Breathing Systems

This provides a general overview of anaesthetic breathing systems. The circle system in particular is covered elsewhere.

Classifications:

  • Open
    Anaesthetic gases not confined to the circuit.
    • Limited current practical application
      Expensive, environmental contamination.
    • e.g. Ether masks
  • Non-rebreathing
    No expired gas is re-inspired; requires a one-way valve.
    • Limited practical application
    • Requires a low-resistance draw-over vaporiser
    • e.g. Triservice apparatus
      • Robust
      • Inexpensive
  • Rebreathing systems
    Expired gas is re-inspired.
    • Absorption systems
      Requires method for CO2 absorption.
      • Circle
        Common anaesthetic circuit, covered in detail under. Can be:
        • Vaporiser Out-of-Circuit
          Common system, covered in detail under circle system.
        • Vaporiser in-circuit
          Uncommon system.
          • In a spontaneous ventilation mode, the patient will increase agent concentration as minute ventilation ↑ This means that as surgical stimulus ↑, depth of anaesthesia also ↑.
      • Waters
        Mapleson B or C with a CO2 absorption canister between bag and FGF.
    • Non-absorption Rebreathing expired gas is part of circuit design.
      • Mapleson Systems

Mapleson System

Properties:

  • Rebreathing of expired gas does not necessarily equate to CO2 retention, provided the FGF is above a certain multiple (circuit dependent) of the patients MV
    • PaCO2 is a function of FGF and CO2 production only
      Increasing MV without increasing FGF will result in rebreathing of CO2 and unchanged PaCO2.
  • In any spontaneous ventilation mode, patients will hyperventilate if FGF is inadequate

Types:

  • Mapleson A
    • Setup
      • APL valve close to mask
      • Tubing between bag and mask
      • FGF close to bag
    • Flow requirements
      • Spontaneous ventilation:
        APL valve is set low. Initial exhalation (which is mostly dead space, and not containing CO2) will fill bag until bag pressure exceeds APL valve opening pressure. Provided the APL valve is set low, the majority of CO2 containing exhalation will exit through the APL valve, and FGF required to clear CO2 from the circuit is low.
      • Controlled ventilation:
        APL valve is set high. More of the exhalation will fill the bag, and so a greater FGF is required to prevent rebreathing.
  • Mapleson B & C
    • Setup
      APL valve and FGF are situated close to the mask.
      • Mapleson B has long tubing between the mask and bag
      • Mapleson C has short tubing between the mask and bag
    • Flow requirements
      Spontaneous and controlled ventilation are similar, at .
  • Mapleson D
    • Setup
      • FGF is is close to mask
      • Valve is close to bag
      • Tubing between FGF and APL valve
        Co-axial versions exist, but are functionally similar.
    • Flow requirements
      Overall, generally the best circuit to maximise efficiency across both spontaneous and controlled ventilation.
      • Spontaneous ventilation:
      • Controlled ventilation:
        Best circuit for controlled ventilation.
  • Mapleson E/Ayre's T-piece
    • Setup
      • T-shaped circuit with no valve or bag
  • Mapleson F/Jackson-Rees modification to the Ayre's T-piece
    • Setup
      • Bag (with hole) added to the stem of the T of a Mapleson E
        Allows monitoring of ventilation, and occluding the hole of the bag allows controlled ventilation.
      • Functionally identical to a Mapleson D, with an operator-controlled APL valve
    • Flow requirements
      As per Mapleson D.
      • Spontaneous ventilation:
      • Controlled ventilation:

References

  1. Westhorpe, R. Paediatric Breathing Systems. RCH Anaesthetic Tutorial Program. 2019.
Last updated 2019-06-07

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