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Physics for Anaesthetists > Oxygen measurement > Flashcards

Oxygen measurement Flashcards

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1
Q

When does cyanosis occur

A

When the concentration of deoxyhaemoglobin is > ± 5 g/dL

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2
Q

Why is cyanosis a poor indicator of arterial oxygenation

A
  1. Subjective - Interobserver variation
  2. Ambient light
  3. Anaemia (reduces conc. of all types Hb)
  4. Low CO orcapillary stasis - isolated cyanosis unrelated to PaO2
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3
Q

What are spectrophotometric techniques

A

Passing radiation, of specific wavelength relevant to measured variable, through a sample and determining the quantity of radiation absorbed.

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4
Q

What is Beer’s law?

A

Beers law states that the absorption of radiation by a given thickness of solution of given concentration is the same as that of twice the thickness of the solution at half the concentration.

Think Beers –> glass of beer versus no beer in the glass

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5
Q

What is Bouguer’s or Lamberts law

A

This law states that each layer of equal thickness absorbs an equal fraction of radiation passing through it

Think Lamberts LENGTH

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6
Q

How does absorption of radiation change with concentration of the sample of interest

A

The absorption of radiation by the compound of interest increases in proportion to the concentration of the compound of interest.

The higher the concentration, the higher the absorption

At sufficiently low concentrations, the absorption is proportional to the concentration.

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7
Q

How does an oximeter work?

A

Light of various wavelengths is transmitted through a haemolysed blood sample and a photocell detects the light absorbances so that the O2 saturation can be calculated.

The light absorbed by blood depends on the quantities of oxyhaemoglobin and deoxyHb present as well as the wavelength.

Red light 660nm
- Absorbance less by oxyHb vs. deoxyHb

Infrared light 940mm
- Absorbance less by deoxyHb vs oxyHb

Draw graph with 2 isobestic points

Comparison of the absorbances at these wavelengths enables an oximeter to measure SaO2

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8
Q

What is an isobestic points

A

Points on the Wavelength (nm) - Absorbance graph where the absorbances for the two forms of Hb are identical. Absorbance at any of these points depends only on the Hb concentration.

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9
Q

How does an oximeter correct for the presence of carboxyHb in smokers and city dwellers

A

Careful selection of wavelengths used in analysis and by electronic computation.

CarboxyHb and MetHb can also be measured by using a series of light filters or diodes for differing frequencies of light

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10
Q

What is the principle used in a pulse oximeter

A

The spectrophometric technique as used in the oximeter just without the haemolysed blood sample.

Radiation from a red (660 nm) and infrared (940nm) LED passes through finger and the radiation transmitted is detected by a photocell.

The output of the sensor is is processed electronically to give a pulse waveform and the SaO2.

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11
Q

How do pulse oximeter’s compensate for penetration of ambient light onto the photocell

A

Red (660nm) on then off. Infrared (940nm) on then off. Then both off. The pause when both lights are off allows the photocell and microprocessor to detect and compensate for any ambient light which may have penetrated around the sensor cover.

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12
Q

How does the pulse oximeter detect arterial rather than venous blood or tissue oxygenation?

A

The diodes are switched on and off at high frequency, several hundred times a second. Hence the device can detect cyclical changes in the signal caused by pulsations during the cardiac cycle. A microprocessor is programmed to analyse these changes of pulsation during arterial pulsatile flow and to ignore nonpulsatile component. Therefore, Arterial O2 can be measured.

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13
Q

Do pulse oximeters compensate for the presence of abnormal Hbs (CarboxyHb and MetHb)? Explain why

A

No, They use only two diodes which do not compensate full for COHb and other abnormal Hb

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14
Q

What can cause an abnormal pulse oximeter reading

A
  1. Excess ambient light
  2. Poor contact
  3. Abnormal Hbs (COHb and MetHb)
  4. Vasoconstriction
  5. Low peripheral skin temperature
  6. fingernail / skin staining
  7. Excessive movement –> movement artefact
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15
Q

Does pulse oximetry measure venous O2?

A

Yes. It measures both arterial and venous but then just excludes the lower value from display

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16
Q

What is NIRS monitoring. How does it work?

A

Cerebral Oximeters use NIRS (Near Infrared Spectroscopy) to obtain continuous non-invasive measurements of cerebral oxygenation values.

The skull is transparent to light in the near-infrared range. LEDs emit near infra-red light through the skull into the brain tissue. This light is either absorbed, reflected, scattered or redirected. When infrared light contacts Hb, a change in the wavelength occurs dependent on the oxygenation status of the Hb molecule. Reflected light is detected by photo detectors on the NIRS apparatus.

