O2, Co2 & H+ Measurement

Front 1

Question 11

2 a) List 6 factors that can affect the functioning of the Clark electrode.

Back 1

Clark electrode needs to be frequently calibrated using solutions with known oxygen tension in order to ensure accuracy and precision

Factors affecting accuracy

1. Temperature – the redox potentials are very sensitive to temperature ∴ the oxygen electrode needs to be kept within 0.1 ºC of known temperature

2. Current-voltage relationship – at 0.6 V, the current-voltage relationship is relatively flat and thus less affected by small variations in the applied voltage. Large deviations from 0.6 V current-voltage relationship is very sensitive to small changes to applied voltage ∴ may produce inaccuracy

3. Voltage-oxygen tension relationship – at 0.6 V, the relationship between voltage and O2 tension is relatively linear. Deviation from external applied voltage of 0.6 V non-linearity in above relationship inaccuracies

4. Protein deposition on plastic membrane – affects O2 equilibration

5. Perforation of the plastic membrane - unreliability

6. Delay from sampling to analysis of the blood.

Front 2

Describe how a clarke electrode functions.

Back 2

(1) Blood comes in contact with plastic membrane

(2) Dissolved O2 diffuses across plastic membrane following difference in oxygen tension

(3) Oxygen tension of electrolyte comes to equilibrium with blood sample

(4) O2 undergoes reduction at cathode → forms OH–, while Ag undergoes oxidation at the anode

(5) Redox reaction produces electromotive force measured by galvanometer produces current in circuit

(6) Electromotive force (i.e. redox potential) is proportional to oxygen tension at cathode ∴ measured current derive redox potential blood sample

Front 3

Label a clarke electrode.

Back 3

Front 4

Write down the reaction that occurs at the cathode and explain how this is used to measure the amount of oxygen present.

Back 4

O2 + 2H20 + 2e = H2O2 + 2OH

The e consumption at platinum electrode & e production at the silver anode results inna current flow between the two half cells, directly proportional to the oxygen consumption, which is what you measure

Anode

Front 5

List 2 causes of measurement error in a clarke electrode.

Back 5

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Front 6

With regard to capnography, explain the physical principles of an infrared gas analyser.

Back 6

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Front 7

State Henrys law.

Back 7

The amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas in equilibrium with the liquid.

The quantity of Vapor which dissolves is proportional to the partial pressure of the vapour.

Front 8

Explain how a piezoelectric crystal can be used to measure Anaesthetic gas concentration.

Back 8

The piezoelectric crystal uses 2 main principles , the piezoelectric effect & Henrys Law.

With the piezoelectric effect, when an electric potential is applied across a crystal of quartz, it contracts slightly.

Anaesthetic gases are soluble in oil.

According to Henrys Law, the quantity of vapour which dissolves in a liquid is directly proportional to the partial pressure of the vapour.

The piezoelectric crystal is coated with oil & made to oscillate at it's resonant frequency by a suitable alternating potential.

Anaesthetic gases dissolve in the oily coating & alter the resonant frequency of the crystal.

This change is measured electrically & displayed as Anaesthetic vapour concentration.

Front 9

Explain how interferometry is used to check vaporizer Calibration.

Back 9

1. Two light beams are passed through the refractometer, one through the sample, & the other through a reference chamber.

2. The two beams are added together at the end, they arrive at a point with their wave motions in phase (in which case reinforcement takes place & the increased amplitude gives rise to a bright fringe, or they combine out of phase & the decreased amplitude gives rise to a dark fringe.

3. Gas passing through a sample chamber reduces the velocity of the light, & alters the phase relationships when the two lights are recombined

4. Displacement of the fringe pattern occurs, observable through the eye piece

5. The displacement of the selected is proportional to the concentration of gas added to the sample chamber.

Front 10

Explain the function of a main-stream Capnograph.

Back 10

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Front 11

State Beer's Law.

Back 11

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Front 12

List 3 factors that may reduce the accuracy of a measurement done by using an oxygen electrode.

