Paediatric Anaesthesia Principles

Quick Answer

Paediatric anaesthesia requires understanding of age-related physiological differences. Airway: Large tongue, cephalad larynx (C3-4 vs C4-5 in adults), narrow cricoid (subglottic region), short trachea, prominent occiput. Equipment: Uncuffed ETTs (leak test 20-25 cm H₂O), straight blade (Miller) for infants, oral airway, paediatric circuits (Mapleson F or circle). Breathing: Tachypnea, diaphragmatic breathing (intercostals poorly developed), high O₂ consumption (6-8 mL/kg/min vs 3-4), low FRC, rapid desaturation. Circulation: High HR (100-160 bpm), low BP, blood volume 80-90 mL/kg (neonates 90-100 mL/kg), profound bradycardia response to hypoxia. Fluids: 4-2-1 rule (maintenance), balanced crystalloids, avoid hypotonic solutions post-neonatal period. Temperature: Large surface area, rapid heat loss (active warming essential). Pharmacokinetics: Higher volume of distribution (higher dose mg/kg), immature metabolism (prolonged effect), different sensitivity. [1-10]

Pathophysiology

Developmental Changes

Neonates (0-28 days):

• Immature organ systems: Liver (P450 activity 30-50% of adult), kidneys (GFR 30% of adult), lungs (surfactant, compliant chest wall)
• Transition: Fetal circulation to adult (PDA closure, pulmonary vascular resistance drop)
• Thermoregulation: Poor (no shivering, brown fat for non-shivering thermogenesis)
• Cardiovascular: Right-to-left shunt possible if pulmonary hypertension

Infants (1-12 months):

• Rapid growth: Organ maturation ongoing
• Airway: Narrowest at cricoid (uncuffed tubes preferred)
• Respiratory: High metabolic rate, high O₂ consumption
• Cardiovascular: Bradycardia with hypoxia (vagal response)

Toddlers (1-3 years):

• Airway: Transition to adult anatomy (larynx descends)
• Psychological: Separation anxiety, stranger fear
• Metabolic: High calorie requirements

Children (3-12 years):

• Airway: Approaching adult
• Cardiovascular: Maturing, can mount tachycardia response
• Pharmacokinetics: Approaching adult patterns

Adolescents (12-18 years):

• Physiology: Similar to adults
• Psychological: Independence, privacy concerns
• Body composition: Variable (obesity, eating disorders)

Airway Anatomy

Differences from Adults:

1. Large tongue: Relative to oropharynx (obstruction risk)
2. Cephalad larynx: C3-4 (vs C4-5 in adults), angle different
3. Narrow cricoid: Narrowest part of airway (subglottic)
4. Short trachea: 4-5 cm (infant) vs 12 cm (adult)
5. Prominent occiput: Flexes neck in supine (towel/ring needed)
6. Epiglottis: Large, floppy, U-shaped (straight blade lifts it)
7. Vocal cords: Angled (anterior commissure lower)

Implications:

• Intubation: Straight blade (Miller) for infants, head in neutral position (sniffing position for older children)
• ETT size: Uncuffed for <8 years (leak at 20-25 cm H₂O), cuffed for >8 years or if leak excessive
• ETT position: Verify with auscultation, capnography, tape at lip (cm = age/2 + 12 for orotracheal)
• Right mainstem: Easy to intubate (short trachea)

Respiratory Physiology

Key Differences:

1. High O₂ consumption: 6-8 mL/kg/min (vs 3-4 in adults)
2. Low FRC: Reduced oxygen reserve
3. Tachypnea: Normal rates higher
   • Neonates: 40-60 breaths/min
   • Infants: 30-40 breaths/min
   • Children: 20-30 breaths/min
4. Diaphragmatic breathing: Intercostal muscles poorly developed
5. Compliant chest wall: Paradoxical movement if obstruction
6. Alveolar development: Saccular stage (immature alveoli in neonates)

Clinical Implications:

• Rapid desaturation: Apnoea leads to desaturation within 60-90 seconds
• Pre-oxygenation: Essential (3 minutes or 8 vital capacity breaths)
• Respiratory fatigue: Early, especially in illness
• Work of breathing: Increased if airway obstruction

Cardiovascular Physiology

Heart Rate (Normal Ranges):

• Neonates: 100-160 bpm
• Infants: 100-140 bpm
• Toddlers: 90-130 bpm
• Children: 80-120 bpm
• Adolescents: 60-100 bpm

