Blood gases + monitoring Flashcards
How is ph measured
ph electrode
potential difference formed across ph senstive glass proportional to the difference in acid conc either side.
Mercury electrode ref. in contact with bloods
Silver electrode kept at constant pH with KCL buffer _ then encasesd in ph sensitive glass
How is pO2 measured
clark electrode
PLatinum catode and silver anode.
Powered by a polarising 0.6v battery
CUrrent generated from electrodes at anode reacting with O2
How is pCO2 measured
Severinghaus electrode
linear relationship between log pCO2 and pH
How is bicarb calculated
using measured ph and pco2
Actual is from the henderson haselback equation
Standard is the bicarb corrected to pCO2 of 5.3 at 37degs. removes any resp component
How is Hb measured
Cooximeter. Spectrophotometer with 4 wavelengths
Photocell is used to measure the amount of light received compared to reference quantitiy
What is measured as part of Hb
Total
Oxy
Carboxy
Methemoglobin
ALso sats
How is BE calculated
using a siggard andersen normoram.
Definition BE
Number of mmoles acide required to titrate 1L of blood to pH 7.4 at 37 degs with co2 of 5.3
What is standard BE
BE calculated for a hb of 5. thought to be better reflection of whole body
WHat errors does heparin introduce
acidic, so spuriously low pCO2 and bicarb with a low pH
What error does delay in analysis introduce
continued metabolic activity of rbc causes a decrease in pH, O2 and increased CO2. HCO3 consequently changes
What errors do air bubbles introduce?
Normallly increased O2 and decreased pCO2 with an increase in pH
If the paO2 of the sample is greater than 21, then the bubbles will cause a reduction
What errors will temperature cause with measurement
Increasing dissociation of H+ ions with rise in temp- pH falls 0.015 units per degree rise.
Solubility of all gases reduce so falselt elevated o2 + co2
