Supplemental Oxygen

Describe different systems to deliver supplemental oxygen and the advantages and disadvantages of these systems

Devices for delivery of oxygen can be classified into:

• Variable performance devices
• Fixed performance devices

Variable Performance Devices

Variable performance devices:

• Do not deliver a fixed FiO₂
  • This is because respiratory flow is non-uniform
    Although minute ventilation may be 5-6L.min^-1, peak inspiratory flows are substantially higher.
  • Delivered FiO₂ is dependent on oxygen flow and inspiratory flow
    • Increasing oxygen flow rate will increase FiO₂, but the effect will vary depending on the device (volume, seal) and the patient
• Include:
  • Nasal Cannulae
    Prongs delivering gas at 1-4L.min^-1.
    • Higher flows may dry mucosa, and lead to epistaxis
    • Nasopharynx acts as an oxygen reservoir, somewhat increasing FiO₂
    • Well tolerated
      • Allow eating, drinking, and talking
  • Hudson Mask
    Simple unsealed mask, allowing gas flow up to 15L.min^-1.
    • Cheap
    • Less well tolerated
    • Rebreathing may occur
  • Non-Rebreather Mask
    Modified version of the Hudson mask, containing a reservoir bag.
    • Reservoir bag is filled during expiration
    • Gas is drawn from the reservoir bag during inspiration, increasing FiO₂
      Some air is entrained from around the mask and so FiO₂ is < 1.

Fixed Performance Devices

Fixed performance devices:

• Theoretically deliver a fixed FiO₂
  These are usually flow limited as well, and so FiO₂ may decrease at higher inspiratory flows.
• Include:
  • Venturi
    Consists of a cone through which oxygen flows. Apertures on the side of the cone entrain room air.
    • Air is entrained via:
      • Frictional drag of molecules
      • The venturi effect (though this is controversial)
        The widening of the cone leads to an increase in fluid velocity and therefore a decrease in pressure, as per the Bernoulli principle.
    • Entrained air is proportional to flow rate, so the ratio of oxygen to air is constant for any given aperture size
      This is known as the entrainment ratio.
    • Will deliver the specified FiO₂ provided oxygen flow is above the minimum rate
      Therefore become variable performance devices when inspiratory flow greatly exceeds oxygen flow.

References

1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.