Introduction 9 Anesthetic Gas Module
175
The patient sample gas or zero calibration gas then flows through the measurement subassemblies:
• the IR Measurement Assembly (for measurement of anesthetic agent, CO
2
and N
2
O)
• the O
2
cell (if present)
• the Agent Identification assembly.
A second solenoid valve between the O
2
cell and the Agent Identification Assembly routes room air
directly to the Agent Identification Assembly for optimal purging of the assembly during zero
calibration.
From the Agent Identification Assembly the patient sample gas or zero calibration gas flows to the
pump. Before reaching the pump, it joins the drainage path again.
From here it is passed through a filter and damper to the flow sensor which consists of a differential
pressure transducer and a flow restrictor. The flow sensor determines, stabilizes and limits the flow rate
of the sampled gas.
After the gas has passed through the flow sensor it is routed through a second damper to the Sample
Gas output.
Agent Identification Assembly
The agent ID analyzer identifies which anesthetic agents are present in a gas sample drawn from the
patients’s airway. The anesthetic agents are identified from a set of known anesthetic gases.
Measurement Principle
Sample gas passes through the agent identification head where the absorption characteristics of the gas
are measured. This is done using NDIR technology as described in “General Measurement Principles”
on page -171. The head outputs analog signals and sends them for processing to identify the anesthetic
agent.
Data averaging is used to ensure accurate measurements when agent concentrations are low. The
information used to calculate the concentrations of the three agents includes:
• The preamplified outputs from the IR detector.
• The thermistor output from the agent identification head.
• Zero calibration constants.
Isoflurane
Enflurane
Halothane
Sevoflurane
Desflurane
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