WM 68071 05/2021
MEDUMAT Standard
2
EN 247
14 Appendix
Functioning of CO
2
monitoring
CO
2
monitoring is performed using the side-stream method by
removing a small amount of gas (80 ml/min) close to the patient
connection of the breathing circuit. The measurement principle
is based on nondispersive infrared technology (NDIR). The CO
2
module provides a time-resolved measurement of the CO
2
concentration (capnogram) and the end-tidal CO
2
(etCO
2
).
This physiological parameter is determined by an internal algorithm
based on the analysis of the capnogram in the time range.
The inspiration and expiration cycles are distinguished by
comparing the differentiated capnogram with dynamically
adjusted threshold values. The etCO
2
values are determined on the
basis of this. No deterioration in the measurement accuracy of the
gas measurement at the end of the ventilation cycle is expected
and the given measurement tolerance applies within the stated
limits of the respiratory rate and I:E ratio. Outside these limits, a
deviation of the measurement accuracy of up to 25 % downwards
is to be expected. This means that the measured value is incorrectly
displayed as too low. If the respiratory rate is greater than
50 min
-1
, e.g., with CCSV, the etCO
2
measurement can be
impaired more significantly.
Measurement range for the respiratory rate
The measurement range for the respiratory rate and the possible
impact of the respiratory rate on the accuracy of the gas
measurement at the end of the ventilation cycle were evaluated
by feeding CO
2
testing gas and compressed air alternatively
through a switch valve at a set rate. The gas was removed by
suction via the CO
2
module and the measurements were
evaluated. The frequency of gas switching was then increased
until the measurement deviations exceeded the set tolerance.
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