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". Remarkably, this clinically apocryphal topic was interrogated in Question 9.1 from the second Fellowship Exam paper of 2009. One can be reasonably confident that it will never appear again, and ignore this chapter in its entirety.
- A silver anode and platinum cathode are suspended in an electrolyte.
- Oxygen is dissolved in the electrolyte.
- A voltage of known magnitude (about 700 mV) is applied to the electrodes.
- Oxygen is reduced at the cathode and silver is oxidised at the anode.
- The resulting current increases as the voltage increases.
- The current reaches a plateau when the rate of reaction is determined by the diffusion of oxygen rather than the voltage.
- This plateau correlates to the oxygen tension in the electrolyte.
Leland C. Clark never called his device "the Clark Oxygen Electrode", as such a gesture would probably have been viewed by his contemporaries as mildly disgusting. The paper he published discusses the "continuous recording of blood oxygen tensions by polarography"- it was a "polarographic" electrode, and this is also how it is referred to in some of the earlier literature. The polarogram is the graphed relationship of current and voltage which is discussed at length elsewhere.
The electrodes nowadays are referred to as "polarographic" as they do not contain a mercury electrode (apparently that is a prerequisite). The Radiometer reference manual describes their electrodes as "amperometric", to reflect the fact that they measure current; whereas the "potentiometric" electrodes are more interested in voltage. The principles of amperometric measurement in general are discussed in broad non-specific terms elsewhere. Like the Clark electrode, other notable members of the amperometric electrode family (the glucose electrode and the lactate electrode) are sufficiently unique to merit their own chapters.
In researching the Clark electrode and its history, one may find it difficult to collect all the desired information, as much of it is locked behind paywalls, or is now out of print. The original article belongs to the Journal of Applied Physiology, as do the recollections of John Severinghaus. Clark's own autobiographical account of the discovery requires a subscription to International Anesthesiology Clinics.
Thankfully, some freegan medical education does exist. 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.
John Kanwisher's article from 1959 discusses the electrode in great detail, even though its relevance is perhaps greatest to oceanography (from his diagrams and discussion, it would appear that Kanwisher measured the respiration of small marine animals by shoving them directly into the electrode). Similarly, it seems one is easily able to retrieve a diagram of it from the United States Patent Office (via Google). Finally, it was possible to retrieve a satisfactory amount of detail from William L. Nastuk's 1962 textbook, "Electrophysiological Methods:Physical Techniques in Biological Research".
Anyway. Apparently, the development of the Clark electrode as a continuous means of measuring oxygenation was driven largely by the popular criticism of Clark's dispersion oxygenator ("bubble oxygenator"), which was being used for the first time for cardiopulmonary bypass in the early 1950s. The critics complained that there was no reliable way to confirm that the blood coming out of the oxygenator was oxygenated. One boggles at the ingratitude; prior to the bubble oxygenator, Clark reports that the academic field of extracorporeal oxygenation was something of an unweeded garden:
"...widely varying means of extrapulmonary oxygen administration have been employed. Oxygen had been injected subcutaneously, intraperitoneally, and intravenously, as well as directly into the intestines, the joints, the renal pelvis, and the urinary bladder."
These complaints about weird methods of oxygen delivery is ironic coming from a man who subsequently went on to become one of the founding members of Oxygen Biotherapeutics, Inc, a company which markets Oxycyte (a perfluorocarbon synthetic oxygen carrier designed to act as a blood substitute).
The principles of amperometric oxygen measurement are discussed at some length in the chapter on the platinum oxygen cathode.
The major difference between this electrode and the earlier oxygen cathode is the addition of an oxygen-permeable membrane. Something resembling the original patent application diagram can be found here.
Its butchered representation can be found below.
A number of design flaws of the platinum oxygen cathode have been addressed by Clark's design;
the membrane is the major change. Its presence both protects the platinum from becoming encrusted in proteinaceous debris, and offers a predictable diffusion distance for the oxygen, without the chances of convection. This protects it from some sources of error (though it must be mentioned that the electrode can still occasionally give confusing results when it starts reducing halothane, for example).
The rate of response of the electrode obviously depends on the membrane thickness. It takes time for those little molecules to make their way to the cathode. This diffusion is obviously going to take longer if the membrane is thicker, or if there is a post-membrane layer of electrolyte to negotiate (that is one of the reasons the electrodes these days are right up against the membrane). The response time of a 5μm Teflon membrane is about 1 second, and this can be increased to 0.4 seconds if the sample is heated to 80° C.
The local machinery uses the Radiometer E799 electrode, images of which can be found at the DOM Medical website. It could be decorating your Christmas tree for only $1200.00 (US).