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Surgical Techniques & Safety > Pulse Oximetry > Flashcards

Pulse Oximetry Flashcards

1
Q

What is pulse oximetry?

A
  • simple
  • non invasive
  • measures oxygen saturation levels
  • also measures HR (pulse)
  • estimate but very good estimate of O2 sats
  • fast and cheap
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2
Q

What is the normal oxygen saturation?

A

95% and above

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3
Q

What are the parts of a pulse oximeter?

A
  • display unit

- probe which attaches to finger/ear

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4
Q

What is the composition of whole blood?

A

Plasma (55%)
WBC & Platelets (<1%)
RBC (45%)

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5
Q

Define SO2

A

Ratio of Hb bound with oxyen to the total Hb

= HbO2/Hb + HbO2

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6
Q

How many molecules of O2 does Hb have?

A

3 or 4

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7
Q

What is the oxygen capacity of normal blood?

A

200ml O2/liter

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8
Q

What is the normal Hb concentration?

A

150g/liter of blood

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9
Q

How much better is Hb at carrying oxygen than plasma?

A

Can carry 65 times more oxygen than plasma at a PO2 of 100mm Mercury
(98-99% of oxygen carried by Hb, very little dissolved in blood plasma)

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10
Q

What gives the most accurate measure of oxygen in the body?

A

Blood gas analysis

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11
Q

What are the limitations of blood gas analysis?

A
  • cannot do in the field (need lab, big machine)
  • takes time for analysis
  • requires blood sample (invasive)
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12
Q

How does a pulse oximeter work?

A
  • shines a light at blood
  • determines oxygen sats from how the light is absorbed
  • goes from air to tissue (low to high refractive index) so TIR will not occur
  • light may be transmitted or reflected or absorbed
  • amount of absorption is slightly different for oxygenated vs. non-oxygenated Hb
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13
Q

What are the parts of the detector?

A
  • 2 LED light sources

- 1 photosensor

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14
Q

What is transmission mode?

A
  • shines LED light through finger
  • determine how much is absorbed
  • detector and light source at different sites
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15
Q

What is reflectance mode?

A
  • if only 1 site
  • detector and light source at 1 site
  • light does not go through, gets reflected by same device on same side
  • photodetectors see what is returned
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16
Q

Difference in colour between oxygenated and non-oxygenated blood

A

Non-oxygenated = blue
Oxygenated = red
- darker colour of venous blood is because Hb absorbs more red & less blue light than HbO2

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17
Q

Define oximetry

A

Optical measurement of oxyhaemoglobin (O2Hb) saturation in the blood

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18
Q

Uses for oximetry

A
  • early diagnosis for hypoxemia

- cannot be used for hypoxia (tissue oxygen levels not blood)

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19
Q

What does hypoxia depend on?

A
  • Hb concentration
  • Hb saturation
  • any further dissolved O2
  • tissue perfusion
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20
Q

What does a pulse oximeter not measure?

A
  • ventilation
  • haemoglobin concentration
  • tissue perfusion/oxygen levels
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21
Q

Define hypoxaemia

A

Low inspired partial pressure of oxygen (ventilation)

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22
Q

Causes of hypoxaemia

A
  • hypoventilation
  • impairment of diffusion across blood-gas membrane
  • ventilation-perfusion inequality
  • airway obstruction (choking)
  • increased airway resistance (asthma)
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23
Q

Dangers of hypoxaemia

A
  • can lead to hypoxia
  • gradual headache, fatigue, SOB, nausea
  • rapid onset = loss of consciousness, seizures, coma, death
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24
Q

What is a detectable sign of hypoxaemia

A

Cyanosis

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25
Q

Survival times in absence of oxygen for different organs

A 
Brain = <1 minute
Heart = 5 minutes
Liver/Kidney = 10 mins
26
Q

Symptoms for hypoxia

A
  • SOB

- cyanosis

27
Q

What oxygen sat level is dangerously low?

A

80% = starves brain & vital organs

28
Q

Episodic hypoxemia

A
  • common
  • common in ITU
  • danger not known
  • 5 minute or more drop in oxygen <90%
  • 3 times higher mortality rate
29
Q

Requirements for pulse oximeter to function

A
  • presence of pulsatile signal from arterial blood

- oxyhaemoglobin (O2Hb) & reduced hemoglobin (Hb) have different absorption spectra for light

30
Q

What law is used to establish drop in light intensity?

A

Beer-Lambert’s Law
Iout = Iin x e^-abc

Iout = intensity out
Iin = intensity in
e = extinction coefficient (specific for material)
a = molecular absorptivity
b = length of light path 
c = concentration of sample
31
Q

How does length of light path affect absorptivity?

A
  • longer length = more absorbed
32
Q

How does concentration of the sample affect absorptivity?

