Components of ABG Flashcards
pH electrode components
potential difference develops across a pH sensitive glass bulb that is caused by, and is proportional to the difference in acid concentration on either side of it.
Two electrodes:
1. Reference electrode - mercury/mercuric chloride, in contact with the blood.
- Sensing electrode - silver/ silver chloride. kept at constant pH with KCL buffer.
pO2 electrode
Clark electrode/ polarographic electrode
platinum cathode
silver/ silver chloride anode
Circuit via KCL solution. O.6V battery. Current generated measured.
pCO2 electrode
Severinghaus electrode
- modifiied pH electrode
- linear relationship between log pCO2 and pH.
pH sensitive glass (which has a thin sodium bicarbonate impregnated nylon mesh fixed to it with an O ring) by a rubber/teflon membrane.
- CO2 diffuses across
- reacts with H2O as in HH equation
- change in pH gives CO2 measurement
How is standard bicarbonate derived?
Siggaard-Anderson nomogram. Plasma bicarb after sample corrected to pCO2 5.3 at 37 degrees.
Hb measurement
Haemoglobin is measured using a co-oximeter.
A co-oximeter is a spectrophotometer that uses four different wavelengths of electromagnetic radiation to measure total haemoglobin, oxyhaemoglobin, carboxyhaemoglobin and methaemoglobin and also gives the oxyhaemoglobin saturation.
How is base excess calculated?
It is calculated from the Siggaard-Andersen nomogram (see next page for more detail). It is the number of milimoles of acid required to titrate one litre of blood to a pH 7.4, at a temperature of 37oC with a PCO2 of 5.3 kPa.
What can give you errors in ABG reading?
Heparin
Air
Temperature
Delay in analysis
Effect of Heparin on ABG reading?
Heparin is acidic, therefore too much heparin causes:
a spuriously low PCO2 and bicarbonate,
with low pH.
Effect of delayed sample in ABG?
The continued metabolic activity of the erythrocytes causes a decrease in pH, and O2, and an increased CO2. The HCO3 changes are secondary to the changes in PCO2.
Effect of temperature on ABG reading?
pH is a strong function of temperature, falling 0.015 units per oC rise in temperature. This is due to increasing dissociation of H+ ions. Therefore maintaining the electrodes at 37oC is very important.
The solubility of all gases decreases with increasing temperature therefore a hypothermic patient whose blood is analyzed at 37oC will have a falsely elevated PO2 and PCO2 and a falsely low pH
Again, the HCO3 changes are secondary to the changes in PCO2.
PaO2 measured by a gas analyzer would be higher than patient’s PaO2 at 32oC?
True. The solubility of all gases decreases with increasing temperature therefore a hypothermic patient whose blood is analysed at 37oC will have a falsely elevated PO2 and PCO2.
