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
Survival times in absence of oxygen for different organs
``` Brain = <1 minute Heart = 5 minutes Liver/Kidney = 10 mins ```
26
Symptoms for hypoxia
- SOB | - cyanosis
27
What oxygen sat level is dangerously low?
80% = starves brain & vital organs
28
Episodic hypoxemia
- common - common in ITU - danger not known - 5 minute or more drop in oxygen <90% - 3 times higher mortality rate
29
Requirements for pulse oximeter to function
- presence of pulsatile signal from arterial blood | - oxyhaemoglobin (O2Hb) & reduced hemoglobin (Hb) have different absorption spectra for light
30
What law is used to establish drop in light intensity?
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
How does length of light path affect absorptivity?
- longer length = more absorbed
32
How does concentration of the sample affect absorptivity?
Higher concentration = greater absorption
33
Formula for transmission
T = Iout/Iin = e^-abc
34
Formula for absorbance
A = -InT = abc
35
What is the total absorbance?
sum of abc's for each molecule - absorption spectra of 2 molecules do not interact - sum of absorptions due to individual chromophores
36
Reasons for wavelength choices
- 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
Which wavelengths are usually chosen?
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
What are some other absorbers in arterial blood?
- carboxyhaemoglobin - methaemoglobin - sulfhaemoglobin - dysfunctional haemoglobin
39
Rearranged formulas
CHbO2 = SO2(CHbO2 + CHb) ChB = (1-SO2)(CHbO2 + CHb)
40
What are the 2 LEDs? How does the timing work?
- 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
What else in the body is absorption affected by?
- constant absorption from surrounding tissues | - affected by pulse (change in diameter of vessels -> different path length and volume of absorbers)
42
Primary light absorbers
- skin pigmentation - bones - arterial blood - venous blood
43
How is absorption affected by systole?
Arteries expanded = less light transmitted
44
How is absorption affected by diastole?
- at basic DC level | - minimum absorption
45
What is the principle of the PPG signal?
- want to separate AC from DC component - AC is 1-2% of DC - photoplethysmograph shows volumetric change - pulse oximeter collects PPG signals
46
Which absorbances vary with pulse?
Hb and HbO2 | DC doesn't really vary with pulse
47
What is normalisation?
??? - 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
What are errors dependent on?
- light scattering, Beer's Law assumes no scattering, occurs off surrounding tissues or rest of blood components - orientation of blood cells
49
What happens to the optical path during systole?
- gets longer - red blood cells change alignment - increased light absorbance - reflectance also changes
50
Define scattering
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
Variables of light scattering
- RBC concentration - size, shape, orientation of RBCs - tissue thickness
52
What does the photodetector do?
- produces current linearly proportional to the light intensity hitting it
53
What does the amplifier do?
- amplifies signal to readable level
54
What does the modulator do?
- controlling timing for R and IR LEDs
55
What does the bandpass filter do?
- 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
What does the demultiplexer do?
- separates and later puts back together 2 wavelengths
57
What is the order of components?
- modulator or sensor -> amp -> demultiplexer -> bandpass filters
58
Calibration
- pulse oximeter needs calibration - normally by manufacture by comparing readings with those from blood sample - generally accurate +- 2% - often lose accuracy below 80%
59
Reasons for inaccuracies
- 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
What things can cause reduction in peripheral pulsatile flow?
- hypovolemia - severe hypotension - cold - cardiac failure - vascular disease - vasoconstrictor drugs