Breathing Systems

Introduction

Breathing systems are the connecting component between the anaesthetic machine and patient. Their role is to deliver oxygen and anaesthetic agent to the patient from the machine, remove exhaled carbon dioxide and provided a means of ventilation via pressure on the reservoir bag.

Basic components of a breathing system are -

• Breathing tubes - Deliver gases to patient and allow for flexibility.
• Reservoir bag - Allows for gas accumulation during exhalation and can be used to assist breathing.
• Carbon dioxide absorbent - To absorb carbon dioxide from the circuit to allow complete rebreathing.
• One way valve - Ensures gases flowing towards the patient do not mix with those exhaled i.e. make a one way system within circuit.
• Pressure relief valve - Limit pressure build up and allows for removal of waste gases.
• Fresh gas inlet - Connects the circuit to the gas outlet on the anaesthetic machine.

Classification

Breathing systems have been classified in a number of different ways, including open and closed, but current classification is -

• Non-rebreathing - The patient breathes from the anaesthetic machine and then exhaled into the atmosphere.
• Rebreathing - The patient rebreathes exhaled gases, but a carbon dioxide absorbant is present to remove any waste gases.

Non Rebreathing Systems

T-piece

• Used on patients less than 10kg.
• Circuit factor - 2.5 - 3 x Minute Volume.
• Reservoir bag on expiratory limb.
• Advantages
  • Low resistance so suitable for small patients.
  • Can use Intermitent Positive Pressure Ventilation (IPPV).
• Disadvantages
  • Cold dry gases delivered to patients.
  • Uneconomical on larger patients.

Bain

• Either parallel or co-axial circuits.
• Used on patients between 10-20kg.
• Circuit factor - 2.5 - 3 x Minute Volume.
• Reservoir bag on expiratory limb.
• Advantages
  • Can use IPPV.
  • In a co-axial circuit the fresh gases are warmed by the exhaled gases surround the tubing.
• Disadvantages
  • Uneconomical as high fresh gas flows required.

Magill

• Used on patients between 10-35kg
• Circuit factor - 1-1.5x Minute Volume.
• Reservoir bag on inspiratory limb.
• Advantages
  • Ecomonical on gas compared to T-piece and Bain.
  • Low resistance
• Disadvantages
  • Cannot use for IPPV.
  • Reservoir bag near patient making positioning difficult.

Lack

• Also parallel and mini circuits available.
• Used on patients between 10-35kg (Mini lack used on patients less than 10kg)
• Circuit factor - 1-1.5x Minute Volume.
• Reservoir bag on inspiratory limb.
• Advantages
  • Economical due to lower flow rates
  • Rebreathe some warm and moist gas from anatomical dead space.
• Disadvantages
  • Cannot use for IPPV.

Rebreathing Systems

Circle

• Also mini circle circuit available.
• Used on patients over 10kg.
• Minimum gas flow = 10ml/kg. It does not have a circuit factor!
• Advantages
  • Patient breathes warm humidified air due to rebreathing.
  • Very economical on gas.
  • Can use IPPV
• Disadvantages
  • Amount of inhalant gas the patient is receiving is not known.
  • Risk of anoxia if system closed.

To-and-Fro

• Not commonly used due to disadvantages.
• Advantages
  • Patient breathes warm humidified air due to rebreathing.
• Disadvantages
  • Amount of inhalant gas the patient is receiving is not known.
  • Risk of anoxia if system closed.
  • Increased dead space as carbon dioxide absorbant used.

Checking Breathing System for Use

1. Check that system has been assembled correctly and that all components are present.
2. Check for any obvious holes, obstructions etc within the tubing - If any found the replace affected part.
3. Securely attached the system to the common gas outlet on the anaesthetic machine.
4. Close the pressure relief valve.
5. Turn on the oxygen and fill the circuit by occluding the end which would usually attach to the patient using thumb, and use oxygen flush to fill if necessary.
6. Listen for leaks and gently squeeze reservoir bag to check that pressure is maintained indicating no leaks.
7. Open the pressure relief valve to ensure pressure relieved.
8. Ensure inspiratory and expiratory valves move freely on a circle circuit by squeezing reservoir bag and releasing thumb off the end of the circuit.

Calculating Fresh Gas Flow

• The formula for working out the fresh gas flow rate is:

Fresh gas flow = Tidal volume (weight x 10 - 15mls) x Respiratory rate x System factor

• For patients under 10kg, the tidal volume is weight times 15mls, for patients over 10kg the tidal volume is weight times 10mls.