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9.11. Technical specifications of gas monitoring module
The gas monitoring module is used to connect other medical devices, provide monitor data,
monitor and display CO
2
, N
2
O and five kinds of anaesthetic gases (halothane, isoflurane,
enflurane, sevoflurane, desflurane).
The gas monitoring module is connected to the patient’s breathing pipeline to monitor the
inspiratory and expiratory gas and can be applied to Adult/Child and Infant.
CAUTION: Gas monitoring module complies with ISO 80601-2-55.
9.11.1. MAC calculation of ISA gas analyzer
Algorithms used in the host monitor to determine MAC values
Minimum alveolar concentration (MAC) is a standard for comparing the potency of inhalation
anaesthetics. The MAC value represents the alveolar concentration of an anaesthetic (at one
atmosphere) that, in 50 percent of a tested population, prevents gross muscular movement in
response to a painful, standardized stimulus.
If a mechanism to determine MAC values is implemented in the host device, the algorithms used
for this calculation must be adequately documented.
The MAC value may be calculated and displayed by using end-tidal (Et) gas concentrations
according to the following formula:
X(AA): HAL=0.77%, ENF=1.7%, ISO=1.15%, DES=6.65%, SEV=2.1%,N
2
O=105%
Note: Altitude, patient age and other individual factors are not considered in the
formula above.
9.11.2. Gas data consideration
Quantitative effects of humidity or condensate
The partial pressure and the volume percentage of CO
2
, N
2
O, O
2
and anaesthetic 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 0.7vol%
H
2
O concentration (at 1013 hPa this equals for example 25°C and 23% RH). The measurement of
CO
2
, N
2
O, and anaesthetic 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 will be about 95%.
If CO
2
values at BTPS are required, the following equation can be used:
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