Devices in this category are used for oxygen therapy and, in some cases, non-invasive ventilation or respiratory support.
Performance of these devices can be described according to
1. the maximum oxygen concentration achievable to be delivered to the patient, and
2. the reliability/ variability with which the devices do that.
Looking at the reliability and variability of oxygen delivery to the patient we can divide these pieces of equipment into fixed performance or variable performance devices.
This means that inspired oxygen concentration is either more dependent on the patient's peak inspiratory flow rate (PIFR) (variable performance devices) or less so (fixed performance devices).
Fixed performance devices
These devices deliver a fixed inspired oxygen concentration to the patient, independent of the PIFR. Even at the time of PIFR no random entrainment of room air will occur to unpredictably alter the FiO2 with this equipment.
Devices in this group include:
- Venturi mask
There is entrainment of room air with these devices, but it is fixed and not dependent on the patient's PIFR. Therefore the resulting delivered FiO2 is also constant.
- Non-rebreather facemask
A reservoir bag supplies 100% oxygen and makes up for the deficit which occurs during times when the patient's PIFR is higher than the oxygen supply flow to the mask.
- Anesthetic facemask (with an anesthetic breathing circuit)
These masks form a complete seal around the patient's airway so no entrainment of room air can occur. The delivered oxygen concentration is the same as in the gas mixture supplied to the mask (assuming no rebreathing occurs in the anesthetic breathing cicuit).
Variable performance devices
These devices deliver a variable inspired oxygen concentration to the patient, which depends on the PIFR. Depending on a patient's inspiratory effort (tidal volume, 'speed' of inspiration and respiratory rate) the PIFR can often exceed the flow rate at which oxygen or an oxygen/air mixture is supplied by the device, meaning that at the time of PIFR more or less entrainment of room air occurs, altering the resulting FiO2 in an unpredictable fashion
Devices in this group are:
- Nasal cannula
- Simple facemask/ 'Hudson' mask