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
How does a fuel cell work?
- Same principle as Clark electrode
- Does not require battery
- Anode = lead, cathode = gold
- e- produced at anode & react with O2 at cathode
How does a paramagnetic analyzer work?
- Principle that O2 is attracted towards magnetic field
- Measures pressure differential between stream of reference gas & sample gas when exposed to alternating magnetic field
- Pressure transducer used to convert pressure difference into electrical signal to give paO2
How does mass spectrometry work?
- Molecules of sample gas enter ionization chamber
- Bombarded with e- moving from cathode to anode
- Forms charged fragments
- These accelerated & focused on to detector using rods/magnetic field
- By varying current different fragments can be analysed
How does gas chromatography work?
- Stationary phase: Column of tiny particles coated in silicone oil
- Mobile phase: Carrier gas (He) sample is injected into
- Separates gas sample into constituent components
- Separation dependent on their differential solubility in the 2 phases
- Detector records the Conc of the components
What are the 3 main detector types in gas chromatography?
- Flame ionization: Current produced by organic vapours ionized in a flame
- Thermal conductivity: Changes in resistance of a heated wire in gas flow (useful for N2O/O2)
- Electron capture: Halogenated compounds reduce e- flow produced by radioactive cathode altering current in proportion to conc
How does Raman spectrometry work?
- Pass laser beam through sample
- Process frequency of resultant scattered radiation
How does IR absorption work?
- Gases with 2+ atoms with absorb IR at characteristic frequencies
- ## CO2, H2O, NO, Volaties all absorb IR
Which gases do not absorb IR?
O2
He
N2
What is collision broadening?
Energy absorbed by CO2 from IR radiation transferred to N2O.
Allows CO2 to absorb more IR leading to falsely high reading
What can UV analyzers measure?
Halothane only
Work similar to UV analyzers
How do piezoelectric crystals work?
- Only measure gas/vapour that is soluble in oil
- Vibrate at specific resonant frequency when current applied
- Once vapour dissolved frequency shifts in proportion to conc present