User's Manual Part 3
10-22
E.2.6.
Gas data considerations
Gas measurement units
Effects of humidity
The partial pressure and the volume percentage of CO
2
,
N
2
O , O
2
and anesthetic agents
depend
on the amount of water vapor in the measured gas. The
O
2 measurement will be
calibrated to show 20.8 vol% at actual ambient temperature and humidity level, instead of
showing actual
partial pressure. 20.8 vol%
O
2
corresponds to the actual
O
2
concentration in
room air with 0.7
vol% H
20
concentration (at 1013 hPa this equals for example 25°C and
23% RH). The measurement of CO
2,
N
2
O, and anesthetic agents (e.g. all gases measured
by the IA-bench) will
always show the actual partial pressure at the current humidity level.
In the alveoli of the patient, the breathing gas is saturated with water vapor at body
temperature (BTPS). When the breathing gas flows through the sampling line, the gas
temperature will adapt to ambient before reaching the gas analyzer. As the NOMO
section removes all condensed water, no water will reach the ISA gas analyzer. The
relative humidity of the sampled gas will be about 95%.
If
CO
2
values at BTPS are required, the following equation can be used:
where :
EtCO2
= EtC
O
2
value sent from ISA [vol %]
Pamb = Ambient pressure sent from ISA [kPa]
3.8 = Typical partial pressure of water vapor condensed between patient circuit and ISA [kPa)
EtC02(BTPS)
= EtCO
2
gas concentration at BTPS [vol%]
Spectral broadening
The presence of oxygen and nitrous oxide can cause some interference in the CO
2
measurement. This is known as spectral broadening.
Nitrous oxide, N
2
O:
ISA sidestream analyzers automatically compensates for spectral broadening caused by
nitrous oxide.
Below is the typical effect if using the default value (0 vol% N20) when measuring on gas
mixtures with different N20 concentrations:
N
2
O concentration
in gas mix
Effect on gas reading
Displayed value if true
concentration is 5.0 vol% CO2
0 vol%
0 % relative
5.0 vol%
30 vol%
5.17 % relative
5.3 vol%
60 vol%
10.34 % relative
5.5 vol%
82 vol%
14.14 % relative
5.7 vol%