Back 12

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Front 13

List the direct methods of CO2 analysis

Back 13

1. Severinghaus electrode

2. Cutaneous electrode

3. Intravascular electrode

Front 14

List the Indirect methods of CO2 analysis

Back 14

Calorimetric devices

Capnograph

Capnometer

Front 15

List 5 errors of infrared analyzers.

Back 15

1. Water vapour absorbs infrared at several wavelengths - > falsely increased readings of CO2, N20 & Anaesthesia Agents concentrations.

2. Oxygen broadens CO2 Absorption spectra - reduced readings.

3. Collision broadening effect- falsely increased readings of CO2.

4. CO2 & N20 have similar absorption spectra. CO2 = 4.2 & 4.4. N20 = 4.4 & 4.6. it requires narrow bandwidth to filter out n2o.

5. Alcohols elevate volatile agent readings.

Front 16

Briefly describe how an infra-red analyser determines carbon dioxide concentration.

Back 16

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Front 17

What are the advantages of using a mainstream infrared analyser?

Back 17

1. Faster response time: transit time is nil.

2. Avoids sample line & its problems

3. Portable.

Front 18

Why must the readings be compensated for the presence of nitrous oxide.

Back 18

This can be explained by the Collision broadening effect. Infrared absorbed by CO2 is emitted as a photon when CO2 collides with N2O. This enables CO2 to absorb more infrared - falsely increasing CO2 concentration reading. This also broadens the absorption spectrum of nitrous.

Front 19

What are the disadvantages of using a mainstream infrared analyser?

Back 19

1. It increases dead space

2. It's bulky & has an increased weight.

3. Used only in ETT or LMA

4. Risk of facial burns

5. May become clogged with secretions.

Front 20

Why does CO2 absorb at infrared wavelengths?

Back 20

It's molecule contains two dissimilar atoms.

Front 21

What are the advantages of a sidestream infrared analyser?

Back 21

1. Light weight

2. Can be used in uncommon positions e. G. Prone

3. Does not require sterilization

4. Can be used with non-intubated patients.

Front 22

What are the disadvantages of of using a sidestream infrared analyser?

Back 22

1. Delayed response time

2. Sample tube obstruction / kinking

3. Loss of anaesthetic gases (if sample scavenged)

4. Needs water trap / filter.

5. Cause bernoulli effect leading to pressure drop, alters partial pressure of sampled gas

6. Dispersion of gas within tube may distort the waveform.

Front 23

Describe how the clarke electrode measures oxygen tension.

Back 23

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Front 24

Briefly describe how a paramagnetic oxygen analyser functions. (5)

Back 24

1. Oxygen is a paramagnetic gas, & is attracted to a strong magnetic field (electrons in outer shell are unpaired)

2. Two nitrogen filled glass spheres are mounted on a rotating suspension placed in a non-uniform magnetic field

3. When O2 is introduced to the cell; the O2 is attracted to the magnetic field & displaces the glass spheres containing nitrogen.

4. The dumbell rotates until the force of this displacement is balanced by tension of the filament

5. Deflection of the dumbell suspension can be measured by attaching a mirror to the dumbell suspension, so that the position of a light beam reflected from this mirror provides an indication on a scale.

6. The scale can be calibrated to show percentage of oxygen present in the measuring cell.

Front 25

Back 25

A. Nitrogen sphere

B. Light source

C. Photocells

D. Amplifier

E. Indicating unit

Front 26

Briefly explain the principles of anaesthetic gas analysis using infrared spectroscopy. (4)

Back 26

1. An infrared analyser uses a hot wire to transmit infrared waves through a sample gas & a photodetector to measure the light that has passsed through the sample chamber.

2. The light emmitted is filtered to a monochromatic light of a specific wavelength which is characteristically absorbed by a specific gas. Example CO2 absorbes wavelength 4.28mcm.

