Blood Gas Analyser


  • machine for point of care measurement of arterial blood gas indices and electrolytes


Direct measurements

  • pH
  • PCO2
  • PO2

Derived measurements

  • HCO3
  • BE
  • O2 saturation


pH measurement

  • glass electrode
  • reference electrode
  • specimen put in a capillary tube surrounded by buffer solution
  • the tube is made of pH sensitive glass across which a potential difference is generated, which is proportional to the pH

PCO2 measurement

  • modified glass electrode
  • comprises of a glass pH electrode that is permeable to CO2
  • CO2 diffuses from the specimen into the HCO3 solution where it dissociates with a change in pH which is measured by the electrode
  • potential difference is proportional to CO2 concentration

PO2 measurement

  • Clark electrode or polargraphic electrode
  • O2 molecules diffuse across a plastic membrane to  small platinum or gold 2nm diameter wire cathode in a glass rod immersed in a phosphate buffer with KCl
  • O2 reduced by 2 hydroxyl ions by 4 electrons after application of 600-800mV

O2 + 2H2O + 4e = 4(OH)

  • this creates current flow between the cathode and silver/silver chloride anode.
  • increasing the voltage increases the current up to a plateau determined by the rate of supply of O2 molecules & proportional to the concentration of O2 in solution.



  • derived from transposition of Hendersen-Hasselbalch equation

[HCO3-] = [K [0.03] x PaCO2] / [H+]

where K is the combined CO2 solubility constant and carbonic acid dissociation constant which can vary with pH, temp & ionic strength of solution.

  • calculated actual HCO3- is assumed to by 24mmol/L


  • titratable acid in mmol/L required to titrate blood to a pH of 7.4 at a PCO2 of 40mmHg
  • used to assess what changes in pH are due to PaCO2 changes.
  • ie. at a pH of 7.4 the change in [HCO3-] from normal bicarb = the amount of HCl added.
  • normal +2.5 to -2.5
  • large negative values = metabolic acidosis

O2 saturation

  • PaO2 ratio of oxyHb to total Hb
  • normally 95-98%

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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