17.2 Paediatric Anaesthesia: Induction, Airway Management, and Emergencies

17.2 Paediatric Anaesthesia: Induction, Airway Management, and Emergencies

Paediatric anaesthesia demands precise comprehension of dynamic anatomical transformations, distinct metabolic and fluid requirements, and unique pharmacological profiles across developmental stages from the neonate to the adolescent.

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1. Preoperative Psychological Preparation and Pharmacological Premedication

Minimizing perioperative anxiety in children reduces distress during induction, lowers postoperative analgesic requirements, and mitigates long-term maladaptive behavioural changes (e.g. night terrors, separation anxiety, and regressive behaviours).

Psychological Factors Across Developmental Stages

• Infants (<8 months): Minimal separation anxiety; tolerate separation if warm, fed, and comfortable.
• Toddlers and Preschoolers (8 months to 5 years): Intense separation anxiety and fear of the unknown. Stranger anxiety peaks between 12 and 24 months.
• School-aged Children (6 to 12 years): Fear of bodily injury, pain, loss of control, and death; benefit from honest, developmentally tailored verbal explanations.
• Adolescents (>12 years): Fear of loss of autonomy, altered body image, and waking during surgery.
• Non-Pharmacological Strategies: Parental presence at induction of anaesthesia (PPIA) can benefit select calm parents and older children, but an anxious parent significantly intensifies child distress. Distraction techniques (tablets, interactive video games, medical play therapy) are highly effective.

Pharmacological Premedication

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2. Inhalational vs Intravenous Induction Mechanics and Physiology

Induction of anaesthesia in children may be achieved via inhalational or intravenous routes, each governed by specific physiological principles.

Inhalational Induction with Sevoflurane

Sevoflurane is the preferred volatile induction agent due to its pleasant, non-pungent sweet odor, absence of airway irritation, and low blood-gas partition coefficient ($0.69$).

• Mechanisms of Accelerated Alveolar Wash-in ($F_A/F_I$ Rise):
  1. High Ratio of Alveolar Ventilation to FRC ($V_A:FRC$): In neonates and infants, the $V_A:FRC$ ratio is approximately $5:1$ (alveolar ventilation $\approx 100 - 150\text{ mL/kg/min}$; $\text{FRC} \approx 25 - 30\text{ mL/kg}$), compared to $1.5:1$ in adults. The alveolar compartment turns over more than three times faster, dramatically accelerating alveolar wash-in of volatile anaesthetic.
  2. Elevated Cardiac Output and Preferential Distribution: Paediatric cardiac index is high ($180 - 200\text{ mL/kg/min}$ in neonates), and a larger proportion of this cardiac output is delivered to the vessel-rich organ group (brain, heart, splanchnic bed, kidneys) than in adults. This ensures rapid cerebral delivery.
  3. Reduced Tissue and Blood Solubility: Lower blood-gas and tissue-gas solubility in young children accelerates blood and brain saturation.
• Complications of Inhalational Induction:
  • Stage II Excitement: Excitatory phase manifested by breath-holding, pupil dilation, conjugate/dysconjugate gaze, involuntary motor movements, and heightened vulnerability to coughing and severe laryngospasm upon tactile stimulation.
  • Dose-Dependent Myocardial Depression: Paediatric myocardium has fewer organized contractile myofibrils ($30\%$ compared to $60\%$ in adults) and immature sarcoplasmic reticulum reliant on extracellular calcium flux. Sevoflurane depresses myocardial contractility and induces profound vasodilation; in hypovolemic infants, rapid $8\%$ sevoflurane induction can cause catastrophic bradycardia and cardiovascular collapse.

