Inhalational induction in paediatric anaesthesia

Question 1

Ideal characteristics of an inhalational induction agent

Answer 1

• High oil:gas coefficient (potent)
• Low blood:gas coefficient (rapid onset)
• Non-flammable, stable
• Minimal adverse effects and toxic metabolites
• Not likely to cause airway irritability or have an unpleasant smell

Question 2

Mechanism of action of sevoflurane

Answer 2

• Agonist of the B sub-unit of GABA-A receptors and glycine receptors
• Causing potentiation of inhibitory neurotransmission

Question 3

Mechanism of action of nitrous oxide

Answer 3

• Inhibition of NMDA receptors in the brain and spinal cord
• Agonism of a-adrenergic, opioid and dopaminergic receptors

Question 4

Anatomical differences of the paediatric/ neonatal airway compared to the adult

Answer 4

• Large occiput
• Large tongue with smaller oral cavity
• Larynx at C3-4 rather than C6 and more anterior
• Long floppy epiglottis
• The narrowest point is the cricoid cartilage
• Diaphragm and intercostal muscles less efficient more likely to fatigue

Question 5

Physiology of inhalational induction in paediatrics

Answer 5

Respiratory
- Influenced by minute ventilation, V/Q matching and diffusion rate across alveolar capillaries
- Tidal volumes around 6-10ml/kg but RR decreases with age
- Airway resistance and oxygen consumption peak in the neonatal period- therefore high risk of desaturation in this group

Cardiac
- Influenced by cardiac output, cerebral perfusion and distribution to other tissues
- CO is predominately influenced by HR in paediatrics.
- A high CO means slower induction time as there will be rapid removal of the agent from the alveoli
- Bradycardia is often a pre-terminal event in paediatrics

Question 6

Indications for inhalational induction

Answer 6

• Difficult venous access
• Potential difficult airway
• Uncooperative child
• Patient or parents choice- including needle phobia

Question 7

Contraindications for inhalational induction

Answer 7

Absolute
- Refusal in a child with capacity
- Malignant hyperthermia

Relative
- Full stomach
- Severe low cardiac output state (cardiomyopathy, valve obstruction, tamponade)
- Severe restrictive or obstructive respiratory disease
- MH susceptibility

Question 8

Advantages and disadvantages of Mapleson F/ Ayre’s T-piece for inhalational induction

Answer 8

Advantages
- Low dead space
- Low resistance circuit
- Ability to provide PEEP

Disadvantages
- Requires a fresh gas flow of 2.5-3x minute volume
- No scavenging and open system so volatiles will leak into the environment
- Unable to measure gas concentrations or pressures

Question 9

Considerations for a gas induction

Answer 9

• Appropriate patient selection (see indications and contraindications), parent education
• Equipment: t-piece or circle, emergency meds (Sux and atropine), tube sizes, NG tube, adjuncts etc
• Staff: paediatric expertise, minimum no. of staff needed to provide a calm environment
• Environment: dedicated paediatric operating theatre, decorations, quiet
• Other: minimise fasting and waiting, avoid drawing up drugs in front of child, stool or bed

Question 10

Hazards and complications of an inhalational induction

Answer 10

• Loss of airway and laryngospasm
• Uncooperative child- can lead to distress, regression, enuresis and night crying
• Bradycardias