
3. Spinal cord ends:
   - L3 neonates
   - L2/3 infant
   - L1/2 (adult) by 8 years of age)
4. Incomplete myelination –> increase sensitivity to local anaesthetics (reduce doses slightly)
5. Immature SNS –> neuraxial blockade well tolerated with hypotension uncommon.

Question 19

Summarise the unique aspects Renal physiology in paediatrics relevant to anaesthesia

Answer 19

Neonatal kidneys are immature and will reach maturity by 2 years of age

1. GFR is 65ml/min (vs adult 120 ml/min)
2. Tubular function immature
3. Concentrating ability reduced (dehydrated infant has a reduced ability to conserve water)
4. Reduced ability to excrete excess sodium

Question 20

Summarise the unique aspects Haemotological physiology in paediatrics relevant to anaesthesia

Answer 20

1. HbF —> 95% HbA by 3 months
2. HbF = 18 g/dL during fetal life to maximise DO2
   - -> First few days of life –> haemoconcentration Hb ± 20 g/dL
   - -> After 3 months falls to 10 g/dL
   - -> Thereafter, gradually increases until adult levels are reached art 12 years
3. Vit K deficient (poor placental/breast milk transfer)
4. Blood volume
   - -> Neonate: 90 ml/kg
   - -> Infant (6/12): 80 ml/kg
   - -> Adult: 70 ml/kg

Question 21

Why is vitamin K given routinely at birth

Answer 21

Vit K barely crosses the placenta and breast milk is very low in Vit K.

Neonates are therefore relatively vit K deficient.

Lack of vitamin K leads to impaired synthesis of clotting factors 2, 7, 9, 10. Potential for bleeding = haemorrhagic disease of the newborn

Question 22

Describe how blood volume changes from birth t adulthood

Answer 22

Neonate: 90 ml/kg
Infant (6/12): 80 ml/kg
Adult: 70 ml/kg

Question 23

Summarise the unique aspects Hepatic physiology in paediatrics relevant to anaesthesia

Answer 23

Hepatic metabolism is immature and reaches adult function at 3 months

1. Glucoronyltransferase for glucuronidation of BR is impaired and crosses immature BBB –> kernicterus
2. Phase 1 and 2 metabolism impaired (e.g. opioids and barbiturates)

Question 24

Summarise the unique aspects Metabolism in paediatrics relevant to anaesthesia

Answer 24

1. BMR 50 kcal/kg.day (vs. adult 25 kcal/kg.day)
   - increased O2 consumption x 2
   - increase CO2 production
   (Growth and thermoregulation)
2. Prone to hypoglycaemia
   High BMR / Immature liver / low glycogen stores / immature gluconeogenesis

Question 25

Is BMR higher in neonate or fetus in utero at term

Answer 25

Neonate. Neonate required energy for gas exchange (25% of BMR is used for this). This is performed passively via the placenta during in utero life.

Question 26

Why are neonates prone to hypoglycaemia

Answer 26

1. High BMR 2. Immature liver - -> low glycogen stores - -> immature gluconeogenesis

Question 27

Why are neonates and infants prone to heat loss

Answer 27

1. Small surface area to body weight ratio 2. Minimal insulating subcutaneous tissue 3. Poorly developed shivering and vascoconstriction mechanisms.

Question 28

What unique mechanism do neonates and infants have for thermoregulation

Answer 28

Non-shivering thermogenesis. --> SNS --> Metabolism of brown adipose tissue for the generation of heat instead of ATP by a process of uncoupling of oxidative phosphorylation. O2 consumption is significantly increased.

Question 29

What is the main mechanism of heat loss under general anaesthesia

Answer 29

Radiation | Increase ambient temp in addition to usual measures to reduce heat loss.

Question 30

What is the physiological effect of hypothermia in neonates and infants

Answer 30

Acidosis Respiratory depression Decreased CO

Question 31

Compare preterm, adult and neonatal % body water with regards to: 1. TBW % 2. ECF as a % of TBW

Answer 31

TBW Preterm water is 85% body weight Neonate water is 75 % body weight Adult water is 60% body weight ECF Neonate ECF 40% TBW Adult ECF 20% TBW

Question 32

When nil by mouth, why is a neonate more at risk of dehydration than an adult

Answer 32

1. Increased surface area to body weight ration --> increased area available for insensible losses 2. Increased respiratory rate --> loss of water vapour 3. Impaired ability to concentrate urine

Question 33

How does increased body water affect drug administration in paediatric anaesthesia

Answer 33

1. Increased Vd of H2O soluble drugs - -> example is Sux: increased 2-3 mg/kg (vs 1 - 2 mg/kg adult) - -> exception is NDMR: water soluble but less Ach at immature NMJ therefore use same dose

Question 34

How does reduced body fat affect pharmacokinetics in neonates and infants < 6 months

Answer 34

Reduced body fat means less redistribution of lipid soluble drugs (e.g. thiopental and propofol). This will cause prolonged effects of the drug as plasma levels will remain higher for longer relative to adult where more fat soluble drug redistributes to higher proportion of adipose tissue relative to body weight

Question 35

Why is the fraction of free/unbound drug higher in neonates and infants < 6/12. Give some examples

Answer 35

Infants and neonates have reduced plasma proteins. This means that there will be a greater proportion of unbound/free/active drug in the plasma. Important examples include: 1. Barbiturates 2. Phenytoin 3. Bupivacaine 4. Diazepam

Question 36

How does MAC vary with age?

Answer 36

Neonate - MAC unchanged Infants - 50% higher MAC --> gradually decrease throughout childhood and returning to adult MAC levels by adolescence.

Question 37

Why is gaseous induction and gaseous emergence quicker in children than in adults

Answer 37

1. Reduced FRC 2. Increased CBF 3. Increased alveolar ventilation