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Chapter 13: GAS Monitoring (Mainstream)
User Manual
For professional use. See instructions for use for full prescribing information,
including indications, contraindications, warnings, precautions and adverse
events.
Measuring principle
The IRMA sensor head snaps in place on the top of the airway adapter that includes the optical
components for measuring all gases. The IRMA airway adapter is inserted between the
endotracheal tube and the Y-piece of the breathing circuit. The respiratory gas measurements are
obtained by continuously measuring the infrared gas absorption through the XTP windows in the
gas flow through the adapter. The XTP windows are transparent to light in the wavelength ranges
of interest and they are specially designed using the latest advances in material technology to
provide a window minimizing the impact of water vapor on light transmission.
To measure the concentrations and identify the gases, absorption of up to nine different
wavelengths of infrared light is measured.
The measurement of CO2, N2O and anaesthetic agents in the breathing gas mixture is based on
the fact that the different gas components absorb infrared light at specific wavelengths. A
microprocessor continuously calculates the CO2, N2O and anaesthetic agent concentrations from
the infrared light absorption measurements. Using matrix calculations to identify which
anaesthetic agents are present in the gas mixture.
(For more information about IRMA sensor, refer to APPENDIX V).
The measured parameters are EtCo2, EtN2O, EtAA (End Tidal CO2/N2O, Anesthesia Agent),
FiCo2, FiN2O, FiAA (Fraction Inspiratory CO2/N2O/Anesthesia Agent), AWRR (Air Way
Respiratory Rate) and MAC.
Fi and Et values are displayed after a breath and average of RESP value is updated regularly. If
the respiration rate (RR) violates 80 bpm, Et value for Anesthesia agent and N2O will fall below
nominal value (Et
) according to below formula:
nom
Et = 80*Et
/RR
nom
EtCO2 value for the respiration rate below 150 bpm will be in the specified range (IRMA CO2
and IRMA AX+).
MAC (Minimum alveolar concentration)
Minimum alveolar concentration or MAC is a concept used to compare the strengths of
anesthetic vapors; in simple terms, it is defined as the concentration of the vapor in the lungs that
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