eLFH - Blood gas machine and Monitoring gas delivery in Anaesthetics Flashcards
Measurements obtained from blood gas
pH
PO2
PCO2
Actual bicarbonate and standard bicarbonate
Electrolytes
Hb
Base excess
Values on blood gas that are calculated rather than measured
Actual / Standard bicarbonate
Base excess
The rest are measured
pH measurement
Potential difference develops across pH sensitive glass bulb - caused by and proportional to the difference in acid concentration on either side of it
Mercury chloride reference electrode
Silver chloride sensing electrode - kept at constant pH with KCl buffer and encased in a pH sensitive glass bulb
PO2 measurement
Clark electrode - platinum cathode + silver chloride anode which form a circuit via an electrolyte solution (usually KCl)
Powered by 0.6 V battery
Electrons formed at anode from reaction with KCl
Electrons react with O2and water at the cathode to produce hydroxyl ions - produces current
Current measured and used to calculate O2 concentration at the cathode
O2 + 4e + 2H2O -> 4(OH)-
Alternate names for Clark electrode
Polarographic electrode
Oxygen electrode
PCO2 measurement
Severinghaus electrode - uses linear relationship between log PCO2 and pH to measure PCO2
Essentially modified pH electrode - measures pH change in electrolyte solution when CO2 diffuses into it
Actual bicarbonate calculation
Calculated using measured pH and PCO2 and Henderson-Hasselbalch equation
Gives true plasma bicarbonate concentration - could be primary or secondary derangement
Standard bicarbonate definition
Plasma bicarbonate concentration after the sample has been corrected to a PCO2 of 5.3 kPa at 37 degrees Celsius
Removes any respiratory component - shows only metabolic component of any derangement
Derived from Siggaard-Andersen nomogram
Electrolytes measurement
Specific ion selective electrodes
Work on similar principle to the pH electrode
Haemoglobin measurement
Measured using co-oximeter - spectrophotometer that uses 4 wavelengths of EM radiation to measure total Hb, oxyHb, carboxyHb and metHb
Also gives oxyhaemoglobin saturation
Works on principle that radiation is absorbed by substances with 2 or more atoms
Photocell measures transmitted radiation and compares to reference photocell as absorption qualities are known
Base excess calculation
Calculated from Siggaard-Andersen nomogram
Measures non respiratory component of an acidosis / alkalosis
Base excess definition
Number of millimoles of acid required to titrate 1 L of blood to a pH 7.4 at 37 degrees Celsius with PCO2 of 5.3 kPa
Standard base excess definition
Base excess value calculated for blood with Hb = 5 g/dL
Hb is effective plasma buffer so standardising to anaemic Hb concentration takes this into account
Siggaard-Andersen nomogram
PCO2 titration line plotted by measuring pH after blood sample has equilibrated with 2 gas mixtures containing different PCO2
Gradient of line determined by buffering capacity of blood - therefore related to Hb concentration
Changes in ABG results with excess heparin
Heparin is acidic
Therefore excess heparin causes:
- Lower pH
- Lower PCO2
- Lower HCO3
Changes in ABG results with delay in analysis
Continued metabolic activity of erythrocytes
Causes:
- Lower pH
- Lower O2
- Higher CO2
- HCO3 changes secondary to PCO2 rise
Changes in ABG results with air bubbles
Lower CO2
Resultant increase in pH
Causes higher O2 unless PaO2 > 21 kPa
Changes in ABG results with temperature
pH falls by 0.015 per degree Celsius rise in temperature - due to increasing H+ ion dissociation
Solubility of all gases decreases with increasing temperature - therefore hypothermic patient ABG measured at 37 degrees will have falsely higher O2 and CO2, and lower pH
Methods of measuring O2 in sample
Clark electrode
Fuel cell
Paramagnetic analyser
Mass spectrometry
Gas chromatography
Raman spectrometry
Fuel cell
Similar to Clark electrode but doesn’t need a battery
Lead anode
Gold cathode
Electrolyte solution potassium hydroxide
Paramagnetic analyser
Works on principle that O2 is attracted towards a magnetic field due to unpaired electrons in outer shell
Measures pressure differential between a stream of reference gas and the sample gas
Mass spectrometry
Can measure any gas in a sample
Sample gas enters ionisation chamber - bombarded by electrons moving from hot cathode to an anode
Forms charged fragments of the gas molecules of varying molecular weight
Fragments are accelerated onto detector by magnetic field or electrically charged rods
Amount of deflection depends on their mass
Concentration of that gas can be measured from output of the detector
Gas chromatography
Can measure all gases if appropriate detector used
Stationary phase and a mobile phase into which sample is injected - separates gas sample into constituent components
Separation is dependent on their differential solubility in the 2 phases
Once separated, detector records the concentration of the components
3 main detector types
3 main detector types for gas chromatography
Flame ionised detector
Thermal conductivity detector
Electron capture detector
Flame ionised detector
Measure current produced by organic vapours ionised in a flame
Thermal conductivity detector
Measures changes in the resistance of a heated wire in the gas flow
Suitable for inorganic gases e.g. N2O and O2
Electron capture detector
Halogenated compounds reduce electron flow produced by a radioactive cathode
Therefore altering current measured is proportional to their concentration
Raman spectrometry
Can measure all gases / vapours
Characteristic alteration in frequency and phase of scattered radiation as it passes through a specific transparent medium
Passing laser through sample and processing frequency of resultant scattered radiation allows concentrations of sample’s components to be estimated from amplitude of shifted peaks
The Raman effect
Occurs when photons interact with atoms / molecules, changing their rotational, vibrational or electrical energy
This alters the frequency of the photon
Infrared analyser uses
Relies in gases with 2 or more different atoms in their molecules absorb IR radiation at characteristic frequencies
CO2, H2O and volatile agents absorb IR radiation
O2 and N2 molecules don’t
Infrared analyser mechanism
Use of ultraviolet analyser
Only can measure Halothane due to absorption characteristics
Same principle as infrared analyser but uses mercury lamp as light source
Piezoelectric crystals use for gas monitoring
Can measure gases / vapours that are soluble in oil
Therefore only measures anaesthetic vapours
Piezoelectric crystals mechanism for anaesthetic gas measurement
Vibrate at specific resonant frequency when current applied
Coat them with oil, then anaesthetic vapour in gas sample dissolves into it, shifting resonant frequency in proportion to concentration of vapour present
Summary table of all gases and commonly used, can be used and cannot be used methods to measure their concentrations
