Patient Monitor User’s manual
14-13
14.8 Adverse effects on Performance
—Quantitative effects of humidity or condensate;
—Quantitative effects of barometric pressure;
—Interfering gases or vapors; (Refer to chapter A.6.8)
—Other sources of interference. (Refer to Appendix D)
Gas measurement units
Gas concentration is reported in units of volume percent. The concentration
is defined as:
The total pressure of the gas mixture is measured by a cuvette pressure
sensor in the ISA sidestream module.
For conversion to other units, the actual atmospheric pressure sent from the
ISA sidestream analyzer may be used, e.g.
CO2 in mmHg = (CO2 concentration)×(atm. pressure value in kPa from ISA)
×(750/100).
Example: 5.0% CO2 @ 101.3 kPa 0.05×101.3×750 / 100 = 38 mmHg
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% at actual ambient temperature
and humidity level, instead of showing actual partial pressure. 20.8% O
2
corresponds to the actual O
2
concentration in room air with 0.7% H
2
O
concentration (at 1013 hPa this equals for example 25℃ and 23% RH). The
measurement of CO
2
, N
2
O, and anesthetic agents (e.g. all gases measured by the
IR-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 is sampled, and passing the sampling line, the gas
temperature will get close to the ambient temperature before reaching the ISA
sidestream gas analyzer. As the Nomoline removed all condensed water, no
water will reach the ISA gas analyzer. The relative humidity of the sampled gas
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