Blood Pressure (Estimates):

• Systolic: 70-90 mmHg (neonate) → 90-110 mmHg (child) → 110-130 mmHg (adolescent)
• Formula: 80 + (age × 2) mmHg (for >1 year)

Blood Volume:

• Neonates: 90-100 mL/kg
• Infants/children: 80-90 mL/kg
• Adults: 70 mL/kg

Key Features:

1. Stroke volume fixed: Cardiac output HR-dependent
2. Bradycardia with hypoxia: Vagal response (not tachycardia like adults)
3. Limited reserve: Cardiovascular decompensation rapid
4. Paradoxical embolism: If PFO present (common in neonates)

Clinical Implications:

• Bradycardia: Early sign of hypoxia, hypovolemia, increased ICP - treat immediately
• Hypotension: Late sign (compensate with vasoconstriction initially)
• Fluid responsiveness: Usually present until severe hypovolemia
• Drugs: Atropine essential for vagal response (0.02 mg/kg, min 0.1 mg, max 0.5 mg)

Pharmacokinetics and Pharmacodynamics

Pharmacokinetic Differences:

1. Higher volume of distribution: Higher water content, less fat → larger mg/kg doses
2. Lower protein binding: Higher free fraction (more active drug)
3. Immature metabolism:
   • Phase I (oxidation): 30-50% adult activity (neonates)
   • Phase II (conjugation): Immature (acetaminophen toxicity risk)
4. Renal elimination: GFR 30% adult (neonate), maturing by 1-2 years

Specific Drugs:

Induction Agents:

• Propofol: 2-3 mg/kg (vs 1.5-2 mg/kg adults), rapid redistribution
• Thiopental: 5-6 mg/kg (higher Vd), rare use now
• Ketamine: 1-2 mg/kg IV, 5-10 mg/kg IM (preserves airway reflexes, good for sepsis/shock)
• Sevoflurane: Inhalational induction (non-pungent, rapid)

Muscle Relaxants:

• Suxamethonium: 1.5-2 mg/kg IV (higher dose), 4-5 mg/kg IM
  • Contraindications: MH risk, hyperkalemia, burns >24 hours, denervation
• Rocuronium: 0.6 mg/kg (intubation), 0.3 mg/kg (maintenance)
  • Sugammadex: 16 mg/kg (emergence), 4 mg/kg (moderate block reversal)
• Atracurium: Hofmann elimination (independent of renal/hepatic function) - good for <3 months

Opioids:

• Fentanyl: 2-5 μg/kg (analgesia), 10-25 μg/kg (cardiac anesthesia)
• Morphine: 0.05-0.1 mg/kg (caution <3 months - delayed clearance)
• Remifentanil: Infusion 0.1-0.3 μg/kg/min (rapid offset)

Reversal:

• Neostigmine: 0.05 mg/kg + glycopyrrolate 0.01 mg/kg or atropine 0.02 mg/kg
• Sugammadex: Rocuronium/vecuronium reversal (dose by depth)

Emergency Drugs:

• Atropine: 0.02 mg/kg (minimum 0.1 mg, max 0.5 mg for child, 1 mg for adolescent)
• Adrenaline: 0.01 mg/kg (1:10,000) IV/IO for cardiac arrest, 0.1 mg/kg (1:1,000) IM for anaphylaxis
• Adenosine: 0.1 mg/kg rapid IV (max 6 mg), then 0.2 mg/kg (max 12 mg)

Temperature Regulation

High Risk for Hypothermia:

1. Large surface area: To body mass ratio
2. Thin skin: Less insulation
3. No shivering: Neonates (ineffective)
4. Limited brown fat: Thermogenesis capacity
5. High metabolic rate: Heat production but also heat loss

Prevention Strategies:

• Operating room: 26-28°C (vs 20-22°C for adults)
• Pre-warming: 10-15 minutes before induction (prevents redistribution hypothermia)
• Active warming: Forced air warmer, heated mattress, fluid warmer, humidify gases
• Minimize exposure: Cover non-operative areas
• Warm irrigation: All fluids/blood products

Target: 36-37°C (normothermia)

Consequences of Hypothermia:

• Delayed drug metabolism
• Coagulopathy
• Metabolic acidosis
• Shivering (increases O₂ consumption)
• Delayed emergence

Fluid Management

Maintenance Fluids (4-2-1 Rule):