Intravenous Induction

• Propofol Pharmacokinetics: Paediatric induction requires larger doses: $2.5 - 3.5\text{ mg/kg}$ IV in children (younger children often need the upper end of this range). Neonates are the exception: immature clearance and a tendency to prolonged hypotension call for smaller, titrated doses. Children have an expanded extracellular fluid volume and larger central volume of distribution ($V_d$), alongside elevated hepatic clearance rates relative to body weight.
• Vagal Tone Dominance and Bradycardia: The parasympathetic nervous system is fully mature at birth, whereas the sympathetic nervous system is anatomically and functionally immature. Because the infant myocardium is non-compliant, stroke volume is relatively fixed; cardiac output is strictly rate-dependent ($CO = HR \times SV$). Airway instrumentation, laryngoscopy, or suctioning stimulates intense vagal reflexes, precipitating profound bradycardia, severe hypotension, and asystolic arrest.
• Anticholinergic Co-Administration: Intravenous atropine ($20\text{ }\mu\text{g/kg}$, minimum dose $100\text{ }\mu\text{g}$ to prevent central paradoxical bradycardia) or glycopyrrolate ($10\text{ }\mu\text{g/kg}$) is strongly recommended during IV induction in neonates and young infants, particularly when co-administering succinylcholine or remifentanil.

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3. Paediatric Airway Equipment, Sizing Formulas, and Micro-Cuff Mechanics

Paediatric airway anatomy possesses critical morphological distinctions from the adult that dictate airway equipment selection and intubation technique.

  [ Adult Larynx (C4-C6) ]                      [ Infant Larynx (C3-C4) ]
  - Broad, flexible epiglottis                  - Long, stiff, omega-shaped epiglottis
  - Vocal cords horizontal                      - Vocal cords slanted anteroinferiorly
  - Narrowest point: Glottic aperture           - Narrowest point: Cricoid ring / subglottis
  - Axis alignment: Elevate head (sniffing)     - Axis alignment: Shoulder roll (large occiput)

Anatomical Distinctives & Laryngoscope Blade Selection

• High Larynx: The infant larynx is positioned more cephalad at C3-C4 (descending to C4-C5 by age 6 and C5-C6 in adults). The vocal cords angle anteroinferiorly.
• Large Occiput: Causes severe neck flexion when supine. A shoulder roll (folded towel under the scapulae) is required to establish the neutral "sniffing" position; head rings or occipital elevation must be avoided.
• Epiglottic Morphology: The paediatric epiglottis is long, narrow, rigid, and omega-shaped ($\Omega$), projecting retroverted at $45^\circ$ over the glottis.
• Blade Selection:
  • Straight Miller Blade (Size 0 for neonates, Size 1 for infants): Preferred in infants under 1-2 years. The straight blade is passed posterior to the epiglottis to directly elevate the stiff, floppy epiglottis, providing an unobstructed view of the anterior vocal cords.
  • Curved Macintosh Blade (Size 2): Introduced in older children ($>2 - 3\text{ years}$) where the blade tip is seated in the vallecula to indirectly elevate the epiglottis.

Equipment Sizing Formulas

$\text{Uncuffed ETT Internal Diameter (mm)} = \frac{\text{Age (years)}}{4} + 4$

$\text{Cuffed ETT Internal Diameter (mm)} = \frac{\text{Age (years)}}{4} + 3.5$

$\text{Oral ETT Depth of Insertion (cm at incisors)} = \frac{\text{Age (years)}}{2} + 12$

$\text{Alternatively: Depth of Insertion (cm)} = 3 \times \text{ETT Internal Diameter (mm)}$

• Neonatal ETT Sizing: Preterm $<1000\text{ g}$: $2.5\text{ mm}$ uncuffed; Preterm $1000 - 2500\text{ g}$: $3.0\text{ mm}$; Term neonate ($>2500\text{ g}$): $3.0 - 3.5\text{ mm}$. Depth at lip: $\text{Weight (kg)} + 6\text{ cm}$.

