Outline the principles of measurement of end-tidal CO2 using infrared radiation (25% of Marks). Describe the potential sources of error when using this modality and
how they may be mitigated (75% of Marks)
The principles of infrared (IR) measurement of expired CO2 was answered in acceptable detail by most candidates. Sources of error were rarely identified nor explained in acceptable detail. Many candidates demonstrated errors of understanding when attempting to answer this section. Important concepts not covered by numerous candidates included side-stream vs mainstream techniques; how the Beer-Lambert law is applied and how concentration is used for partial pressure; what is the role of sapphire glass and the difference between collision broadening and additional IR absorption by N2O.
The "sources of error" section here was weighted 75%, which is what must have killed most of the answers. The only other SAQ on this topic was Question 10 from the second paper of 2019, where the pass rate was also dismal (31%).
Principle of IR spectroscopy:
- CO2 is a good absorber of a near-IR wavelength of light (4.26 μm)
- A sapphire lens can be used to transmit light of the correct wavelength through the sample (as saphire has good transmittance for near-IR light)
- Concentration of CO2 in the sample can be determined by the Beer-Lambert law on the basis of this absorption
- Measurement may be by mainstream detectors (circuit components introduced into the path of respiratory gases) or by sidestream detectors (which sample a small fraction of the circuit gas mixture)
Sources of error, and how they might be mitigated
- Of IR spectroscopy:
- Sensor is susceptible to blockage by secretions or condensation (this can be detected and prevented by circuit heating and equipment changes)
- Sensor calibration can drift from factory settings (the sensor is usually impossible to re-calibrate at the bedside, and may need to be returned for servicing)
- Ambient infra-red light can confuse the monitor (a well-shielded sensor should be able to defend against this)
- Spurious readings can be produced by N2O (though some devices are designed to measure both gases)
- Sidestream devices have a delay in measurement (this needs to be accounted for when interpreting the waveform)
- The collision broadening effect can confuse the measurement, where the IR absorption band widens due to the increase in pressure in the gas sample, or due to the presence of other gases. In scenarios where this is likely to happen (eg. in anaesthetic machines where nitrous oxide is used) this source of error is usually corrected for at the level of software, as the magnitude of the effect is predictable.
- Cross-interference of absorption bands can occur, where other gases (eg. nitrous oxide o oxygen) are present in such high concentrations that their absorption bands overlap with that of CO2. You can usually avoid this problem by using an emitter with a sufficiently narrow wavelength spectrum.
- Of capnometry in general:
- The end-tidal CO2 value is not pathology-specific or diagnostic.
- Bias flow can dilute the sample
- False-positive CO2 measurements can occur (eg. gastric gas)
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Kennell, Eric, Raymond Andrews, and Harry Wollman. "Correction factors for nitrous oxide in the infrared analysis of carbon dioxide." Anesthesiology 39.4 (1973): 441-443.
Rosencwaig, Allan. Photoacoustics and photoacoustic spectroscopy. Wiley, 1980.