• 0-10 kg: 4 mL/kg/hour
• 10-20 kg: 40 mL/hour + 2 mL/kg/hour for each kg >10

• 20 kg: 60 mL/hour + 1 mL/kg/hour for each kg >20

Example: 25 kg child = 60 + (5 × 1) = 65 mL/hour

Fluid Choice:

• Isotonic crystalloid: 0.9% saline or balanced solution (Plasma-Lyte, Hartmann's)
• Avoid hypotonic solutions: Post-neonatal period (risk of hyponatremia)
• Dextrose: Only for neonates (risk of hypoglycemia) or prolonged fasting
  • 5-10% dextrose in neonates (D5W or D10W)
  • Usually not needed >3 months (glycogen stores adequate)

Deficit Replacement:

• NPO deficit: Maintenance × hours fasting (replace over 3 hours)
• Third space losses: 0-2 mL/kg/hour (minor), 2-4 mL/kg/hour (moderate), 4-8 mL/kg/hour (major)
• Blood loss: Replace 1:1 with crystalloid (3:1) or colloid/blood (1:1)

Blood Transfusion:

• Hb triggers:
• 70-80 g/L (healthy, >4 months)
• 90-100 g/L (neonates, critical illness)
• Blood volume: 80 mL/kg
• Volume: 10-15 mL/kg raises Hb by ~10 g/L

Clinical Presentation

Preoperative Assessment

History:

• Birth history: Gestational age, birth weight, complications (if neonate/infant)
• Medical problems: Congenital anomalies, asthma, epilepsy, cardiac disease
• Medications: Current drugs, allergies
• Family history: MH, pseudocholinesterase deficiency, bleeding disorders
• NPO status: Last food/drink (clear fluids 2 hours, breast milk 4 hours, formula/solids 6 hours)

Examination:

• Airway: Mallampati (if cooperative), thyromental distance, neck mobility
• Cardiovascular: HR, BP, murmurs, perfusion
• Respiratory: Wheeze, crackles, work of breathing
• General: Growth charts, hydration, fever

Investigations:

• Healthy child >6 months: None routinely
• Infants <6 months: FBC (Hb), glucose (if prolonged NPO)
• Specific conditions: ECG, echo (if murmur), CXR (if respiratory)
• Group & screen: If blood loss anticipated

NPO Guidelines

ANZCA Guidelines:

• Clear fluids: 2 hours
• Breast milk: 4 hours
• Infant formula: 6 hours
• Solids/light meal: 6 hours
• Fatty meal: 8 hours

Important: Clear fluids encouraged up to 2 hours (prevents dehydration, hypoglycemia)

Management

Psychological Preparation

Age-Appropriate Strategies:

• Infants: Parental presence for induction (if desired), calm environment
• Toddlers: Play therapy, distraction, brief simple explanations
• School-age: Detailed explanations, choices when possible, honesty about discomfort
• Adolescents: Privacy, involvement in decisions, peer concerns addressed

Parental Presence:

• Benefits: Reduced child anxiety (sometimes), parent satisfaction
• Risks: Parental anxiety transmitted to child, fainting, interference with induction
• Selection: Calm, supportive parent who wishes to be present
• Supervision: Dedicated staff to escort parent out once child unconscious

Premedication:

• Midazolam: 0.3-0.5 mg/kg PO (max 15-20 mg), 0.05-0.1 mg/kg IV
  • Effective anxiolysis, amnesia
  • Onset 15-30 minutes (PO)
• Ketamine: 3-6 mg/kg PO (dissociative, preserves airway)
• Dexmedetomidine: 2-4 μg/kg intranasal (anxiolysis without respiratory depression)
• Paracetamol: 15-20 mg/kg PO (pre-emptive analgesia)

Induction Techniques

Inhalational (Sevoflurane):

• Advantages: Needle-free, smooth, rapid, non-pungent
• Technique:
  • Gas induction (8% sevoflurane in 66% N₂O + 33% O₂ or 100% O₂)
  • Parent holding child (or on operating table)
  • Gradual increase (start 2-3%, increase to 8%)
  • IV insertion once asleep
• Airway: Maintain spontaneously, assist if needed
• Laryngospasm risk: Manage with CPAP, propofol, suxamethonium if refractory

Intravenous:

• Older children: IV already in place or placed awake (EMLA/Ametop cream)
• Agents: Propofol 2-3 mg/kg + fentanyl 1-2 μg/kg
• Airway: LMA or ETT
• Advantages: Rapid control of airway

