9-1
9 AG and O2 Concentration Monitoring
9.1 Introduction
The anaesthetic gas (AG) module measures the patient’s anesthetic and respiratory gases, and
incorporates the features of the O
2
module and BIS module as well.
The AG (anesthesia gas) module determines the concentrations of certain gases using the
infrared (IR) light absorption measurement. The gases that can be measured by the AG
module absorb IR light. Each gas has its own absorption characteristic. The gas is transported
into a sample cell, and an optical IR filter selects a specific band of IR light to pass through
the gas. For multiple gas measurement, there are multiple IR filters. This means that higher
concentration of IR absorbing gas causes a lower transmission of IR light. The amount of IR
light transmitted after it has been passed though an IR absorbing gas is measured. From the
amount of IR light measured, the concentration of gas present can be calculated.
Oxygen does not absorb IR light as other breathing gases and is therefore measured relying
on its paramagnetic properties. Inside the O
2
sensor are two nitrogen-filled glass spheres
mounted on a strong rare metal taut-band suspension. This assembly is suspended in a
symmetrical non-uniform magnetic field. In the presence of paramagnetic oxygen, the glass
spheres are pushed further away from the strongest part of the magnetic field. The strength of
the torque acting on the suspension is proportional to the oxygen concentration. From the
strength of the torque, the concentration of oxygen is calculated.
The measurement provides:
1. An EtCO
2
waveform;
2. Measured parameters: EtCO
2
, FiCO
2
, EtN
2
O, FiN
2
O, EtAA, FiAA and MAC,
where, AA stands for any of the five anesthetic agents: Des (desflurane), Iso (isoflurane), Enf
(enflurane), Sev (sevoflurane), or Hal (halothane),
NOTE
z Perform AG
monitoring when using this anesthesia machine to ensure patient
safety. If your anesthesia machine is not configured with AG
module, use the
monitor with
CO
2
monitoring function in compliance with the relevant
international standard to perform AG
monitoring.
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