J2ii / F12iii / 22B16: Describe how the values for PaO2, PaCO2, pH and bicarbonate are determined on a blood gas sample

22B16: Exam Report

Describe how the values for PaO2, PaCO2, pH and bicarbonate are determined on a blood gas sample

36% of candidates passed this question.

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.

JJ2ii / F12iii / 22B16: Describe how the values for PaO2, PaCO2, pH and bicarbonate are determined on a blood gas sample

The Clark electrode

The Clark electrode detects PaO2

  • Consists of a platinum cathode and a silver/silver chloride anode in a buffered solution of potassium chloride
  • A gas permeable membrane separates the sensor from blood, oxygen tension in blood equilibrates with the electrolyte solution
  • A voltage of 0.6V is applied between electrodes
  • At the anode Ag is oxidised: 4Ag + 4Cl– → 4AgCl + 4e–
  • At the cathode O2 is reduced: O2 + 4H+ + 4e– —> 2H2O
  • Current from anode to cathode is measured by a microammeter and is proportional to the oxygen tension in the electrolyte solution (and therefore blood)

The glass (Sanz) electrode

The glass (Sanz) electrode detects pH

  • Consists of a measurement electrode (Ag/AgCl) and a reference electrode (Hg/HgCl2 in KCl solution)
  • The measurement electrode bathed in a pH stable buffer solution which is then encased in a pH sensitive glass bulb
  • A potential difference is generated by movement of H+ from blood through the pH sensitive glass into the buffer solution
  • The magnitude of which is proportional to the difference in [H+] between the blood and buffer solution on either side of the glass membrane
  • This is measured and converted to a either a pH reading

The Severinghaus electrode

The Severinghaus electrode is a a modified glass electrode which detects PaCO2

  • Measurement electrode (Ag/AgCl) and a reference electrode (Hg/HgCl2)
  • The measurement electrode is bathed in a pH stable buffer solution which is bathed in a bicarbonate solution
  • CO2 from blood crosses a CO2 permeable membrane, reacting with water to form H+ ions (CO2 + H2O → H2CO3 → H+ + HCO3-)
  • This changes the pH of the bicarbonate solution
  • A potential difference is measured which is proportional to the [H+] of the bicarbonate solution which, in turn, is proportional to the PaCO2 given a stable ToC and pressure

Bicarbonate

  • HH equation states pH = pKA + log ([conjugate base]/[acid])
  • In this instance:

pH (calculated value) = pKA of HCO3– (known) + log ([HCO3–]/[H2CO3])

  • [H2CO3] = 0.03 x PaCO2 (known) based on Henry’s Law
  • Thus, [HCO3–] is calculated

Author: Andrew Wang