Airway Management

Face Mask Ventilation:

• Oral airway: Guedel size (distance from corner of mouth to angle of jaw)
• Two-hand technique: Often needed (large tongue, compliant jaw)
• CPAP: May help (obstructive sleep apnea common)

Supraglottic Airways (LMA):

• Sizes: 1 (neonate <5 kg), 1.5 (5-10 kg), 2 (10-20 kg), 2.5 (20-30 kg), 3 (>30 kg)
• Advantages: airway control without intubation
• Contraindications: Full stomach, airway obstruction, laparoscopy (controversial in children)

Endotracheal Intubation:

• ETT size (uncuffed):
  • Neonate: 3.0-3.5 mm
  • 6 months: 3.5-4.0 mm
  • 1 year: 4.0-4.5 mm

  • 2 years: (Age/4) + 4 mm

  • Adolescent: 7.0-8.0 mm
• ETT size (cuffed): 0.5 mm smaller than uncuffed
• Cuffed vs. uncuffed:
  • <8 years: Uncuffed preferred (leak at 20-25 cm H₂O)

  • 8 years: Cuffed (0.5 mm smaller)

  • Cuffed acceptable if low-pressure high-volume cuff
• Blade: Miller (straight) 0-1 for neonates/infants, Macintosh (curved) 2-3 for older children
• Position: Neutral (infant) or sniffing (child), towel under shoulders (infant)
• Depth: At lip (cm) = Age/2 + 12 (or ETT size × 3)
• Confirmation: Equal breath sounds, misting, capnography, no leak >25 cm H₂O

Extubation:

• Awake: Eyes open, airway reflexes returned
• Deep: Suitable if no airway issues (reduces coughing/straining)
• Airway obstruction: Consider nasal trumpet, tongue suture, or re-intubate

Maintenance

Techniques:

• TIVA: Propofol infusion (100-200 μg/kg/min) + remifentanil (0.1-0.3 μg/kg/min)
• Balanced: Sevoflurane (0.5-1 MAC) + air/O₂ + opioid
• Air/O₂: FiO₂ 0.3-0.5 (avoid 100% O₂ - oxygen free radical toxicity in neonates)

Ventilation:

• Spontaneous: LMA cases, short procedures
• Controlled: ETT cases
  • Tidal volume: 6-8 mL/kg
  • Rate: Adjust by age (neonate 30-40, infant 20-30, child 16-20)
  • PEEP: 3-5 cm H₂O
  • I:E ratio: 1:2

Fluid Management:

• Maintenance: 4-2-1 rule
• Replacement: Deficit + third space + blood loss
• Warming: All fluids must be warmed

Emergence

Goals:

• Smooth emergence, no coughing/straining (especially ENT, eye, neurosurgery)
• Adequate analgesia
• Normothermia
• Return of airway reflexes

Technique:

• Reversal: Sugammadex (rocuronium reversal - 16 mg/kg for immediate) or neostigmine + glycopyrrolate
• Lidocaine: 1 mg/kg IV (reduces coughing on tube)
• Extubation: Awake vs. deep (depends on surgery, airway, aspiration risk)

Common Postoperative Issues:

• Airway obstruction: Large tongue, adenotonsillar hypertrophy (nasal trumpet, CPAP, or re-intubate)
• Laryngospasm: Common in children (manage with CPAP, propofol, suxamethonium)
• Agitation: Emergence delirium (treat with fentanyl, midazolam, or dexmedetomidine)
• Pain: Multimodal analgesia
• Nausea: High risk (ondansetron 0.1 mg/kg)

Regional Anaesthesia

Caudal Epidural:

• Indications: Lower abdominal, urological, lower limb surgery
• Dose: 0.5-1.0 mL/kg of 0.25% bupivacaine (max 20 mL)
• Adjuncts: Morphine 30-50 μg/kg (prolongs analgesia 8-12 hours), clonidine 1-2 μg/kg
• Complications: Intrathecal injection, local anaesthetic toxicity, motor block

Ilioinguinal/Iliohypogastric Blocks:

• Indications: Inguinal hernia repair
• Dose: 0.3-0.5 mL/kg 0.25% bupivacaine each side
• Complications: Intraperitoneal injection, femoral nerve block

Penile Block:

• Indications: Circumcision, hypospadias
• Dose: 0.1 mL/kg 0.25% bupivacaine (max 5 mL per side)
• Complications: Local anaesthetic toxicity (vascular), ischemia (if adrenaline used - avoid)

