Principles of measuring PCO₂ with the Severinghaus electrode

This chapter is most relevant to Section F12(iii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the methods of measurement of oxygen and carbon dioxide tension in blood". By some weird cruelty of the CICM examiners, it had come up once in Question 9.1 from the second Fellowship Exam paper of 2009. One can be reasonably confident that it will never appear again.

In summary:

The recollections of John Severinghaus are not available to most people, as they are locked behind paywalls. This is unfortunate, because he has co-authored some awesomely wacky articles. Thankfully, John W Severinghaus and A. Freeman Bradley's 1958 paper detailing the design and performance characteristics of their first ABG analyser can still be seen at the Journal of Applied Physiology.

The Severinghaus Electrode

This thing is essentially a slightly modified glass electrode. Observe:

The CO₂ dissolved in the sample diffuses into the middle compartment of the electrode via a thin membrane. After experimenting with several thin films of different polymers, Severinghaus settled on Teflon; however subsequent generations have changed over to silicon polymer.

Once inside, the CO₂ finds itself in an aqueous solution. For convenience, there may or may not be a bicarbonate solution added to this chamber. The reaction which takes place is an old familiar carbonic acid dissociation equilibrium:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^-

Thus, the pH of the solution in the middle chamber changes. The change in pH is completely dependent on the pCO₂, provided the temperature and pressure remain constant:

This results in a change in potential difference in the glass electrode; and the function of this item has already been discussed at some length in another chapter. Thus, from the change in pH, one can calculate the pCO₂.