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 vapouriser
  • e.g. Tri-service 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:
      • Vapouriser Out-of-Circuit
        Common system, covered in detail under circle system.
        • $[Alv_{agent}] \propto {FGF \over MV}$
      • Vapouriser in-circuit
        Uncommon system.
        • $[Alv_{agent}] \propto {MV \over FGF}$
        • 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 re-breathing 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: $~0.7 \times MV$
      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: $~>3 \times MV$
      APL valve is set high. More of the exhalation will fill the bag, and so a greater FGF is required to prevent re-breathing.
• 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 $~3 \times MV$.
• 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: $~2 \times MV$
    • Controlled ventilation: $~0.8-1 \times MV$
      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: $~2 \times MV$
    • Controlled ventilation: $~0.8-1 \times MV$

References

1. Westhorpe, R. Paediatric Breathing Systems. RCH Anaesthetic Tutorial Program. 2019.