Oxygen therapy devices for spontaneously breathing patients fall into two categories, Variable Performance or Fixed Performance.
1. Variable Performance
Variable performance devices, such as nasal cannulae and masks with or without a rebreathing bag, deliver uncontrolled oxygen therapy because the patient’s breathing affects the concentration of the inspired mixture.
Various factors influence the inspired oxygen concentration when using variable performance devices including an irregular breathing pattern coupled with the use of an oxygen flow less than the patient’s inspiratory flow rate, so the patient inhales an unpredictable mixture of oxygen and ambient air. In addition, the size and fit of the mask, and size and placement of the mask vents will cause variation in inspired oxygen concentrations.
Therefore the delivered oxygen concentration varies not only on a breath by breath basis, but also from patient to patient.
2. Fixed Performance
These devices allow controlled (known, fixed and selectable) oxygen dosage. They create a constant proportion of air/oxygen mixture in excess of patient inspiratory flow rate and are independent of patient factors or fit to the face. With gas flow constantly in excess of patient demand and with enhanced CO2 washout, rebreathing is virtually eliminated.
To work efficiently and to prevent air dilution a fixed performance device should incorporate:
- A large volume face piece (not less than 280ml)
- Gas mixture flowing directly towards the nose and mouth
- Vents positioned well away from the patient airway.
Fixed performance devices are sometimes referred to as venturi or air entrainment masks.
Calculating total gas mixture flow rates
Using the mixture graph below, the total gas flow rate available to the patient can be assessed.