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17
Q

What are the isobestic points used for by the pulse oximeter

A

Used to calculate total Hb concentration

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18
Q

Describe the components and function of an oximeter

A

Light source –> monochromatic filter –> Beam splitter –> reference detector (reads fraction of deflected light by beam splitter) –> through cuvette (with sample and connected to pump and haemolyser) –> radiation absorbed –> remaining radiation received by detector –> electrical signal transduced by photocell and sent to microprocessor –> comparison of output of two detectors (reference and photocell) to establish how much radiation was absorbed at a specific wavelength.

19
Q

How is error introduced by traces of COHb in smokers and city dwellers reduced?

A
  1. Careful selection of specific light wavelengyhs
  2. Electronic computation

Can get filters specific for COHb and measure it in more modern oximeters

20
Q

How does a pulse oximeter work?

A

Radiation from red (660nm) and infrared (940nm) LED passes through the finger and the photocell detects the transmitted radiation. The output of the sensor is processed electronically to give a pulse waveform and the arterial O2 saturation.

21
Q

How do pulse oximeters detect arterial rather than venous O2 saturation

A

The microprocessor is programmed to only analyse the light absorption changes during arterial pulsatile flow and to ignore the non-pulsatile component of the signal in tissues and venous blood

22
Q

What is the oxygen content equation?

A

CaO2 = 1.34 x Hb x SaO2%/100 + [PaO2 x 0.023]

23
Q

How can O2 content of blood be measured more accurately than can be calculated by the CaO2 equation

A

O2 measured in blood sample directly:

O2 flushed out of a haemolysed blood sample by means of an O2 free carrier gas and the total O2 content in the gas measured with a fuel cell

24
Q

What is a Clarke electrode, what is an alternative name for this and how does it work

A

The Clark electrode or polarographic electrode is the apparatus incorporated into a blood gas machine to measure the oxygen tension in blood.

A platinum cathode and a Ag/AgCl anode submerged in a KCl solution

  1. A voltage of 0.6 V is applied between the electrodes and the current is measured.
  2. Anode: Ag reacts with Cl (from KCl) –> AgCl + e-
  3. Cathode: 4e- from the anode combine with O2 and 2H2O to produce 4OH-

THE MORE O2 around, the more e- can be taken up at the cathode and hence the greater the current that flows. so the current flow through the cathode is dependent on the O2 tension at the platinum cathode.

25
Q

How is the reaction occuring at the platinum cathode in a Clark (polarographic) electrode analgous to that occurring in cell mitochondria

A

Motochondria
–> Electrons produced in the KREBS cycle flow down the cytochrome enzyme chain until reaching ATP synthetase. Here O2 combines with electrons and water to form OH- ions. ATP synthetase uses these electrons to synthesze ATP from ADP and P storing energy in this bond.

Platinum cathode (in Clark electrode)
--> electrons travel through the cathode to combine with O2 and H2O to form OH - ions

O2 + 4e- + H2O –> 4 OH-

26
Q

Can the platinum cathode of a Clark electrode be inserted into the blood and why?

A

No. protein deposits will affect the cathodes function. Instead there is a plastic membrane which divides the electrolyte bath from the blood sample. This allows O2 to equilibrate with the electrolyte solution but prevents protein build up and distortion of function of the cathode and anode.

27
Q

How is accuracy using the Clark (polarographic) electrode optimized?

A
  1. Regular maintenance –> Keep it scrupulously clean and free from protein deposits
  2. Temperature –> 37 deg C (most accurate)
    (ice for transfer reduce accuracy)
  3. Time sensitive (esp. at temp 37 deg C): cells within blood metabolise O2
  4. Anaerobic sampling with heparin to prevent clotting and ambient gas contamination
28
Q

Can the Clark (polarographic) electrode be used to measure O2 concentration in gas mixtures. If so, how and what are some of the problems with this. how are these problems resolved

A

YES. Electrolyte is in the form of a gel containing the cathode and anode. O2 in the gas mixture equalizes with this electrolyte gel.

The membrane in this system as well as the gel deteriorate fairly rapidly and hence need to be replaced regularly

Also, halothane may cause false high readings as it is reduced by the polarizing voltage 0.6V. An electrode membrane not fully permeable to halothane may reduce this problem

29
Q

What are the techniques used to measure the partial pressure of oxygen in gas mixtures

A
  1. The Fuel Cell
  2. The Clark (Polarographic) Electrode
  3. Paramagnetic analyser
30
Q

What are the differences and similarities between the Clark (Polarographic electrode) and the Fuel Cell

A

Fuel cell Clark electrode

  • Cathode gold mesh Platinum
  • Cathode equation O2 + 4e- + H2O –> 4 OH- (same
  • Anode Lead (Pb) in KOH Ag/AgCl in KCl
  • Anode equation Pb + 2OH- –> PbO2 + H2O + 2e-

So OVERALL

  1. Fuel cell gold cathode with lead anode
  2. Fuel cell no need for potential as creates own (it is a battery or ‘cell’)
  3. Clark electrode platinum cathode and silver anode
  4. Clark electrode requires applied potential
31
Q

How long does a Fuel cell last

A

It depends on the period during which it is exposed to O2. Several months.