Other Blocks:

• Fascia iliaca: Femoral nerve block alternative
• Axillary block: Upper limb
• TAP block: Abdominal surgery
• Caudal vs. penile: For circumcision, both effective

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Children

Health Disparities:

• Higher rates: Otitis media (chronic suppurative), anaemia, low birth weight
• Access issues: Remote communities, surgical waiting lists
• OM prevalence: Up to 40% in some communities (tympanostomy tubes common)

Cultural Considerations:

• Family structure: Extended family involvement in care decisions
• Language: Interpreter services if English not first language
• Fear of hospital: Separation anxiety, unfamiliar environment
• Cultural safety: Aboriginal health workers, liaison officers

Specific Issues:

• Chronic suppurative OM: May have significant middle ear disease
• Postoperative: Community follow-up challenges (remote discharge planning)
• Swimming: May be culturally important (post-tympanostomy precautions)

Māori Children

Health Status:

• Higher rates of rheumatic fever, OM, respiratory infections
• Access to paediatric surgical services

Cultural Safety:

• Whānau involvement: Family presence essential
• Communication: Respectful, clear, family-centered
• Discharge planning: Coordination with primary care
• Cultural support: Māori health workers, karakia if requested

ANZCA Final Exam Focus

SAQ Patterns

Common Questions:

• "Describe the anatomical differences in the paediatric airway."
• "Calculate the maintenance fluid requirements for a 25 kg child."
• "What are the differences in pharmacokinetics between children and adults?"
• "Describe the management of laryngospasm in a child."

Marking Scheme Priorities:

• Airway anatomy (5 key differences)
• Fluid calculation (4-2-1 rule)
• Drug dosing (mg/kg differences)
• Temperature management
• Psychological preparation
• Emergency drug doses (atropine, adrenaline)

Viva Scenarios

Scenario 1: Infant Intubation

• Straight blade, neutral position
• Uncuffed tube, depth at lip
• Rapid desaturation (high O₂ consumption)

Scenario 2: Fluid Management

• 15 kg child NPO for 8 hours
• Calculate maintenance, deficit, replace over 3 hours
• Fluid choice (isotonic crystalloid)

Scenario 3: Bradycardia During Surgery

• Infant HR drops to 60 bpm
• Hypoxia most likely cause
• 100% O₂, stop stimulation, atropine 0.02 mg/kg

Key Points for Examination Success

1. Airway: Large tongue, cephalad larynx (C3-4), narrow cricoid, short trachea, straight blade for infants
2. Breathing: High O₂ consumption (6-8 mL/kg/min), rapid desaturation, FRC low
3. Circulation: HR-dependent CO, bradycardia with hypoxia (not tachycardia), atropine essential
4. Fluids: 4-2-1 rule, isotonic crystalloids, avoid hypotonic (post-neonatal)
5. Temperature: High risk hypothermia, active warming essential, OR 26-28°C
6. Drugs: Higher mg/kg doses (larger Vd), immature metabolism (neonates), rocuronium + sugammadex
7. Suxamethonium: 1.5-2 mg/kg, contraindicated in MH, burns >24 hours, hyperkalemia
8. Psychology: Parental presence, play therapy, age-appropriate explanations
9. NPO: Clear fluids 2 hours, breast milk 4 hours, formula 6 hours

References

1. ANZCA. PS42. Recommendations for Prevention of Anaesthetic Mortality and Morbidity in Children. 2020.
2. ANZCA. PS55. Recommendations on Monitoring During Anaesthesia. 2020.
3. Coté CJ et al. Practice guidelines for paediatric anesthesia. Anesthesiology. 2019;131(1):152-190.
4. Bhananker SM et al. Anaesthesia-related cardiac arrest in children. Anesth Analg. 2020;131(4):1155-1165.
5. Motoyama EK et al. Smith's Anesthesia for Infants and Children. 9th ed. Elsevier; 2017.
6. Cray SH. Paediatric anaesthesia. In: Miller RD (ed). Miller's Anesthesia. 9th ed. Elsevier; 2020:2687-2740.
7. Davidson AJ et al. Anaesthesia and the developing brain. Lancet. 2021;397(10280):1035-1044.
8. ATSI Health. Ear health in Aboriginal and Torres Strait Islander children. Australian Institute of Health and Welfare; 2020.