Modern Micro-Cuff Paediatric Endotracheal Tubes

• Historical Dogma: Classical pediatric teaching asserted that the circular, rigid cricoid cartilage formed a natural subglottic anatomical seal, mandating uncuffed tubes in children under 8 years to avoid subglottic stenosis.
• Contemporary Evidence: High-resolution MRI, CT, and dynamic videobronchoscopy demonstrate that the paediatric cricoid lumen is elliptical (narrower transversely than anteroposteriorly), and the functional narrowest point of the upper airway is dynamic at the glottis. Uncuffed tubes produce unpredictable air leaks, requiring tube exchange in $20 - 30\%$ of cases, increasing mechanical airway trauma, operating room gas pollution, and aspiration risk.
• Micro-Cuff Tube Design: Modern cuffed tubes incorporate an ultrathin ($10\text{ }\mu\text{m}$) polyurethane cuff located distally on the shaft. This allows effective low-pressure sealing ($<20\text{ cmH}_2\text{O}$) without causing capillary ischaemic necrosis of the tracheal mucosa (capillary perfusion pressure is $\sim 25 - 30\text{ cmH}_2\text{O}$). Micro-cuff tubes safely seal the airway, drastically reduce re-intubation rates, protect against pulmonary aspiration, and allow accurate low-flow capnography and tidal volume delivery.

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4. Fluid Management and Metabolic Considerations

Paediatric fluid therapy requires precise calculation because children have higher metabolic rates, greater total body water percentages ($75 - 80\%$ in neonates vs $60\%$ in adults), and limited renal concentrating capacities.

The Holliday-Segar 4-2-1 Maintenance Rule

$\text{First 10 kg of body weight: } 4\text{ mL/kg/h}$

$\text{Second 10 kg (10 to 20 kg): } + 2\text{ mL/kg/h } (\text{i.e. } 40\text{ mL/h for first 10 kg} + 2\text{ mL/kg/h for each kg above 10})$

$\text{Weight above 20 kg: } + 1\text{ mL/kg/h } (\text{i.e. } 60\text{ mL/h for first 20 kg} + 1\text{ mL/kg/h for each kg above 20})$

Worked Example: A $26\text{ kg}$ child requires:
$(10 \times 4) + (10 \times 2) + (6 \times 1) = 40 + 20 + 6 = 66\text{ mL/h}$.

The Mandate for Isotonic Balanced Crystalloids

• Danger of Hypotonic Solutions: Historical practice utilized hypotonic solutions (e.g., $0.18\%$ or $0.45\%$ sodium chloride with $5\%$ dextrose). Perioperative distress, pain, nausea, and surgical stress stimulate non-osmotic secretion of antidiuretic hormone (ADH / arginine vasopressin). High ADH levels impair free-water excretion by the kidneys. Infusing hypotonic fluids in the presence of elevated ADH leads to rapid water retention, acute severe hyponatremic encephalopathy, cerebral oedema, brain herniation, and death in previously healthy children.
• Modern Guidelines: European and international consensus mandates the exclusive use of isotonic balanced crystalloids (e.g., Plasmalyte, Hartmann's solution, Ringer's lactate) for routine perioperative maintenance and volume replacement.

Dextrose Requirements

• Neonates and infants under 12 months (or children $<10\text{ kg}$) possess low hepatic glycogen stores, higher metabolic consumption, and immature gluconeogenic enzyme pathways. In this cohort, baseline maintenance fluids should contain $1.0\% - 2.5\%$ dextrose in balanced isotonic crystalloid to prevent hypoglycemia ($<2.6\text{ mmol/L}$ or $<45\text{ mg/dL}$).
• Routine infusion of $5\%$ dextrose must be avoided, as it reliably triggers hyperosmolar hyperglycemia, osmotic diuresis, and exacerbates cerebral injury in ischaemic events.

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5. Common Paediatric Airway Emergencies

Paediatric airway emergencies demand structured algorithmic intervention to avert catastrophic hypoxemia and cardiac arrest.

