eLFH - Blood gas machine and Monitoring gas delivery in Anaesthetics

Question 1

Measurements obtained from blood gas

Answer 1

pH
PO2
PCO2
Actual bicarbonate and standard bicarbonate
Electrolytes
Hb
Base excess

Question 2

Values on blood gas that are calculated rather than measured

Answer 2

Actual / Standard bicarbonate
Base excess

The rest are measured

Question 3

pH measurement

Answer 3

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

Question 4

PO2 measurement

Answer 4

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)-

Question 5

Alternate names for Clark electrode

Answer 5

Polarographic electrode

Oxygen electrode

Question 6

PCO2 measurement

Answer 6

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

Question 7

Actual bicarbonate calculation

Answer 7

Calculated using measured pH and PCO2 and Henderson-Hasselbalch equation

Gives true plasma bicarbonate concentration - could be primary or secondary derangement

Question 8

Standard bicarbonate definition

Answer 8

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

Question 9

Electrolytes measurement

Answer 9

Specific ion selective electrodes

Work on similar principle to the pH electrode

Question 10

Haemoglobin measurement

Answer 10

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

Question 11

Base excess calculation

Answer 11

Calculated from Siggaard-Andersen nomogram

Measures non respiratory component of an acidosis / alkalosis

Question 12

Base excess definition

Answer 12

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

Question 13

Standard base excess definition

Answer 13

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

Question 14

Siggaard-Andersen nomogram

Answer 14

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

Question 15

Changes in ABG results with excess heparin

Answer 15

Heparin is acidic

Therefore excess heparin causes:
- Lower pH
- Lower PCO2
- Lower HCO3

Question 16

Changes in ABG results with delay in analysis

Answer 16

Continued metabolic activity of erythrocytes

Causes:
- Lower pH
- Lower O2
- Higher CO2
- HCO3 changes secondary to PCO2 rise

Question 17

Changes in ABG results with air bubbles

Answer 17

Lower CO2
Resultant increase in pH

Causes higher O2 unless PaO2 > 21 kPa

Question 18

Changes in ABG results with temperature

Answer 18

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

Question 19

Methods of measuring O2 in sample

Answer 19

Clark electrode

Fuel cell

Paramagnetic analyser

Mass spectrometry

Gas chromatography

Raman spectrometry

Question 20

Fuel cell

Answer 20

Similar to Clark electrode but doesn’t need a battery

Lead anode
Gold cathode
Electrolyte solution potassium hydroxide

Question 21

Paramagnetic analyser

Answer 21

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

Question 22

Mass spectrometry

Answer 22

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

Question 23

Gas chromatography

Answer 23

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

Question 24

3 main detector types for gas chromatography

Answer 24

Flame ionised detector

Thermal conductivity detector

Electron capture detector

Question 25

Flame ionised detector

Answer 25

Measure current produced by organic vapours ionised in a flame

Question 26

Thermal conductivity detector

Answer 26

Measures changes in the resistance of a heated wire in the gas flow

Suitable for inorganic gases e.g. N2O and O2

Question 27

Electron capture detector

Answer 27

Halogenated compounds reduce electron flow produced by a radioactive cathode

Therefore altering current measured is proportional to their concentration

Question 28

Raman spectrometry

Answer 28

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

Question 29

The Raman effect

Answer 29

Occurs when photons interact with atoms / molecules, changing their rotational, vibrational or electrical energy

This alters the frequency of the photon

Question 30

Infrared analyser uses

Answer 30

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

Question 31

Infrared analyser mechanism

Answer 31

Question 32

Use of ultraviolet analyser

Answer 32

Only can measure Halothane due to absorption characteristics

Same principle as infrared analyser but uses mercury lamp as light source

Question 33

Piezoelectric crystals use for gas monitoring

Answer 33

Can measure gases / vapours that are soluble in oil

Therefore only measures anaesthetic vapours

Question 34

Piezoelectric crystals mechanism for anaesthetic gas measurement

Answer 34

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

Question 35

Summary table of all gases and commonly used, can be used and cannot be used methods to measure their concentrations

Answer 35