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
2
Q
What is the normal oxygen saturation?
A
95% and above
3
Q
What are the parts of a pulse oximeter?
A
- display unit
- probe which attaches to finger/ear
4
Q
What is the composition of whole blood?
A
Plasma (55%)
WBC & Platelets (<1%)
RBC (45%)
5
Q
Define SO2
A
Ratio of Hb bound with oxyen to the total Hb
= HbO2/Hb + HbO2
6
Q
How many molecules of O2 does Hb have?
A
3 or 4
7
Q
What is the oxygen capacity of normal blood?
A
200ml O2/liter
8
Q
What is the normal Hb concentration?
A
150g/liter of blood
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)
10
Q
What gives the most accurate measure of oxygen in the body?
A
Blood gas analysis
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)
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
13
Q
What are the parts of the detector?
A
- 2 LED light sources
- 1 photosensor
14
Q
What is transmission mode?
A
- shines LED light through finger
- determine how much is absorbed
- detector and light source at different sites
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
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
17
Q
Define oximetry
A
Optical measurement of oxyhaemoglobin (O2Hb) saturation in the blood
18
Q
Uses for oximetry
A
- early diagnosis for hypoxemia
- cannot be used for hypoxia (tissue oxygen levels not blood)
19
Q
What does hypoxia depend on?
A
- Hb concentration
- Hb saturation
- any further dissolved O2
- tissue perfusion
20
Q
What does a pulse oximeter not measure?
A
- ventilation
- haemoglobin concentration
- tissue perfusion/oxygen levels
21
Q
Define hypoxaemia
A
Low inspired partial pressure of oxygen (ventilation)
22
Q
Causes of hypoxaemia
A
- hypoventilation
- impairment of diffusion across blood-gas membrane
- ventilation-perfusion inequality
- airway obstruction (choking)
- increased airway resistance (asthma)
23
Q
Dangers of hypoxaemia
A
- can lead to hypoxia
- gradual headache, fatigue, SOB, nausea
- rapid onset = loss of consciousness, seizures, coma, death
24
Q
What is a detectable sign of hypoxaemia
A
Cyanosis
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
Surgical Techniques & Safety
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