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Venturi Mask (air entrainment mask)
provides 24% to 50% O2
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Increasing the flow rate on the device will not alter FiO2.
The jet size and entrainment port alter FiO2.
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The larger the entrainment port, the more air entrained and
the lower the FiO2.
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Likewise, the smaller the entrainment port, the less air
entrained and the higher the FiO2.
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The larger the jet size, the less air entrained and the
higher the FiO2.
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The smaller the jet size, the more air entrained and the
lower the FiO2.
The entrainment port must be prevented from becoming occluded (e.g.,
by the patient's hand or bed sheet), because this decreases the
amount of air entrainment and thus increases the delivered O2
percentage.
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O2 Percentage
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Air/O2 Entrainment Ratio
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24%
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25:1
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28%
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10:1
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30%
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8:1
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35%
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5:1
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40%
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3:1
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45%
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2:1
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50%
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1.7:1
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60%
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1:1
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These ratios may be calculated by the following formula:
100-x =
parts of air entrained
x-20*
1 part O2
*Use 21 on percentages of less than 40%
Example:
Calculate the air/O2
ratio for 40% O2
100 - 40 =
60 =
3 = 3:1
40-20 20 1
This means that for every liter of O2 (source gas) delivered from
the flowmeter, 3 L of air are entrained into the device.
Another method to calculate air/O2 ratios is the "magic box" method.
It is not really magic, but most people like it better than the
above equation.
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Draw a tic-tac-toe box and place 20 or 21 (depending on
percentage you are calculating) in the upper left corner.
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Place 100 in the lower left corner.
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Place the percentage you are calculating in the middle box
of the middle row (in this example, use 40).
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Subtract 20 from 40 and place the answer in the lower right
corner, and subtract 40 from 100 and place the answer in the
upper right corner.
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After subtracting, you should have 60 in the upper right
corner (representing air) and 20 in the lower right corner
(representing O2). Now divide 60 by 20, which equals 3, or a 3:1
air/O2 ratio.
Calculating Total Flow
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If a Venturi mask is set on 40% oxygen
and a flow rate of 12 L/min, the total flow delivered
would be:
12 L/min of O2
+36 L/min of
air (12 x 3)
48 L/min of total flow
FASTER METHOD: Add the ratio parts together and multiply
by the flow.
40% (3:1 air/O2 ratio)
3 + 1 = 4 and 4 x 12 = 48 L/min |
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Example:
A Venturi mask is set on 24% O2 and a flow of 4 L/min.
Calculate the total flow.
100-24 =
76 =
approx. 25
= 25:1 air/O2 ratio
24-21 3 1
4 L/min of O2
+ 100 L/min of
air
104 L/min of total flow
OR
Sum of the ratio of parts multiplied by flow:
[(25+1)=26] x 4 = 104 L/min |
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Example:
The physician has ordered a 40% aerosol mask to be
placed on a patient who has a total inspiratory flow
of 44 L/min.
What is the minimum flow rate setting on the
flowmeter that will meet this patient's inspiratory
flow demands?
Because the air/O2 ratio for 40% O2 is 3:1, add
the ratio parts together:
3 + 1 = 4
Set the flowmeter on the lowest L/min flow that
multiplied by 4 delivers a total flow of at least 44
L/min, or 4 x ? = 44.
Therefore, the flowmeter must be set at a minimum of
11 L/min.
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To determine a patient's inspiratory
flow, use the following equation:
Inspiratory flow = ______VT
(L)_____
inspiratory time (sec)
The flow will be in L/sec, so multiply by 60 to change
to L/min.
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Example:
The physician has ordered O2 for a patient with a
spontaneous VT of 550 mL and an inspiratory time of
1 sec. The O2 device must deliver what flow rate to
meet the patient's inspiratory demands?
Inspiratory flow = .55 L/1.0 sec. = .55 L/sec x 60
33 L/min
In other words, the O2 delivery device used must be
able to deliver a total flow of at least 33 L/min to
meet the patient's inspiratory flow demands. Which
of the following would produce the necessary flow?
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40% air
entrainment mask at 8 L/min
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50% air
entrainment mask at 12 L/min
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35% air
entrainment mask at 6 L/min
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50% air
entrainment mask at 10 L/min
Flow rates are:
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32 L/min
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32 L/min
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36 L/min
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27 L/min
Choice "C" exceeds the patient's inspiratory flow
demands, and therefore is the correct choice.
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