Anesthesia Gas Monitoring
18-4
18.2 Measurement Principles and Procedure
Measurement principle of AGs
The AG concentration is measured based on the rationale that the AGs have the
property of absorbing the infrared.
The AG module can measure gases that have various properties of absorbing the
infrared. To measure the concentration of a gas, send it to the sampling room, select
the infrared of a specific wavelength with an optical infrared filter, and transmit it
through the gas. For a given volume of gas, the higher its concentration is, the more
the infrared that will be absorbed by the gas is, and the less the infrared that will be
transmitted through the gas is. The concentration of the measured gas is in inverse
proportion to the volume of the infrared that is transmitted through the gas.
Therefore, the AG concentration can be obtained by calculating the infrared. For the
AG module that implements the measurements of multiple gases, multiple infrared
filters are necessary.
Measurement principle of O
2
The oxygen (O
2
) does not absorb the infrared within the above-mentioned
wavebands, so the oxygen is measured based on its paramagnetism. Inside the
sensor of the O
2
module, there are two crystal balls full of nitrogen. They are
suspended in the symmetrical magnetic field, and they are designed to point to the
strongest outgoing part of the magnetic field. Outside the balls is the paramagnetic
oxygen. Therefore, the balls are forced, by the relatively stronger paramagnetic
oxygen, out of the magnetic field. The moment of the force acting on the balls is
proportional to the paramagnetic strength as well as to the concentration of the
oxygen.
Figure 18-3 Connections for AG measurements
Patient Monitor
AG module
Exhaust line to
scavenging system
Patient sample line
Airway
adapter
Endotracheal tube
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