CO2 analysis in medical

Anesthesia & ventilation during surgery

During intraoperative ventilation, low-flow anesthesia and lung-protective ventilation strategies enable precise control of general anesthesia.  The concentrations of the respiratory gases O2 and CO2 are the most important vital parameters for this, which are measured and output with sensor-controlled monitoring devices, so-called anesthesia systems. The precise indication of values in real time is absolutely essential for the anesthesiologist to take the right measures and decisions for a successful operation.

Capnometry measurement

Capnometry is used to measure the carbon dioxide content of the respiratory air during the patient's entire respiratory cycle. The determined in- and expiratory CO2 concentration is graphically displayed as a continuous curve on a monitor - "capnography".  The end-expiratory CO2 concentration is of particular interest to the anesthesiologist. This value is therefore shown digitally, preferably as partial pressure (PECO2) in "mmHg", on the display of an anesthesia machine. Today, capnometry/graphy is one of the most important parameters of intraoperative anesthesiological monitoring of the anesthetized patient.

It is used to

• Monitoring ventilation and the respiratory system
• Assessment of CO2 production
• Assessment of CO2 reabsorption
• Assessment of the circulation-dependent CO2 transport

Which measurement method is used?

Infrared spectrometry has become widely used in clinical practice. Here one uses the physical optical fact that CO2 can absorb infrared light in a narrow wavelength range (maximum at 4.26 µm) due to its molecular properties. An IR emitter transmits light of this wavelength through a measuring chamber containing the breath sample (gas cuvette). The transmitted portion is then detected by an IR detector and quantitatively analyzed. Since the amount of light absorbed is proportional to the number of CO2 molecules present in the sample, the CO2 concentration can be calculated. A reference chamber contains CO2 free gas. It serves as a zero value for calibration. The zero calibration is performed by modern instruments in short intervals (ca.2-4 times per h) during operation.

Interpretation and application of measurement results

With the help of capnometry basically 3 physiological processes can be evaluated qualitatively and partly also quantitatively:

1. CO2 exhalation (ventilation)
2. cellular CO2 production (metabolism)
3. CO2 transport in the blood and thus the HZV (hemodynamics)

The interpretation of the values of ventilation allows to determine whether ventilation or respiratory activity is taking place at all. For example, a missing CO2 curve is considered to be the fastest and most reliable criterion for determining false intubation or extubation.  In such critical situations, fast medical action is absolute urgent.

What basic technologies are needed for gas analysis in medical devices?

In order to obtain highly reliable measured values, NDIR gas sensors serve as sensor modules in anesthesia systems. In each module, an IR emitter and IR detector form the metrological basis of the sensor. In the described application, special technical requirements apply to the infrared components: they must be extremely stable over time and powerful in order to output highly accurate and absolutely reliable measured values.

Micro-Hybrid offers medical technology manufacturers here for the complete product and technology spectrum:

• Powerful IR emitters
• Highly sensitive IR detectors
• Customized gas sensors, incl. development of sensor electronics

We are specialized in customization and development of infrared components and sensor modules. Send us your inquiry about current development projects. Our sales team will be happy to advise you.