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Anaesthetic FRCA Primary - eLFH Physics > eLFH - Light > Flashcards

eLFH - Light Flashcards

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1
Q

SI unit of luminous intensity

A

Candela (cd)

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2
Q

Definition of Candela

A

Luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 10^12 Hz and has radiant intensity in that direction of 1/683 watt per steradian

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3
Q

Properties of light travelling

A

Light travels in straight line until it encounters a surface, where it can then transmit, reflect, refract or diffract

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4
Q

Relationship between light angle of incidence and angle of reflection

A

They are equal

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5
Q

Refraction definition

A

When light travels from one medium to another, it can bend / change direction at the interface between the two mediums

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6
Q

Factors which determine the degree of refraction of light

A

Angle of incidence of incoming light ray

Nature of the mediums

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7
Q

Index of refraction

A

Obtained by comparing speed of light in a particular substance to the speed of light in a vacuum

E.g. index of refraction for air = 1.0, for water = 1.3

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8
Q

Snell’s law

A
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9
Q

Total internal reflection definition

A

If ray of light strikes a medium boundary at an angle larger than the critical angle, it bounces back and does not pass through resulting in total internal reflection

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10
Q

How to determine critical angle

A

Determined by refractive indices of the two substances

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11
Q

Equipment that rely on total internal reflection

A

Fibreoptic laryngoscope / bronchoscope

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12
Q

Diffraction definition

A

Spreading out of light waves as they pass through a gap

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13
Q

Factors which affect extent of diffraction

A

Wavelength of waves and width of gap

Narrower gap causes wider spread

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14
Q

Construction of fibreoptic laryngoscope

A

Bundles of fine glass fibres 8 - 10 micrometre diameter
Each coated in 1 micrometre thick cladding glass layer

Cladding glass has lower index of refraction to ensure total internal reflection occurs

Glass fibres arranged into bundles

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15
Q

Number of glass fibres in each bundle in fibreoptic scope

A

36,000 to 85,000 fibres in viewing bundle depending on scope size

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16
Q

Spectrophotometric analysis

A

Method of measuring gas concentrations in the anaesthetic gas analyser according to optical density

Involves shining light (radiation) through a sample and determining the quantity of radiation absorbed and therefore gas concentrations

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17
Q

Laws which describe absorption of radiation as it passes through a substance

A

Beer’s law

Bouguer’s (Lambert’s) law

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18
Q

Beer’s law

A

Absorption of radiation by a given thickness of solution of a given concentration is the same as that of double the thickness and half the concentration of the solution

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19
Q

Bouguer’s / Lambert’s law

A

Each layer of a substance of equal thickness absorbs an equal fraction of the radiation that passes through it

20
Q

Wavelength range of infrared radiation

A

1 to 40 micrometres

21
Q

Wavelength at which CO2 maximally absorbs infrared radiation

A

4.26 micrometre

22
Q

Gas analyser construction and mechanism

A

Infrared light emitted
Known path length and wavelength
Changes in infrared light reaching detector must be due to changes in gas concentration

23
Q

Optical density definition

A

Unitless measure of the absorbance of a substance

E.g. clear vs foggy evening and light absorption

24
Q

Factors which determine optical density

A

Length of light path

Wavelength of light

Substance concentration

25
Q

2 main types of infrared gas analysers

A

Side-stream analyser

Mainstream analyser

26
Q

Side-stream analyser

A

Sample of gas (typically at 150 ml/m) drawn from breathing circuit and analysed

Moisture trap removes water from sample gas

Sampled gas either returned to circuit or scavenged

27
Q

Advantages of side-stream analyser

A

Lightweight at patient end

Multiple gases can be analysed simultaneously

28
Q

Disadvantages of side-stream analyser

A

Lag time while it draws off sample

29
Q

Mainstream analyser

A

Special connector sits in the breathing circuit at patient end

Analyser shines infrared light across breathing circuit usually across a sapphire window

30
Q

Advantages of mainstream analyser

A

No lag time for result

31
Q

Disadvantages of mainstream analyser

A

Bulky at patient end

Usually only measure CO2

32
Q

Issues that can arise with infrared gas analysers

A

Oxygen can broaden the CO2 absorption spectra

Nitrous oxide can interfere with CO2 absorption and vice versa

Water vapour absorbs infrared light causing falsely high CO2 readings

33
Q

How do modern gas analysers compensate for nitrous oxide when measuring CO2 from infrared absorption

A

Most modern gas analysers measure amount of nitrous oxide and automatically compensate for it

34
Q

How does pulse oximeter measure level of oxygenated Hb

A

Spectrophotometric technique

Light (red light and infrared light) from two emitting diodes shone through sample every 5 to 10 microseconds and quantity of radiation absorbed is determined by a detector on other side of sample

35
Q

Wavelength of red light in pulse oximeter

A

660 nm

36
Q

Wavelength of infrared light in pulse oximeter

A

910 nm

37
Q

Analysis of red and infrared light absorption in pulse oximeter

A

660 nm red light absorbed less by oxyHb vs deoxyHb (hence why oxyHb is more red as less red light is absorbed)

910 nm infrared light absorbed less by deoxyHb vs oxyHb

Pulsatile component (i.e. arterial blood flow) measured for each wavelength and constant component which is not from arterial blood is subtracted from it

Oxygen saturations are calculated by comparing absorption against measured values from experimental studies

38
Q

Isobestic points definition

A

Wavelength at which the absorbance is identical for two chemical substances (e.g. both oxyHb and deoxy Hb)

39
Q

Isobestic points for oxyhaemoglobin and deoxyhaemoglobin

A

590 nm and 805 nm

40
Q

Potential causes for false readings with pulse oximeter

A

Errors calculating the pulsatile component of light absorption

Machine dysfunction

Increased ratio of non-pulsatile component of light absorption

41
Q

Causes of errors calculating the pulsatile component of light absorption in pulse oximeter

A

Hypoperfusion - more difficult for pulse oximeter to determine points of maximum and minimum absorption

Abnormal haemoglobins / IV compounds that interfere with light absorption

Arrythmias - harder for pulse oximeter to predict points of minimum and maximum absorption

42
Q

Examples of hypoperfusion which cause failure of pulse oximeter

A

Low pulse pressure

Profound vasoconstriction

Venous pulsation - e.g. torrential tricuspid regurgitation

43
Q

Examples of abnormal haemoglobins and IV compounds with interfere with light absorption in pulse oximeter

A

Carboxyhaemoglobin - misleadingly high reading

MetHb (methaemoglobinaemia) - misleadingly low reading of 85%

Methylene blue and Indocyanine green

44
Q

Causes of machine dysfunction in pulse oximeter

A

Electrical interference - e.g. surgical diathermy

45
Q

Causes of increased ratio of non-pulsatile component of light absorption in pulse oximeter

A

Nail varnish
Dirty fingers

Spurious non-constant background light levels may cause optical interference - e.g. flickering room lights

Anaesthetic FRCA Primary - eLFH Physics (33 decks)

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