- machine for point of care measurement of arterial blood gas indices and electrolytes
- O2 saturation
METHOD OF INSERTION AND/OR USE
- 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
- 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
- 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
- PaO2 ratio of oxyHb to total Hb
- normally 95-98%