Arterial Blood Gas Measurement

How does an arterial blood gas machine measure pH in the blood?

  • pH electrode
  • Reading electrode made of silver/silver chloride
    • Immersed in buffer solution to maintain constant H+ concentration
    • Separated from blood sample by ion-sensitive glass
      • The difference in H+ concentration between blood sample and buffer solution generates an electromotive potential across the glass
        • This electrical signal is amplified and calibrated to give a pH value
  • Reference electrode is silver/silver chloride in potassium chloride
    • This is connected via a semipermeable membrane to complete the circuit
  • System is kept at 37°C

How does an ABG machine measure CO2?

  • Severinghous electrode
    • This is a modified pH electrode
  • CO2 diffuses from the blood sample across a CO2-permeable membrane into a bicarbonate solution
  • This causes it to dissociate to form H+ ions
  • The rest of the electrode then behaves in the same way as a pH electrode, but calibrates the change in pH to the PaCO2
  • Because of the extra time needed to allow the CO2 to diffuse and equalise in solution, the Severinghaus electrode is relatively slow
  • Needs calibrating and regular maintenance
  • Gas solubility in water increases as temperature decreases
    • Therefore hypothermia will cause a lower PaCO2, and therefore increased pH

How does an ABG machine measure oxygen tension?

How does an anaesthetic machine measure oxygen tension in a gas sample?

Paramagnetic analyser

  • This method has four main advantages and three main disadvantages
    • Advantages
      • Fast
      • Accurate
      • Sensitive
      • Works continuously
    • Disadvantages
      • Needs a power supply
      • Can suffer from interference such as from water vapour, so gas needs to be passed through a drying agent such as silica gel
      • Needs calibration

Fuel Cell

  • Also known as a galvanic oxygen analyser
  • It has a lead anode and a gold mesh cathode in an electrolyte solution with potassium hydroxide surrounded by an oxygen permeable membrane
    • Advantages
      • Cheap
      • Compact
      • No power supply required as the apparatus itself generates a voltage
      • Unaffected by water vapour
    • Disadvantages
      • Lead anode is gradually consumed and needs replacing
      • Needs regular servicing
      • N2O can affect accuracy and damage the electrodes
      • Slow (30 seconds)

Haldane Apparatus

  • Haldane apparatus
    • By measuring the volume of a sample of gas before and after the removal of an anaesthetic volatile agent, it can work out the concentration of that agent
      • Uses potassium pyrogallate absorbant to absorb oxygen
      • Potassium hydroxide used to absorb CO2
      • Not really used much any more