Outline  how pH, PCO2 and PO2  are measured in a blood gas analyser and briefly state the underlying principle behind  each of those measurements.

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College Answer

pH  –                glass electrode,   The specimen is put in a capillary tube surrounded by buffer solution. The tube is made of pH sensitive glass across which a potential difference is generated,           Measures potential difference across the electrodes


PCO2                    modified glass electrode , comprises a glass pH electrode which is in contact with a thin film of NaHCO3 solution. This is separated from the specimen by a membrane that is permeable to CO2 – CO2 diffuses from specimen into the HCO3 solution where it dissociates with a change in pH which is measured by the electrode. Measures potential difference across the electrodes


PO2 –               Clark electrode or polarographic electrode,   polarographic or measures current generated (an amperometric system) or the current flow across the Clark electrode is determined by the PO2 of the solution

Discussion

The scientific principles behind the measurements which magically occur behind the shiny glass of the ABG machine are discussed in greater detail elsewhere

In brief:

  • pH is measured by a glass electrode
    • The potential difference across the electrode is proportional to the pH difference, and this can be measured.
  • PaCO2 is measured by a modified glass electrode
    • The electrode contains some sodium bicarbonate, which reacts with the CO2; the reaction changes the pH in the electrode, which corresponds to a change in potential difference, and this is measured. The CO2 is then inferred from the change in pH.
  • PO2 is measured by a Clark electrode or polarographic electrode
    • O2 in an aqueous buffer is reduced to OH- ions with the application of current (600-800mV); this causes a current to flow between two submerged electrodes. Increasing the voltage across this system also increases the current - up to a plateau. The plateau level depends upon, and is proportional to, the concentration of oxygen.

References

References

 

LITFL give a good overview, in sufficient detail for the time-poor exam candidate:

The best resource for this sort of stuff is the handbook and user guide for the blood gas machine. The brief brochure for our hometown machine can be found here, at the Radiometer website.

There is also a comprehensive operations manual.

The Radiometer Blood Gas Handbook is also a valuable resource. However, it tries to explain what the parameters mean for clinical decisionmaking, rather than how they are measured.

An excellent professional resource on this topic can be found at The Deep Picture.

And if one wants to go to town on this topic, Severinghaus (yes, THE Severinghaus) has published a series of articles on this theme:

Severinghaus, John W., and Paul B. Astrup. "History of blood gas analysis. I. The development of electrochemistry." Journal of clinical monitoring 1.3 (1985): 180-192.

Severinghaus, John W., and Poul B. Astrup. "History of blood gas analysis. II. pH and acid-base balance measurements." Journal of clinical monitoring 1.4 (1985): 259-277.

Severinghaus, John W., and Poul B. Astrup. "History of blood gas analysis. III. Carbon dioxide tension." Journal of clinical monitoring 2.1 (1986): 60-73.

Severinghaus, John W., and Poul B. Astrup. "History of blood gas analysis. IV. Leland Clark's oxygen electrode." Journal of clinical monitoring 2.2 (1986): 125-139.

Severinghaus, John W., and Poul B. Astrup. "History of blood gas analysis. V. Oxygen measurement." Journal of clinical monitoring 2.3 (1986): 174-189.

Severinghaus, John W., and Poul B. Astrup. "History of blood gas analysis. VI. Oximetry." Journal of clinical monitoring 2.4 (1986): 270-288.

Severinghaus, John W., and Yoshiyuki Honda. "History of blood gas analysis. VII. Pulse oximetry." Journal of clinical monitoring 3.2 (1987): 135-138.