Describe how high-airflow oxygen enrichment masks work

High-airflow oxygen enrichment (HAFOE) masks are also known as fixed performance or venturi masks. Their function is based on the Venturi effect.

These devices use air entrainment to deliver fixed concentrations of oxygen to patients at flow rates that exceed the patient’s peak inspiratory flow rate. Oxygen flows through a constriction to create a pressure drop in accordance with the Bernoulli principle. The fall in pressure is used to entrain ambient air through side openings in the venturi device.

The oxygen flow rate required is specified by the manufacturer for different aperture sizes, which result in a predictable fraction of inspired oxygen (FiO₂). They are capable of achieving inspired oxygen concentrations of 24%, 28%, 31%, 35%, 40% and 60% depending on the aperture size and the driving oxygen flow rate.

Flow rates higher than the patient’s peak inspiratory flow rate are required to ensure accurate oxygen concentration delivery. The entrainment ratio refers to the amount of air entrained for the driving oxygen flow. A high entrainment ratio produces a relatively low FiO₂, while a low ratio produces a higher FiO₂. To achieve high oxygen concentrations, the entrained volume of air needs to be limited, resulting in a reduced total flow of gas to the patient. Therefore, HAFOE devices are not suitable for the administration of oxygen concentrations greater than 60%, as the gas flow may be inadequate.