Blood gas machine & gas monitoring Flashcards
How is pH measured on a gas machine?
- Using pH probe
- Constructed with 2 electrodes (mercury reference electrode & silver sensing electrode)
- Mercury in contact with blood
- Silver at constant pH with KCl buffer
- Potential difference measured by sensitive glass bulb at silver
How is paO2 measured on a gas machine?
- Clark electrode made of platinum cathode & silver anode
- Form circuit via electrolyte solution
- Powered by battery
- e- formed at anode
- React with O2 + H2O at cathode
- Produces hydroxyl ions & generates current
- Current measurement = paO2
How is paCO2 measured on a gas machine?
- Severinghaus electrode (similar to pH electrode)
- Blood separated from electrode by NaHCO3 semi-permeable membrane
- CO2 diffuses across membrane
- Reacts with H2O on other side creating H+ ions
- Change in pH measured by electrode
- PaCO2 measured due to linear relationship between PCO2 & pH
What is the Henderson Hasselbach equation?
CO2 + H2O = H2CO3 = H+ +HCO3-
How is actual bicarb measured on a gas machine?
- Using measured pH & PaCO2 & Hasselbach equation
Define standard bicarb
The plasma bicarb conc after the sample has been corrected to PaCO2 of 5 at 37 degrees
Why use a standarb bicarb instead of actual?
Removes any resp component of acid/alkalosis
Shows only metabolic component in any derangement
How is Hb measured on a gas machine?
- Using co-oximeter (spectrophotometer)
- Uses 4 wavelengths of electromagnetic radiation to measure
- Works by knowing absorption characteristics of substances
What do the 4 wavelengths in Hb measuring monitor?
- Total Hb
- OxyHb
- CarboxyHb
- Methaemoglobin
How is BE calculated?
- Siggaard- Andersen nomogram
- Number of milimoles of acid required to titrate 1L blood to a pH 7.4 at 37 degrees with PCO2 5
What is standard BE?
BE value calculated for blood with a Hb = 5
How does excess Heparin effect an ABG?
- Heparin is acidic
- Causes low PaCO2, pH, HCO3
How does a delay in analysis effect an ABG?
- Continued metabolic activity of erythrocytes
- Low pH, paO2
- Increased CO2
- HCO3 changes secondary to CO2 changes
How do air bubbles effect an ABG?
- Increase O2
- Decrease CO2 with resulting pH change
- If PaO2 of sample >21 (air) then will cause decrease in measured paO2
How does temperature effect an ABG?
- Due to increasing dissociation of H+ ions
- pH decreases as temp rises
- Solubility of all gases decreases with increasing temp