Question 9.1 from the second paper of 2008 asks of the bewildered candidates:
"Outline how pH, PCO2 and PO2 are measured in a blood gas analyser and briefly state the underlying principle behind each of those measurements."
The pass rate of 47% was pleasantly surprising, suggesting that many (if not all) of the candidates arose from an intellectually robust anaesthetic background. Judging by the college model answer, the bare minimum of expected knowledge resembles the contents of the following three paragraphs:
- pH is measured with a glass electrode suspended in the blood sample.
- The blood sample acts a a conducting electrolyte.
- The potential difference across the electrode is proportional to the pH difference, and this can be measured.
- PaCO2 is measured with a modified glass electrode.
- The electrode is bathed in a solution which contains some sodium bicarbonate, and generates a known potential difference.
- The CO2 from the blood sample diffuses across a semipermeable membrane into the bicarbonate solution,
- 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.
- PaO2 is measured with a Clark electrode
- The Clark electrode measures the change in current flowing through a reaction chamber where O2 is reduced to OH- ions by a change in voltage.
- O2 from the blood sample diffuses through a semipermeable membrane into an aqueous buffer.
- In the aqueous buffer it is reduced to OH- ions with the application of a potential difference (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.
- The rate of increase of current in proportion to increase in voltage becomes non-linear at a PaO2 above 150mmHg, and the ABG machine is usually clever enough to compensate for this known fact.
- HCO3- is calculated from the Henderson-Hasselbalch equation.