32
Q

What is a Fuel cell and how does it measure O2 concentration in a gas mixture

A

A Fuel Cell is an apparatus used to measure the concentration of O2 in a gas mixture. A lead (Pb) anode is submerged in a KOH electrolyte solution to undergo the following reaction: Pb + 2 OH- –> PBO+ H2O + 2e-.Unlike the Clark electrode, no battery is needed as the cell produces a voltage. Oxygen in the gas mixture equilibrates through a plastic membrane and layered gold (Au) mush with the KOH electrolyte solution. In the presence of O2 the following equation takes place: O2 + 4e- + 2H2O –> 4 OH-, This induces a current in the system which is displayed and recorded by an ammeter. this current is proportional to the amount of O2 present . The Fuel cell is connected to a small indicator unit to display the O2 concentration.

33
Q

What are the pros and cons of Fuel cell vs. Clark Electrode

A

Fuel cell

  1. No battery required
  2. Less maintenance
  3. Guaranteed lifespan

But slow response time (30s)

Clark Electrode

  1. Fast response time
  2. High maintenance
  3. Shorter life span
34
Q

Define paramagnetic
Define ferromagnetic
Define diamagnetic

A

Diamagnetic - material not attracted to external magnetic field. Weakly repelled by external magnetic filed

Paramagnetic - material weakly attracted to external magnetic filed

Ferromagnetic - material strongly attracted to external magnetic field

35
Q

What is a Paramagnetic analyser and how does it work

A

Oxygen is paramagnetic and is therefore attracted into a magnetic field

Most other gases (e.g. nitrogen) are diamagnetic and are therefore repelled from a magnetic field.

  1. Chamber with inlet and outlet
  2. Two glass spheres filled with nitrogen connected in the middle to form the shape of a dumbell placed in a nonuniform magnetic field
  3. A mirror in the middle of the dumbell shaped spheres
  4. The dumbell spheres attached to a filament
  5. Light source
  6. Calibrated scale for reflection of light or photocells connected to a circuit around the dumbells that generates an opposite magnetic force correcting the displacement of the dumbells and causing. The magnitude of this current required to replace the dumbell shaped spheres to a neutral position is proportional to the O2 concentration within the sample chamber

The paramagnetic property of O2 moves in a particular path through the chamber causing displacement of the dumbell sphere apparatus.

36
Q

Why is O2 paramagnetic

A

It has 2 unpaired electrons in its outer shell

37
Q

What are the advantages of paramagnetic oxygen anaylsers

A

Pros

  1. Very fast
  2. Very accurate
  3. Minimal maintenance
  4. Minimal interference from other gases

Cons
1. Lots of moving parts and susceptible to vibration / shocks

38
Q

How does a pulsed field paramagnetic oxygen analyser work and what is its advantage

A

Two streams of gas : Sample and Reference gas are separated by a pressure transducer but connected downstream and influenced by a electromagnet which forms a magnetic field around both streams. the magnetic filed is pulsed. when the magnetic filed is on O2 is moved into the magnetic field from the sample line containing more O2. This alters the relative pressure and the magnitude of this pressure change is measured and this is propotional to the concentration of O2 in the sample gas mixture.

39
Q

WHat is the advantage of a pulse-field paramagnetic oxygen analyzer

A

There are no moving parts. More resistant to shock and vibration.

40
Q

How are the inaccuracies of analysis of gases containing water vapour avoided

A
  1. Special tubing which removes water vapour before the sample reaches the sample cell
  2. Calculation accounting for SVP H2O
  3. Pass gases through drying agent before analysis (silica gel)
41
Q

What is the problem with continous in vivo O2 measurement. What are the problems with the devices designed to monitor continuous arterial PaO2

A

Calibration remains the problem with continuous recording systems.

42
Q

How does a transcutaneous O2 electrode work

A

Local skin heating 43 - 44 deg C –> adequate vasodilation so that O2 used in the skin is negligible compared to arterial O2 tension.

O2 diffuses through the skin –> electrode placed on the skin surface can measure the arterial O2 tension.

The complete system forms a polarographic electrode with a heater and thermostat.

43
Q

What are the limitations of a transcutaneous O2 electrode

A
  1. Burns with malfunction of thermistor/heater
  2. Less accurate
    - Metabolism O2 as as it diffuses through skin
    - OHDC shifts with temp
    - Reduction in skin perfusion: oedema/shock/cold/CO etc
  3. Slower response time (takes 1 minute)
  4. there is a lag

Physics for Anaesthetists (26 decks)

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