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2.2 Pre-use Test
2.2.1 Fresh Gas
Fresh gas (FG) fl ow adds to the delivered Tidal Volume (TV) during the
inspiratory period. To compensate, the delivered volume must be reduced.
The formula for this reduction is:
For example, let
Therefore,
2.2.2 Compliance
Compression of gas in the dead space within the breathing system reduces the
tidal volume delivered to the patient. In an ideal ventilator, the Set TV would
be the volume of gas that is delivered to the patient’s lungs. This cannot be
achieved because the anatomy of the patient is unknown. However, the Set
TV can be accurately delivered from the catheter mount, thus reducing Set TV
errors to a minimum.
To calculate the effect of breathing system compliance on
the delivered TV, it is necessary to measure the capacity or
compliance (Cs) of the system. This can only be done as part
of a pre-use check procedure.
Effective TV = TV - TV due to FG
TV
E
=TV - ((FG fl ow rate (mL)/60)* Inspired time (sec))
TV
E
= TV - TV
FG
FG fl ow rate = 5LPM, TV = 600mL, Frequency = 10BPM, I:E = 1:20
Inspired time = * = 2 Sec
TVfg = * 2 = 83 1/3 = 167ml
10
3
1
5000
60
60
Effective TV = 600 - 167 = 433mL
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