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This section is supported by an unrestricted educational grant from Intersurgical

Oxygen therapy

Created: 27/4/2010
Updated: 6/12/2017

Fixed performance devices

FiO2 is constant, despite changes in inspiratory flow rate. Examples include an oxygen tent, anaesthetic breathing circuit, high air flow oxygen enrichers (HAFOE). The feed connector incorporates holes to allow entrainment of atmospheric air into the oxygen stream by jet mixing. 

Figure 1: Fixed performance devices

The Venturi effect

The pressure drop induced by the increase in velocity of a fluid passing through a narrow orifice can be used to entrain air or a nebuliser solution for treating our patients.

Figure 2: Section through a Venturi mask 

At point A, 100% oxygen flows into the wider point B via a narrow orifice. Because of the narrowing, the oxygen speeds up and the pressure drop at that point is below atmospheric pressure and room air is drawn to this low pressure point, hence diluting the 100% oxygen to the calibrated value set by the coloured nozzle. The nozzle has a varying aperture open to room air that sets the entrainment ratio and hence the inspired concentration given to the patient.

Variable performance devices

Figure 3: Plastic mask

Actual FiO2 depends on inspiratory flow rate. Examples include nasal cannulae and plastic masks. The British Thoracic Society guidelines advise flow rates of no less than 5 L/min (to avoid rebreathing).


Figure 4: Nasal cannula

Nasal cannulae are more comfortable and less claustrophobic than facemasks. However, oxygen flow greater than 2 L/min can cause discomfort and drying of the nasal mucosa.

Figure 5: Sentri ETCO2mask

The Sentri ETCO2 mask is a conventional medium concentration oxygen mask with the addition of a sampling port. This port permits sampling of exhaled CO2 from both the mouth and nose in non-intubated patients during the administration of supplementary oxygen.

Figure 6: High capacity oxygen delivery device

High capacity oxygen delivery devices (capacity 250-1500 ml)

With these devices, O2 flows directly into the reservoir bag, which fills whenever the patient's inspiratory flow rate is lower than the flow rate of delivered O2. The patient thus inhales O2 preferentially from the reservoir bag and ambient air is not entrained (avoiding dilution of the FiO2).

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