Inhalational Induction in Paediatric Anaesthesia
Highlights
- The key properties of the ideal inhalational agent include physical stability and non-flammability, a high oil:gas (O:G) partition coefficient denoting potency, with a low blood:gas (B:G) partition coefficient enabling rapid onset, and the absence of toxic metabolites or adverse effects (View Highlight)
- In modern UK practice the polyfluorinated methyl ether sevoflurane is the only practical volatile agent used for inhalational induction, which approaches ideal characteristics favourably.2 It has an acceptable odour, is non-irritant and exerts its anaesthetic effects through agonism at the β subunit of the gamma-aminobutyric acid A (GABAA) receptor and at glycine receptors where it potentiates inhibitory neurotransmission (View Highlight)
- The main adverse effects are a predictable reduction in minute volume, a decrease in systemic, cerebral and coronary vascular resistance, and emergence agitation and delirium (View Highlight)
- Sevoflurane undergoes hepatic metabolism by the cytochrome P450 system to a greater extent than other commonly used volatile agents, though the nephrotoxic fluoride ions produced are at sufficiently low levels as to be non-toxic.3 Similarly, the production of Compound A and B in the presence of certain CO2 absorbents has also been shown not to affect renal function in animal studies, making it a safe agent even after prolonged use. (View Highlight)
- Nitrous oxide exerts its analgesic and anaesthetic properties by inhibition of N-methyl-d-aspartate (NMDA) receptors in the brain and spinal cord, and agonism at alpha adrenergic, opioid and dopaminergic receptors (View Highlight)
- Neonates and young babies have a relatively large occiput (Fig. 1) and tongue, with a small oral cavity and narrow airways. The infant larynx is high, at the vertebral level C3–4, with a large, floppy and acutely angled epiglottis; it is narrowest at the cricoid cartilage. Attention to tracheal tube size and cuff pressure is important to prevent subglottic trauma and stenosis. (View Highlight)
- In the lower airways alveolar size, alveolar number and diaphragmatic smooth muscle mass increase significantly in the first year of life (View Highlight)
- The rib position in infants and young children is relatively horizontal, limiting the capacity to increase intrathoracic volume through elevation (View Highlight)
- The chest wall compliance and functional residual capacity (FRC) are maintained above closing volume from ∼12 months of age (View Highlight)
- These anatomical factors contribute to a poor functional reserve and tendency to early fatigue in neonates and infants, which is exaggerated in prematurity. Continuous positive airway pressure (CPAP) improves oxygenation and reduces the work of breathing during inhalational induction. (View Highlight)
- The pulmonary phase of inhalational induction is influenced by alveolar minute ventilation, ventilation/perfusion matching and diffusion across the alveolar capillaries (View Highlight)
- In the lungs tidal volume stays relatively stable at 6–10 ml kg−1 with age, whereas ventilatory frequency and therefore alveolar minute ventilation decrease with increasing age (View Highlight)
- Airway resistance and oxygen consumption peak in the neonatal period which explains why desaturation and fatigue occur early in this group (View Highlight)
- A smaller FRC relative to minute ventilation concentrates the alveolar partial pressure of the volatile agent; crying or shouting increase alveolar minute ventilation: both factors increase the speed of induction of anaesthesia (View Highlight)
- The circulatory phase of inhalational induction is the transport of volatile in blood to the brain and is influenced by cardiac output, cerebral perfusion and distribution to other tissues (View Highlight)
- It is important to recognise that severe bradycardia in children can herald a preterminal event and any induction technique in a low cardiac output state should be undertaken with extreme caution. (View Highlight)
- most agitated behaviours are expected physiological features of anaesthesia and not signs of pain or distress.
Fig. 2. Guedel's four stages of anaesthesia (View Highlight)
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- when an inhalational induction is performed in the absence of i.v. access, suxamethonium should be ready to use for i.m. injection at a dose of up to 4 mg kg−1 in the event of severe laryngospasm and complete loss of the airway (View Highlight)
- To prevent rebreathing the T-piece requires a fresh gas flow of 2.5–3 times the patient's minute volume, and the volume of the reservoir tubing should approximate the child's tidal volume (View Highlight)
- Resistance to breathing is higher in the circle circuit than with the T-piece mostly because of unidirectional valves, particularly in spontaneous ventilation, though modern fan-assisted iterations go some way to overcome this (View Highlight)
- Laryngospasm is more common in the reactive airway and so a preoperative history of asthma, prematurity, recent respiratory tract infection, obstructive sleep apnoea and passive smoking should prompt extra caution and appropriate discussion of risk with the carer. (View Highlight)