Describe the principles of capnography, including calibration, sources of error and limitations.
Answers that scored well followed the structure outlined in the question and explained the principles of each component of the question.
It is hard to tell, but it seems that by "structure outlined in the question", it appears the examiners wanted something with the headings "principles", "calibration", "sources of error" and "limitations". This puts one in a position where one must identify sources of error which are not limitations, and limitations which are not sources of error, which would be hard to do over a ten-minute answer. Perhaps because of this, the pass rate was 31%. The marker's expectations are of course difficult to reconstruct from the examiner's elliptic comments. What is presented here hopefully represents a passing answer, but how would you ever know.
- Capnometry is the measurement of the concentration of CO2
- Capnography refers to the graphic display of this measurement over time.
- The most common methods in routine use are IR spectroscopy and colour change colourimetry.
- IR spectroscopy:
- CO2 is a good absorber of a near-IR wavelength of light (4.26 μm)
- 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)
- Colour change colourimetry:
- The fact that CO2 changes the pH of a solution it passes through can be used to detect CO2 in expired gas mixtures
- These capnometers are usually used to immediately detect the correct placement of an endotracheal tube
- Limitations of IR spectroscopy:
- Sensor is susceptible to blockage by secretions or condensation
- Spurious readings can be produced by N2O
- Sidestream devices have a delay in measurement
- Mainstream devices increase dead space
- Limitations of colour change colourimetry:
- Not quantitative
- Only useful over a short lifespan of the detector
- Highly sensitive to CO2, which might result in colour change with oesophageal intubation
- Limitations 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
- Calibration of capnometry
- Capnometers are zeroed to room air
- A gas of known CO2 concentration is used for calibration
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