Flow devices for oxygen therapy are of two types.
- High flow:- Some of the common examples include venturi mask, jet nebulizer, high flow nasal cannula, etc.
- Low flow:- Some of the common examples include nasal cannula, partial rebreathing mask, normal (aerosol) mask, etc.
- Flow rate:- Flow rate for low flow devices is lower than the normal inspiratory flow rate of the patient which is usually from 20 liters/min to 30 liters/min. The patient gets the remaining portion of the air from the atmosphere. On the other hand, high flow devices supply more air than the required inspiratory flow rate of the patient.
- Fraction of inspired oxygen (FiO2):- FiO2 is the amount of oxygen present in the air. For example, FiO2 of the atmospheric air is 21%.
Since the remaining portion of the air is supplied from the atmosphere the value of FiO2 for low flow devices cannot remain fixed. Factors affecting the value of FiO2 are as follows.
- Breathing rate:- Breathing rate differs from person to person. For a fixed supply of oxygen, the value of FiO2 is inversely proportional to the breathing rate. This is because of the dilution of pure oxygen by atmospheric gases increases. So, FiO2 decreases.
- Supply flow rate:- If the supply flow rate increases then the value of FiO2 will also increase.
- The purity of supplied oxygen:- The purity of supplied oxygen from oxygen sources is not always 100%. It may differ from machine to machine. For example, an oxygen concentrator supplies variable oxygen, ranging from 90 to 96 percent purity.
In the case of high flow devices, the value of FiO2 remains controlled.
- Temperature:- Air is supplied at a normal atmospheric temperature in case of low flow devices. But, in the case of high flow devices, it supplies air which is generally warm and humid. This allows maximizing the tolerance of the patient.