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.
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.
Example
24% Ventimask:
At the minimum O flow rate of 2 liters/min, this mask will
generate a total flow rate of 52 liters/min.
To calculate the total flow rate from the same mask if O2 flow
is increased to 4 liters/min, simply look for 4 liters/min on the
vertical axis of the graph and follow the horizontal line to where
it intersects the 24% line. At this point of the intersection look
to see what the air flow rate on the horizontal axis is
appropriate. In this case it is 100 liters/min. Add O2 flow rate
for the final mixture:
100 (air entrained) + 4 (O2 flow) = 104
liters/min.
NOTE: Increasing O2 flow rate maintains the O2 concentration but
delivers it at a higher flow rate.