27.6 Inhalation Sedation in Paediatric Dentistry

2. Pharmacological Behaviour Management: Conscious Sedation

When non-pharmacological techniques prove insufficient for an anxious or phobic child, conscious sedation is indicated. In UK paediatric practice, Inhalation Sedation with Nitrous Oxide and Oxygen ($N_2O / O_2$), also termed Relative Analgesia (RA), is the sedation modality of choice.

Pharmacological Properties of Nitrous Oxide

• Mechanism of Action: Central nervous system depressant acting as a non-competitive NMDA receptor antagonist and GABA-A receptor facilitator. Induces anxiolysis, altered somatic sensation, mild analgesia, and altered time perception, without loss of protective airway reflexes.
• Pharmacokinetics: Insoluble in blood (blood-gas partition coefficient = $0.47$). This extremely low solubility produces rapid alveolar equilibration, rapid onset of action (clinical effect within 2 to 3 minutes), and rapid recovery following cessation.

Relative Analgesia (RA) Titration Protocol

1. Pre-Sedation Checks: Verify written parental consent, confirm light meal taken (2 hours for clear fluids, 4 hours for solid food), and test the fail-safe emergency oxygen mechanism and scavenging flow.
2. Mask Positioning: Select a correctly fitting nasal mask. Establish a total gas flow rate matching the child's physiological minute volume (typically 5 to 6 litres per minute).
3. Baseline 100% Oxygen: Administer 100% $O_2$ for 1 to 2 minutes. Confirm nasal breathing and observe reservoir bag movement.
4. Incremental Nitrous Oxide Titration: Introduce $N_2O$ at 10% for 1 minute. Increase the $N_2O$ concentration in 10% increments every 1 to 2 minutes until the optimal clinical sedation plane is achieved.
   • Sedative Plane: Most paediatric patients achieve effective sedation between 30% and 50% $N_2O$.
   • Safety Limit: Never exceed 50% $N_2O$ in primary care paediatric practice without advanced sedation training.
   • Fail-Safe Protection: British Standard dental sedation machines have an internal mechanical interlock ensuring a minimum delivery of 30% oxygen at all times, preventing delivery of hypoxic mixtures.
5. Clinical Signs of Ideal Sedation: Patient remains fully conscious, responsive to verbal command, with relaxed limbs, tingling in hands/feet, peripheral warmth, glazed eyes, reduced blink rate, and delayed response to stimuli.
6. Scavenging and Health & Safety: UK Control of Substances Hazardous to Health (COSHH) regulations mandate an active scavenging flow rate of 45 L/min, maintaining staff occupational exposure below the statutory maximum 8-hour Time Weighted Average (TWA) of 100 ppm.

Inhalation Sedation Gas Flow and Safety Architecture
  ┌─────────────────────────────────────────────────────────────────────────┐
  │ Gas Titration: 100% O2 (1–2 min) ──> 10% N2O / 90% O2 (1 min)          │
  │   ──> 20% N2O / 80% O2 (1 min) ──> 30% N2O / 70% O2 (Titrate to effect)│
  │ Maximum N2O Delivery: 50% in primary care (Minimum 30% O2 safety stop)  │
  ├─────────────────────────────────────────────────────────────────────────┤
  │ Active Scavenging: 45 L/min flow rate (COSHH limit: 100 ppm 8-hr TWA)   │
  ├─────────────────────────────────────────────────────────────────────────┤
  │ Recovery Phase: 100% O2 for 3–5 minutes (Eliminates Diffusion Hypoxia)   │
  └─────────────────────────────────────────────────────────────────────────┘

[!IMPORTANT] Diffusion Hypoxia (The Fink Effect) and Discharge Protocol: Upon termination of inhalation sedation, nitrous oxide must be turned off immediately while maintaining 100% oxygen delivery for 3 to 5 minutes.

• Pathophysiology: Nitrous oxide is 34 times more soluble in blood than nitrogen. When administration ceases, high concentrations of $N_2O$ rapidly diffuse down their concentration gradient from the capillary blood across the alveolar-capillary membrane into the pulmonary alveoli.
• Mechanism: This sudden influx of $N_2O$ into the alveoli dilutes the alveolar oxygen concentration ($PAO_2$) and alveolar carbon dioxide tension ($PACO_2$). The diluted $PAO_2$ causes arterial hypoxaemia, while the reduced $PACO_2$ blunts the central respiratory drive.
• Clinical Sequelae: If the patient breathes room air (21% $O_2$) immediately post-sedation, diffusion hypoxia causes dizziness, nausea, headache, disorientation, and acute arterial desaturation.
• Prevention: Flushing with 100% oxygen for 3 to 5 minutes washes out diffusing $N_2O$ from the lungs, maintaining alveolar hyper-oxygenation.

Contraindications to Inhalation Sedation

• Severe chronic obstructive pulmonary disease (COPD) or active lower respiratory infection.
• Nasal obstruction (common cold, seasonal rhinitis, severe adenoidal hypertrophy) that prevents nasal breathing.
• Mouth breathers or crying/screaming children (mouth breathing dilutes the gas mixture with ambient air, leading to sedation failure).
• First trimester of pregnancy (potential teratogenicity) in female adolescent patients or staff.
• Bleomycin chemotherapy (sensitises lungs to oxygen toxicity).
• Recent eye surgery involving intraocular gas bubble placement (perfluoropropane, $SF_6$) within the previous 3 months ($N_2O$ diffuses into closed gas cavities, expanding volume and precipitating blindness).

Practical Requirements and Recovery

Inhalation sedation requires a machine with a minimum oxygen delivery of 30 per cent, an oxygen fail-safe, an air-entrainment valve and active scavenging to protect staff from chronic nitrous oxide exposure. The child must be able to breathe nasally and to cooperate with the mask, so a blocked nose or an inability to understand instructions is a practical contraindication. At the end of the procedure the patient receives 100 per cent oxygen for several minutes to prevent diffusion hypoxia, and recovery is rapid; the child is assessed against discharge criteria and released to a responsible adult with written postoperative instructions.