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1. Principles of operation
A fixed performance device utilises the HAFOE principle (High Air Flow with Oxygen Enrichment). O2 (A) flows into a jet (B) to entrain air through apertures (C) in the venturi barrel (D). The resultant air/oxygen mixture containing the prescribed Oxygen concentration flows into the face piece (E) for patient breathing. Surplus gas leaves the mask through the holes (F) to flush out expired CO2.
A fixed performance device utilises the HAFOE principle (High Air Flow with Oxygen Enrichment). O2 (A) flows into a jet (B) to entrain air through apertures (C) in the venturi barrel (D). The resultant air/oxygen mixture containing the prescribed Oxygen concentration flows into the face piece (E) for patient breathing. Surplus gas leaves the mask through the holes (F) to flush out expired CO2.
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How a fixed performance mask works
2. Calculating total gas mixture flow rates
In most clinical situations the minimum recommended oxygen flow rate will entrain sufficient air to provide a gas mixture flow rate to match or exceed the patients inspiratory flow rate. If a patient is known to be hyperventilating it may be necessary to increase the oxygen flow rate to generate a greater gas mixture flow to ensure this is in excess of patients demand. Using the mixture graph below, the total gas flow rate available to the patient can be assessed.
In most clinical situations the minimum recommended oxygen flow rate will entrain sufficient air to provide a gas mixture flow rate to match or exceed the patients inspiratory flow rate. If a patient is known to be hyperventilating it may be necessary to increase the oxygen flow rate to generate a greater gas mixture flow to ensure this is in excess of patients demand. Using the mixture graph below, the total gas flow rate available to the patient can be assessed.
