Pulse oximetry

Question 1

Pulse oximeter vs co-oximeter

in-vivo vs in-vitro, mechanism

Answer 1

Pulse oximeter is an in-vivo monitor

Co-oximeter is an in-vitro monitor which measures the concentration of different forms of haemoglobin (oxyHb, DeoxyHb, MetHb, COHb) in a haemolysed blood sample - can therefore derive oxygen saturation.

Both function by the absorption of light
* Pulse oximeter: light absorbed by whole human tissue – see diagram
* Co-oximeter: light absorbed by blood alone. Relies on the fact that there is a linear relationship between light absorbance and hte concentration of an absorbing substance, and that different haemoglobins absorb light at different frequencies.

Question 2

Beer-Lambert law

Answer 2

There is a linear relationship between light absorbance and the concentration of an absorbing substance

Question 3

Pulse oximeter probe

Structure, mechanism. At which frequencies do the LEDs emit light?

Answer 3

• Probe contains 2 light-emitting diodes (LED), emitting light at red (660nm) and infrared (940nm) frequencies
• There is a photodetector either on opposite side to LEDs (transmission) or same side as LEDs (reflectance) to measure the transmitted or reflected light

Measures how the two frequencies (red and infrared) of light are absorbed across a pulsatile tissue bed therefore inferring saturation
* Photodectector produces current linearly proportional to the intensity of the light striking it
* Photodetector cannot distinguish between red and infrared light, so to accomodate this, micoprocessor turns each LED on and off in sequence
* When both LEDs are off, ambient light is measured so it can be extracted from the signal

Question 4

Pulse oximeter computerised unit

Basic components (5)

Answer 4

• Switching circuit: controls sequencing of LEDs
• Automatic gain controllers: control intensity of emitted light to compensate for thickness of finger, skin colour
• Bypass filters: extract the AC (pulsatile, due to arterial pulsation) component from the DC (constant) component -> can be displayed as pulse waveform -> pulse rate calculated from waveform
• Analogue to digital signal converters (microprocessor requires digital signal)
• Microprocessor and monitor: processes filtered data, allows setting of alarms, display shows pulse rate, saturation, plethysmograph

Question 5

How is pulse rate calculated from pulse oximeter

Answer 5

• Photodetector generates a current proportional to the transmitted light
• Using bypass filters, computerised unit can extract pulsatile (AC) component due to arterial pulsation from the constant component (DC)
• –> plethysmograph (pulse waveform)
• Pulse rate is calculated from this waveform

Question 6

Absorption spectrogram

Points of similar absorption for different forms

Answer 6

Absorption spectrogram shows how light at different frequencies is absorbed by haemoglobins. Note y axis is logarithmic

• Oxyhaemoglobin has much greater absorption at 940nm (infrared) than 660nm (red)
• Deoxyhaemoglobin has much greater absorption at 660nm than 940nm
• Isobestic point = 810nm, both oxyhaemoglobin and deoxyhaemoglobin have same absorption
• At 660nm carboxyhaemoglobin has similar absorption to oxyhaemoglobin, and methaemoglobin has similar absorption to deoxyhaemoglobin - therefore methaemoglobin mimics deoxyhaemoglobin and carboxyhaemoglobin mimics oxyhaemoglobin

Question 7

How does the pulse oximeter calculate saturation

Formula, averaging

Answer 7

Processor calculates ratio of absorption at 660nm to 940nm (R/IR ratio):

R/IR = (ac660 / dc660) / (ac940/dc940)

This is compared to a ‘look up’ table pre-programmed into the machines memory. See graph of saturation vs R/IR ratio. Note curve is extrapolated below SpO2 70%

Most oximeters average out saturation readings over 5 to 20 seconds.

Question 8

Accuracy of pulse oximeter

Range, acute changes, effect of poor perfusion/ venous pulsation

Answer 8

Accuracy of +/-2% between 100-70% SpO2. Below 70% calibration curve of R/IR vs SpO2 is extrapolated, and calibration between pulse oximeter and in-vivo oximeter is less accurate

Most oximeters average out saturation readings over 5-20 seconds - if sampling rate up to 20s, limited ability to detect acute changes

Accuracy/ measurement may be limited by conditions that affect the pulsatile (AC) component
* Poor perfusion and low BP -> pulsatile (AC) component is harder to extract - see picture
* Venous pulsation e.g. tricuspid regurgitation and impaired venous return may be misinterpreted as part of the pulsatile (AC) component

Question 9

Effect of carboxyhaemoglobin and methaemoglobin on pulse oximeter readings

Answer 9

Carboxyhaemoglobin
* e.g. carbon monoxide poisoning and heavy smokers
* causes falsely high SpO2, as it resembles oxyhaemoglobin at 660 nm.

Methaemoglobin
* caused by drugs, such as nitrates and local anaesthetics e.g. prilocaine
* can lead to misreading as it resembles deoxyhaemoglobin at 660 nm

Question 10

External causes of error in pulse oximeter readings (4)

Answer 10

• Excessive movement
• Poor positioning of probe
• External fluorescent lighting
• Nail polish - can cause under-reading of pulse oximeter. Especially blue and black polish. Degree of artificial desaturation correlates with the difference in the polish’s absorbance at 660nm and absorbance at 940nm. Turning oximeter through 90 degrees can eliminate problem

Question 11

Complications of pulse oximeter use (2)

Answer 11

• Pressure sores may be caused by long term use due to local decreased perfusion from pressure on the tissues.
• Burns from the LEDs may occur

Poor perfusion is a risk factor for both

Question 12

Identify the different absorption frequencies shown on the absorption spectogram.

Answer 12