3. The infrared light passes through a transparent window e.g. Sapphire window into the sample chamber.

4. According to Beers law, the absorbance of the infrared light is directly proportional to the concentration of the medium (Anaesthetic gas).

5. The photodetector then measures the light passed through the chamber, compares it to the emitted light and uses this to calculate the concentration of the Anaesthetic gas. The less light present at the detector, the more absorbance occured & the greater the concentration of the gas.

6. A second beam from the same source may be passed through a reference cell to a reference detector in order to avoid Variations (in the components of the analyser) being interpreted as changes in the gas. This is called the Double beam instrument.

Front 27

Which type of gases can be measured using infrared spectroscopy? Give an example of such a gas.

Back 27

Gases with molecules that have different atoms. For example N2O & carbon dioxide.

NB: Oxygen and nitrogen are not measured because they do not contain dissimilar atoms

Front 28

Define “collision broadening” with the aid of a simple diagram/graph. (3)

Back 28

Front 29

How is collision broadening compensated for? (1)

Back 29

Modern analysers can compensate by measuring the concentrations of interfering agents.

Front 30

Describe how a pH electrode functions.

Back 30

1. The pH electrode measures pH.

2. It consists of a silver measuring electrode which is coated with silver chloride and enclosed in a buffer solution of HCl which maintains the pH at 7.

3. The electrode tip is composed of specialised H+ ion-sensitive glass. When placed in a solution the glass softens, forming a semi-solid gel membrane.

4. Hydrogen ions diffuse into the outer layer of the glass, displacing ions such as lithium and sodium toward the inner layers. This creates a positively charged glass surface, which attracts anions (Cl−) from the buffer solution.

5. An ion gradient is established within the glass electrode; this is dependent on the H+ concentration in the sample.

5. Thus a potential difference is created. The potential difference is measured against a reference electrode that is connected by a lead wire and placed into the same sample being measured.

6. The silver/silver chloride reference electrode is contained in a saturated 3 mol l−1 KCl solution, which gives it a fixed potential. It is encased in a non-permeable shell with a diaphragm at its tip, allowing continuity with the sample. Calomel electrodes were used previously, but are now avoided due to the toxicity of mercury.

Front 31

List the sources of errors of a pH electrode 5.

Back 31

1. Temperature affects the dissociation of H+ ions from bicarbonate. Therefore, the apparatus must be kept at 37°C.

• 2. Blockage of ion-sensitive channels or reference electrode diaphragm by precipitated silver chloride prevents functioning. Regular cleaning and maintenance are recommended.

3. • Damage or leakage from the electrode into the sample negates potentials developed, requiring electrode replacement.

4 • Ion selectivity of a membrane is not 100% specific. The presence of other ions in high concentrations can influence the results. Standard addition can be used to overcome this, whereby defined volumes of the ion to be measured are added to the sample in steps, and the effect measured. The initial concentration can then be calculated by extrapolation.

5• Drift affects the precision and accuracy over time, and can be prevented by regular 2-point calibration with buffer solutions of known pH.

Front 32

What is standard bicarbonate?

Back 32

Standard bicarbonate is the concentration of plasma bicarbonate under standardised conditions: temperature 37°C, pCO2 of 5.3 kPa and fully saturated with oxygen. By keeping pCO2 constant, the respiratory component is eliminated, resulting in a figure that reflects the metabolic contribution of an acid/base disturbance.

Front 33

Define base excess.

What is a normal BE?

Why is it valuable to measure BE?

Back 33

Base excess (BE) is the amount of strong acid needed to return a litre of fully oxygenated blood at 37°C and pCO2 of 5.3 kPa to pH 7.40.

A normal value is between −2 and +2 milliequivalents per litre (mEq·l−1).

Base excess is a useful measure, because it accounts for all acids, including those that do not predictably affect standard bicarbonate concentrations.

Front 34

Define Standard Base Excess.

Back 34

Standard base excess is base excess corrected for an Hb of 5 g·dl−1. This removes the buffering effect of blood, better reflecting the acid/base status of plasma and extracellular fluid.