Pulse Oximetry Flashcards
What is pulse oximetry?
- simple
- non invasive
- measures oxygen saturation levels
- also measures HR (pulse)
- estimate but very good estimate of O2 sats
- fast and cheap
What is the normal oxygen saturation?
95% and above
What are the parts of a pulse oximeter?
- display unit
- probe which attaches to finger/ear
What is the composition of whole blood?
Plasma (55%)
WBC & Platelets (<1%)
RBC (45%)
Define SO2
Ratio of Hb bound with oxyen to the total Hb
= HbO2/Hb + HbO2
How many molecules of O2 does Hb have?
3 or 4
What is the oxygen capacity of normal blood?
200ml O2/liter
What is the normal Hb concentration?
150g/liter of blood
How much better is Hb at carrying oxygen than plasma?
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)
What gives the most accurate measure of oxygen in the body?
Blood gas analysis
What are the limitations of blood gas analysis?
- cannot do in the field (need lab, big machine)
- takes time for analysis
- requires blood sample (invasive)
How does a pulse oximeter work?
- 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
What are the parts of the detector?
- 2 LED light sources
- 1 photosensor
What is transmission mode?
- shines LED light through finger
- determine how much is absorbed
- detector and light source at different sites
What is reflectance mode?
- 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
Difference in colour between oxygenated and non-oxygenated blood
Non-oxygenated = blue
Oxygenated = red
- darker colour of venous blood is because Hb absorbs more red & less blue light than HbO2
Define oximetry
Optical measurement of oxyhaemoglobin (O2Hb) saturation in the blood
Uses for oximetry
- early diagnosis for hypoxemia
- cannot be used for hypoxia (tissue oxygen levels not blood)
What does hypoxia depend on?
- Hb concentration
- Hb saturation
- any further dissolved O2
- tissue perfusion
What does a pulse oximeter not measure?
- ventilation
- haemoglobin concentration
- tissue perfusion/oxygen levels
Define hypoxaemia
Low inspired partial pressure of oxygen (ventilation)
Causes of hypoxaemia
- hypoventilation
- impairment of diffusion across blood-gas membrane
- ventilation-perfusion inequality
- airway obstruction (choking)
- increased airway resistance (asthma)
Dangers of hypoxaemia
- can lead to hypoxia
- gradual headache, fatigue, SOB, nausea
- rapid onset = loss of consciousness, seizures, coma, death
What is a detectable sign of hypoxaemia
Cyanosis
Survival times in absence of oxygen for different organs
Brain = <1 minute Heart = 5 minutes Liver/Kidney = 10 mins
Symptoms for hypoxia
- SOB
- cyanosis
What oxygen sat level is dangerously low?
80% = starves brain & vital organs
Episodic hypoxemia
- common
- common in ITU
- danger not known
- 5 minute or more drop in oxygen <90%
- 3 times higher mortality rate
Requirements for pulse oximeter to function
- presence of pulsatile signal from arterial blood
- oxyhaemoglobin (O2Hb) & reduced hemoglobin (Hb) have different absorption spectra for light
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
How does length of light path affect absorptivity?
- longer length = more absorbed
How does concentration of the sample affect absorptivity?
Higher concentration = greater absorption
Formula for transmission
T = Iout/Iin = e^-abc
Formula for absorbance
A = -InT = abc
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
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)
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
What are some other absorbers in arterial blood?
- carboxyhaemoglobin
- methaemoglobin
- sulfhaemoglobin
- dysfunctional haemoglobin
Rearranged formulas
CHbO2 = SO2(CHbO2 + CHb)
ChB = (1-SO2)(CHbO2 + CHb)
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
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)
Primary light absorbers
- skin pigmentation
- bones
- arterial blood
- venous blood
How is absorption affected by systole?
Arteries expanded = less light transmitted
How is absorption affected by diastole?
- at basic DC level
- minimum absorption
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
Which absorbances vary with pulse?
Hb and HbO2
DC doesn’t really vary with pulse
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
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
What happens to the optical path during systole?
- gets longer
- red blood cells change alignment
- increased light absorbance
- reflectance also changes
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
Variables of light scattering
- RBC concentration
- size, shape, orientation of RBCs
- tissue thickness
What does the photodetector do?
- produces current linearly proportional to the light intensity hitting it
What does the amplifier do?
- amplifies signal to readable level
What does the modulator do?
- controlling timing for R and IR LEDs
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
What does the demultiplexer do?
- separates and later puts back together 2 wavelengths
What is the order of components?
- modulator or sensor -> amp -> demultiplexer -> bandpass filters
Calibration
- pulse oximeter needs calibration
- normally by manufacture by comparing readings with those from blood sample
- generally accurate +- 2%
- often lose accuracy below 80%
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)
What things can cause reduction in peripheral pulsatile flow?
- hypovolemia
- severe hypotension
- cold
- cardiac failure
- vascular disease
- vasoconstrictor drugs
