Blood Gas Measurement

Blood Gas: pO₂

• Measured using Clark Electrode
  • Platinum cathode
    • Coated in gas permeable membrane à permeable to O₂
    • Membrane impermeable to other oxidizing molecules
    • O₂ + 4e^– → 2O^–
      2O^– + 2H₂O → 4OH^–
  • Silver chloride anode
    • 4Ag → 4Ag⁺ + 4e^–
      4Ag⁺ + 4Cl^– → 4AgCl
  • Phosphate buffer with KCl
• Cathode voltage potential to -0.65v
• In absence of O₂ current = 0v
• In presence of O₂, O₂ diffuses through gas-permeable membrane and is reduced, absorbing 4e^–
• Micro-ampmeter measures movement of electrones between anode and cathode
• The current produced is proportional to the pO₂ of the test solution
• Sensitivity – related to thickness of membrane and size of cathode area

Blood Gas: pCO₂

• Measured using Stowe-Severinghaus Electrode
• Modified pH electrode with outer semi-permeable membrane (Teflon or Silicon Elastic)
• CO₂ diffuses into electrolyte layer
  • CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁺
• H⁺ diffuses across glass electrode and alter pH
• Change in pH measured
• Henderson-Hasselbach Equation used to determine pCO₂

Blood Gas: pH

• Measured using Sanz Electrode
• 2 half-cells connected by KCl bridge
  • Measurement half-cell contain glass membrane with layers of hydrated and non-hydrated glass, permeable or sensitive to H⁺
  • Measurement electrode consists of Silver-Silver chloride bathed in phosphate buffer at known pH 6.84
  • Reference half cell contains mercury-mercury chloride in KCl saturated solution
  • Measuring electrode responds to the H⁺ in the sample and the difference between reference and measurement cells measured by a voltmeter or pH meter
  • At 37 degrees – change in one pH unit alters voltage by 61.5mV

Blood Gas: Bicarbonate

• Calculated from pH and CO₂ using Henderson-Hasselbach Equation
• Actual bicarbonate
  • Aerobically drawn arterial sample
• Standard bicarbonate
  • Bicarbonate level in an oxygenated plasma specimen at 37 degrees and a pCO₂ of 40mmHg

Sakurai 2016

Answer by Sakurai