Question 16

College Answer

This question about how PaO2, PaCO2, pH, and HCO3 are obtained was not well answered by most candidates. Arterial blood gasses are routinely performed in most ICU on a daily basis. This question relates to a Level 1 (L1) topic in the CICM First Part Syllabus. Most answers simply lacked enough information. Details of how the Clark, Severinghaus, and Sanz electrode’s function was expected. Many candidates confused the pH and PaCO2 electrodes and confused the Clark (Polarographic) electrode with a Fuel Cell. Some knowledge about the types of electrodes and chemical reactions (e.g. reduction of O2 at the Platinum cathode in the Clark electrode) occurring in these devices was expected.

Discussion

This closely resembles Question 9.1 from the second paper of 2008, except that was from the Second Part Exam.

• PaO₂ measurement:
  • 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.
• PaCO₂ measurement:
  • ​​​​​​​PaCO₂ is measured with a modified glass electrode (Severinghaus 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.
• pH measurement:
  • ​​​​​​​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.
• Bicarbonate is not measured, it is derived:
  • ​​​​​​​The pH and PaCO₂ measurements are known
  • From these, using the Henderson Hasselbalch equation, it is possible to calculate the bicarbonate value

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.