A

Higher concentration = greater absorption

33
Q

Formula for transmission

A

T = Iout/Iin = e^-abc

34
Q

Formula for absorbance

A

A = -InT = abc

35
Q

What is the total absorbance?

A

sum of abc’s for each molecule

  • absorption spectra of 2 molecules do not interact
  • sum of absorptions due to individual chromophores
36
Q

Reasons for wavelength choices

A
  • below 600 nm = skin absorbs large amounts of light
  • 660nm = largest difference between spectra
  • 805nm = isosbestic point
  • 660 & 940 nm = both flat response in that wavelength range (less error)
37
Q

Which wavelengths are usually chosen?

A

660 = large difference between artieral and venous Hb
940 = large difference but not red oxygenated arterial Hb is above blue venous dexoygenated
- can balance flat response in this range (less errior) with large differences in absorption

38
Q

What are some other absorbers in arterial blood?

A
  • carboxyhaemoglobin
  • methaemoglobin
  • sulfhaemoglobin
  • dysfunctional haemoglobin
39
Q

Rearranged formulas

A

CHbO2 = SO2(CHbO2 + CHb)

ChB = (1-SO2)(CHbO2 + CHb)

40
Q

What are the 2 LEDs? How does the timing work?

A
  • red (660nm) = O2Hb absorbs less light than Hb
  • infrared (940nm) = Hb absorbs less light than O2Hb
  • can only turn one on at a time, read one then other as photodiode cannot distinguish between light sources
  • switch one and off consecutively as we know which wavelength light is for which colour
  • we then see how absorbance changes with oxygen saturation at 2 different wavelengths
41
Q

What else in the body is absorption affected by?

A
  • constant absorption from surrounding tissues

- affected by pulse (change in diameter of vessels -> different path length and volume of absorbers)

42
Q

Primary light absorbers

A
  • skin pigmentation
  • bones
  • arterial blood
  • venous blood
43
Q

How is absorption affected by systole?

A

Arteries expanded = less light transmitted

44
Q

How is absorption affected by diastole?

A
  • at basic DC level

- minimum absorption

45
Q

What is the principle of the PPG signal?

A
  • want to separate AC from DC component
  • AC is 1-2% of DC
  • photoplethysmograph shows volumetric change
  • pulse oximeter collects PPG signals
46
Q

Which absorbances vary with pulse?

A

Hb and HbO2

DC doesn’t really vary with pulse

47
Q

What is normalisation?

A

???
- different LED intensities
- different photodetector sensitivity to 2 wavelengths
- need to normalise the light intensities for each wavelength, so result isn’t set up specific
normalised intensity -> In = Iout/IH
IH = highest intensity
- anything smaller than IH is smaller than 1 (as intensity is 0 to 1)
- can compare 2 AC components from 2 wavelengths

48
Q

What are errors dependent on?

A
  • light scattering, Beer’s Law assumes no scattering, occurs off surrounding tissues or rest of blood components
  • orientation of blood cells
49
Q

What happens to the optical path during systole?

A
  • gets longer
  • red blood cells change alignment
  • increased light absorbance
  • reflectance also changes
50
Q

Define scattering

A

deviation of light beam from initial direction

  • when light is refracted by object of similar dimension to its wavelength
  • wavelengths of red & infrared light similar size to RBCs
51
Q

Variables of light scattering

A
  • RBC concentration
  • size, shape, orientation of RBCs
  • tissue thickness
52
Q

What does the photodetector do?

A
  • produces current linearly proportional to the light intensity hitting it
53
Q

What does the amplifier do?

A
  • amplifies signal to readable level
54
Q

What does the modulator do?

A
  • controlling timing for R and IR LEDs
55
Q

What does the bandpass filter do?

A
  • allows signal processing
  • bandpass or high & low pass
  • separating DC and AC signal components
  • converts current to a voltage, and separates DC and AC
56
Q

What does the demultiplexer do?

A
  • separates and later puts back together 2 wavelengths
57
Q

What is the order of components?

A
  • modulator or sensor -> amp -> demultiplexer -> bandpass filters
58
Q

Calibration

A
  • pulse oximeter needs calibration
  • normally by manufacture by comparing readings with those from blood sample
  • generally accurate +- 2%
  • often lose accuracy below 80%
59
Q

Reasons for inaccuracies

A
  • reduction in peripheral pulsatile flow (peripheral vasoconstriction) = inadequate signal for analysis
  • venous congestion (TR = venous pulsations)
  • badly positioned probe (reposition if readings lower than expected, if waveform is good reading is accurate)
  • motion artefacts (shivering can make it difficult to pick up adequate signal, major cause of false alarms in ITU)
60
Q

What things can cause reduction in peripheral pulsatile flow?

A
  • hypovolemia
  • severe hypotension
  • cold
  • cardiac failure
  • vascular disease
  • vasoconstrictor drugs

Surgical Techniques & Safety (5 decks)

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