ABG = arterial blood gas
- pH, PaO2 & PaCO2 are all directly measured
- HCO3-, base excess, SaO2 are derived
PaO2 = partial pressure (tension) of O2 in arterial blood
Oxygen Tension Methods
(1) oxygen (Clarke’s) electrode: amount of O2 producing a voltage
(2) transcutaneous electrodes
(3) fluorescence-based blood gas analysis: filtration of xenon light being proportional to O2 tension
(4) ion selective or pH electrode
O2 Content and Capacity Methods
(2) blood haemolysis
(3) galvanic cell
(4) calorimetric method
- CO2 diffuses through a selectively permeable membrane of silicone rubber or Teflon into an aqueous electrolyte (NaHCO3).
- this causes a change in the H+ ion concentration -> measured by pH sensitive glass electrode.
- output voltage is logarithmically related to PCO2 in blood.
- negative log of H+ activity
- measured by pH sensitive glass with oxides of silicone, lithium & calcium, porous only to H+.
- the glass has its metal cations displaced by the H+ ions in the test solution.
-> development of an EMF and can be calculated by a modification of the Nernst equation.
-> log [H+] in the sample of blood when connected to a reference electrode of mercury beads suspended in mercurous chloride (HgCl2) and surrounded by a saturated KCl solution.
-> this potential is compared with the potential developed using a standard solution of selected pH value.
- derived from the Henderson Hasselbalch equation
Base excess (BE)
- = amount of titratable acid in mmol/L needed to titrate one litre of blood to a pH of 7.4.
- with Hb of 150g/L
- PCO2 of 40mmHg
- @ 37C
-> represents the non-respiratory component of a pH disturbance.
Standard Base Excess (SBE)
- same as above but corrected for haemoglobin as Hb is a buffer of acid (more accurately reflects the BE in the ECF)
- ratio of oxyHb to total Hb
- normal 95 to 98%
- a laboratory test involving a blood sample heated to 37 C and subjected to light of various length and assesses absorption spectra.
- does not require pulsatile flow
- measures MetHb, COHb and other forms of Hb
- uses many other wavelengths
- measures either venous, arterial or capillary oxygenation
- contamination of line with flush solution
- extreme leukocytosis -> pseudohypoxaemia (from excessive in vitro O2 consumption)
- ice storage in polypropylene syringes (rather than glass) -> artefactual PaCO2 elevation
- inter-analyser variability
- inadequate heparinization
- non-linearity of Clark electrode when PaO2 > 150mmHg
- lack of appropriate electrolyte temperature
- interference of NO and halothane
- poor quality control
- ABG tensions fluctuate constantly even in stable patients