Blood Gas Analyser
USES
- machine for point of care measurement of arterial blood gas indices and electrolytes
DESCRIPTION
Direct measurements
- pH
- PCO2
- PO2
Derived measurements
- HCO3–
- BE
- O2 saturation
METHOD OF INSERTION AND/OR USE
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.
OTHER INFORMATION
HCO3-
- 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
BE
- 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%
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
| INTENSIVE | RAGE | Resuscitology | SMACC