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Appendix
Flow Measurement
-
Measuring
principle and signal processing
The sensor works on the principle
of a constant
temperarure
hot-wire
anemometer.
The breathing
gas flows through
a tube which contains
two
platinum wires. One of the wires is heated while the other compensates
for
gas temperature.
The transfer of energy from the wire to the breathing
gas is dependent
on
mass flow. As a result the measurement
of volume is influenced
by ambient
air pressure.
If ambient
air pressure
changes
by 20mbar the measurement
accuracy
changes
by 2% (relative to normal pressure
1013 hPa)
Gas compensation
PM 8030 automatically
corrects
the influence
of breathing
gases of different
compositions
(Oz/NzO
mixture) on flow measurement.
During inspiration
(no
gas flow to sensor) the gas composition
is identified.
Linearisation
is carried out using different
calibration
tables for gas mixtures
of Oz/NzO
or of air and OZ.
Classification
of breathing
models
The calibration
of the sensor applies to defined
flow conditions.
Artefacts,
arising from fluctuations
of the gas column in the hoses or pressure
surges
during the closing or opening
of valves are eliminated
by using a specific
model of the breathing
process.
This process
is based on the physiological
expiration
flow model where there is a steep increase
in ilow at the beginning
of expiration.
A minimum flow and a minimum volume must be achieved
to
establish
a valid breathing
model:
\jE minute volume Umin
Minimum volume mL
VE < 2
15
2<V~<4
30
4<V~<6
50
VE > 6
90
Determination
of \SE
i/E measurement
is independent
of the breathing
model. The measured
value
VE is integrated
from the flow for a 30 second time period.
Definition of PEEP and Plateau pressure
PEEP (positive endexpiratory
pressure)
is the airway pressure at the end of
expiration.
Plateau pressure
is the airway pressure
measured
16 milliseconds
before the
start of expiration.
This definition of PEEP and Plateau pressure
is related :o the breathing
model described
above.
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