Developmental - Paediatric physiology Flashcards
What is the difference between a neonate, infant, child and adolescent?
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
Summarise the unique airway attributes in childhood relevant to anaesthesia
- Big HEAD (neutral > sniffing)
- Big TONGUE
- Big EPIGLOTTIS
- Short neck
- Short trachea
- Narrowest at cricoid ring (not cords)
- Cephalad larynx (C3 neonate –> C4 infant –> C5/C6 adult)
Why do excessive secretions/NGTs affect neonatal/infant breathing more than adults
Before 4 months of age. Neonates and infants are obligate nasal breathers
Why is the shorter trachea problematic for the anaesthetist
Increase risk of endobronchial intubation
What is an appropriate position for ETT in paeds
Tip 1 cam superior to the carina
Why is it relevant that the narrowest point of the airway is the carina and not the vocal cords in children
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
What is the formula for ETT size
(Age/4) + 4
Summarise the unique respiratory physiology in children relevant to anaesthesia
- High VO2 (High BMR) 6ml/kg in kids (3 ml/kg adults)
- High Va (achieves normal PaCO2 from high BMR)
- High RR to achieve increase Va
- Normal Vt (6 - 8 ml/kg)
- Diaphragm dominant (minimal bucket handle & intercostal recession in RDS)
- Low FRC (Increased chest wall compliance)
- CC > FRC (neonates) –> V:Q mismatch
- Easy fatigued muscles of respiration
How is PaCO2 different in children? Discuss the physiological mechanism for this.
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.
Which muscle of respiration is responsible for the majority of the breathing mechanics in paediatric physiology. Explain the mechanism and implications of this
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
What is the mechanism for reduced FRC in children? What are the implications of this?
- 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:
- Rapid desaturation during apnoea
- CC can exceed FRC (esp. in neonates) –> V:Q mismatch.
What are the factors that determine FRC
- 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
- intrinsic PEEP –> partial adduction of vocal cords during expiration
- Inspiratory muscle tone
Why are the muscles of respiration easily fatigued in paediatrics
- 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
- Increased chest wall compliance + Inadequate becket handle mechanism = reduced breathing efficiency
- Lack of type 1 muscle fibres –> easy fatiguability.
Summarise the unique aspects of cardiovascular physiology in children relevant to anaesthesia
The following cardiovascular differences are most pronounced in neonates and become more adult-like with age
- Cardiac Index (CO/BSA) increased 30 - 60%
- Limited Frank-Starling response (CO is HR dependent)
- HR decreases with age (HR 120 –> 70)
- Sinus arrhythmia
- BP increases with age (SBP 70 –>120)
- PSNS mature. SNS immature in neonates –> Bradycardia response to hypoxia (Rx with ventilation rather than atropine)
Why is the Frank-Starling response limited in neonates and infants?
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
What is sinus arrhythmia
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
Why does bradycardia occur during hypoxia in children r(especially infants and neonates) rather than tachycardia as is usually observed in adults
PSNS is mature at term
SNS remains relatively immature
Neonatal response to stress is therefore predominantly parasympathetic –> bradycardia during hypoxia
Summarise the unique aspects of the CNS in paediatrics relevant to anaesthesia
- ICP –> anterior fonatelle (compensation/palpation
- BBB –> Immature and incomplete in neonates (Bilirubin and drugs - increased sensitivity to opioids/barbiturates)
- Spinal cord ends:
- L3 neonates
- L2/3 infant
- L1/2 (adult) by 8 years of age) - Incomplete myelination –> increase sensitivity to local anaesthetics (reduce doses slightly)
- Immature SNS –> neuraxial blockade well tolerated with hypotension uncommon.
Summarise the unique aspects Renal physiology in paediatrics relevant to anaesthesia
Neonatal kidneys are immature and will reach maturity by 2 years of age
- GFR is 65ml/min (vs adult 120 ml/min)
- Tubular function immature
- Concentrating ability reduced (dehydrated infant has a reduced ability to conserve water)
- Reduced ability to excrete excess sodium
Summarise the unique aspects Haemotological physiology in paediatrics relevant to anaesthesia
- HbF —> 95% HbA by 3 months
- 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 - Vit K deficient (poor placental/breast milk transfer)
- Blood volume
- -> Neonate: 90 ml/kg
- -> Infant (6/12): 80 ml/kg
- -> Adult: 70 ml/kg
Why is vitamin K given routinely at birth
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
Describe how blood volume changes from birth t adulthood
Neonate: 90 ml/kg
Infant (6/12): 80 ml/kg
Adult: 70 ml/kg
Summarise the unique aspects Hepatic physiology in paediatrics relevant to anaesthesia
Hepatic metabolism is immature and reaches adult function at 3 months
- Glucoronyltransferase for glucuronidation of BR is impaired and crosses immature BBB –> kernicterus
- Phase 1 and 2 metabolism impaired (e.g. opioids and barbiturates)
Summarise the unique aspects Metabolism in paediatrics relevant to anaesthesia
- BMR 50 kcal/kg.day (vs. adult 25 kcal/kg.day)
- increased O2 consumption x 2
- increase CO2 production
(Growth and thermoregulation) - Prone to hypoglycaemia
High BMR / Immature liver / low glycogen stores / immature gluconeogenesis
Is BMR higher in neonate or fetus in utero at term
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.
Why are neonates prone to hypoglycaemia
- High BMR
- Immature liver
- -> low glycogen stores
- -> immature gluconeogenesis
Why are neonates and infants prone to heat loss
- Small surface area to body weight ratio
- Minimal insulating subcutaneous tissue
- Poorly developed shivering and vascoconstriction mechanisms.
What unique mechanism do neonates and infants have for thermoregulation
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.
What is the main mechanism of heat loss under general anaesthesia
Radiation
Increase ambient temp in addition to usual measures to reduce heat loss.
What is the physiological effect of hypothermia in neonates and infants
Acidosis
Respiratory depression
Decreased CO
Compare preterm, adult and neonatal % body water with regards to:
- TBW %
- ECF as a % of TBW
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
When nil by mouth, why is a neonate more at risk of dehydration than an adult
- Increased surface area to body weight ration –> increased area available for insensible losses
- Increased respiratory rate –> loss of water vapour
- Impaired ability to concentrate urine
How does increased body water affect drug administration in paediatric anaesthesia
- 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
How does reduced body fat affect pharmacokinetics in neonates and infants < 6 months
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
Why is the fraction of free/unbound drug higher in neonates and infants < 6/12. Give some examples
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:
- Barbiturates
- Phenytoin
- Bupivacaine
- Diazepam
How does MAC vary with age?
Neonate - MAC unchanged
Infants - 50% higher MAC
–> gradually decrease throughout childhood and returning to adult MAC levels by adolescence.
Why is gaseous induction and gaseous emergence quicker in children than in adults
- Reduced FRC
- Increased CBF
- Increased alveolar ventilation
