Sometimes, one does not wish to be bogged down in elaborate discussions of definitions, or in the minutae of historical developments surrounding a specific concept. Sometimes one only has a ten-minute viva session, or a paragraph in a short answer question to devote to an explanation of pH measurement.
For these situations, a brief summary of ABG machine function is available in the "Required Reading" section of the CICM Fellowship Exam Preparation chapters. That, in fact, is enough. The rambling digression on this page certainly does not represent the level of familiarity with ABG analysis expected from the ICU trainee in Australia.
The glass electrode in a modern ABG machine
Though acknowledging that many ABG machines are available and a variety of methods are around, the author has decided to focus selfishly on the Radiometer ABL800 FLEX which uses the E777 pH-sensitive glass electrode and the E1001 reference electrode. One can inform oneself regarding these devices to an insane level of detail with the aid of the reference manual, which is available from the manufacturer's website. Grainy images of the pH electrode itself are available from DOM Medical, where one may purchase it as a replacement backup unit, or as a unique piece of jewellery.
These retailers and their competitors all have nice explanatory documents to precisely describe the function of their products. However, most of these materials involve a near-PhD level of expected chemistry background. Thus, instead of resorting to image burglary I have decided to construct some sort of informative diagram, which might illustrate the methods of pH measurement employed by modern pH electrodes for the lay person.
Without digressing interminably about glass electrode engineering, or the measurement of "hydrogen ion activity", or whether or not there actually is such a thing, one can summarise the critical points in the following manner:
- pH is proportional to the activity of hydrogen ions in solution.
- Differences in hydrogen ion activity between two solutions can generate a potential difference netween those solutions.
- This potential difference can be compared to the potential difference of a reference sample, which has a known pH.
- From the difference between these two measurements, the pH of an unknown sample can be determined.
Thus, the ABG machine measures the activity rather than the concentration
Each electrode is essentially a little sealed vial of carefully controlled buffer solution (about 0.6ml of it). The reference electrode maintains a fixed potential, by virtue of an equilibrium reaction:
AgCl ↔ Ag+ + Cl- .... as well as Ag + + e- ↔ Ag
The glass electrode generates a potential difference which depends on the pH difference between the internal buffer solution and the blood sample. In this way, using the Nernst equation, one can calculate the pH of blood if one knows that at 37° the potential difference changes by -61.5 mV per every pH unit.
E(sample) = E0 - 61.5 × pH
This is not just an irritating delay. It is in fact a process which determines the sensitivity of the pH electrode. Two calibration solutions are washed across the electrode, and their (known) pH is measured. Specifically, Solution 1 has a pH of 7.4, and Solution 2 has a pH of 6.9.
The acceptable limits for sensitivity are 92-103%.
The measuring range for pH on the local ABG machine is 6.300 to 8.000. If your patient's pH is outside of these ranges, you will need to contact your local industrial chemist for advice.