Laryngospasm

• Pathophysiology: Involuntary, sustained reflex closure of true and false vocal cords mediated by superior laryngeal nerve sensory afferents and recurrent laryngeal nerve motor efferents.
• Risk Factors: Peak incidence in infants under 1 year; active or recent upper respiratory tract infection (URTI within 2 to 4 weeks, conferring bronchial hyperreactivity for up to 6 weeks); exposure to environmental tobacco smoke; light plane of anaesthesia during airway manipulation; blood or secretions touching the glottic aperture.
• Systematic Treatment Ladder:
  1. Stop surgical stimulation; clear blood and secretions with gentle suction.
  2. Administer $100\%\text{ O}_2$ via tight-fitting mask; apply continuous positive airway pressure (CPAP at $10 - 15\text{ cmH}_2\text{O}$) to pneumatically separate the vocal folds.
  3. Larson's Maneuver (Laryngospasm Notch Pressure): Apply firm, bilateral, sustained anterior-cephalad force to the laryngospasm notch (behind the condyle and ramus of the mandible, anterior to the mastoid process). This forcefully displaces the tongue anteriorly while intense periosteal stimulation reflexively terminates adductor spasm.
  4. Deepening Anaesthesia: If spasm persists, administer intravenous propofol ($0.5 - 1.0\text{ mg/kg}$). Deepening anaesthesia breaks the reflex arc.
  5. Neuromuscular Blockade: Administer intravenous succinylcholine ($0.5 - 1.0\text{ mg/kg}$ IV for rapid partial cord relaxation, or $1.0 - 2.0\text{ mg/kg}$ IV for full paralysis). If IV access is lost, administer intramuscular succinylcholine ($4.0\text{ mg/kg}$ IM) co-administered with atropine ($20\text{ }\mu\text{g/kg}$ IM) to prevent succinylcholine-induced severe bradycardia.

Croup vs Acute Epiglottitis

Post-Tonsillectomy Haemorrhage

• Clinical Timing: Primary haemorrhage occurs within 24 hours (peak 4 to 8 hours postoperatively, usually surgical slipped vessel); secondary haemorrhage occurs at 5 to 10 days (sloughing of fibrin slough/eschar or local infection).
• Resuscitation and Airway Dilemma:
  • Children swallow large quantities of blood: the stomach is completely full of blood clots, creating an extreme aspiration risk.
  • Occult hypovolemia: Young children maintain normal arterial pressure despite substantial blood loss ($>30\%$ volume loss) through intense sympathetic peripheral vasoconstriction. Tachycardia, pallor, delayed capillary refill ($>3\text{ s}$), and agitation are early signs of profound hypovolemia.
  • Management Strategy: Aggressive intravascular volume resuscitation with warmed balanced crystalloids (or PRBCs if unstable) BEFORE induction. Two working, high-volume suction units (Yankauer) must be verified. Induction is performed with a slight head-down tilt ($15^\circ$ Trendelenburg) using a rapid sequence induction (RSI) technique with rocuronium or succinylcholine by an experienced anaesthetist. Pass a wide-bore orogastric tube under direct visualization to decompress the stomach before extubation. Extubate only when fully awake and coughing, in the lateral head-down (recovery) position.

Foreign Body Aspiration

• Presentation: Most common in toddlers ($1 - 3\text{ years}$). Sudden paroxysm of coughing, choking, followed by unilateral wheezing, stridor, or diminished breath sounds. Expiratory chest radiography frequently demonstrates air-trapping (unilateral lung hyperlucency) due to a ball-valve mechanism.
• Anaesthetic Management:
  • Inhalational induction with sevoflurane in $100\%\text{ O}_2$ while maintaining spontaneous respiration throughout.
  • Critical Rationale: Positive pressure ventilation (PPV) is avoided where possible before bronchoscopy (gentle assisted ventilation may be needed if oxygenation fails); positive pressure can propel the foreign body from a main bronchus into distal segmental branches, convert a partial ball-valve obstruction into complete occlusion, or induce tension pneumothorax.
  • Spray vocal cords with topical lidocaine; rigid ventilating bronchoscopy is performed by the surgeon to retrieve the foreign body.