Question 10

College Answer

Answers that scored well followed the structure outlined in the question and explained the principles of each component of the question.

Discussion

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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Gravenstein, Joachim S., et al., eds. Capnography. Cambridge University Press, 2011.

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.

Marriott, W. McKim. "The determination of alveolar carbon dioxid tension by a simple method." Journal of the American Medical Association 66.21 (1916): 1594-1596.

Berman, J. A., J. J. Furgiuele, and G. F. Marx. "The Einstein carbon dioxide detector." Anesthesiology (Philadelphia) 60.6 (1984): 613-614.

Puntervoll, S. A., et al. "Rapid detection of oesophageal intubation: take care when using colorimetric capnometry." Acta anaesthesiologica scandinavica 46.4 (2002): 455-457.

Westenskow, D. R., et al. "Clinical evaluation of a Raman scattering multiple gas analyzer for the operating room." Anesthesiology 70.2 (1989): 350-355.