33
Pulse Oximeter
Pulse Oximeter
Pulse Oximeter
• If someone has a lung disease, the blood oxygen level may be lower than normal. This is important to know because when the oxygen level is low, the cells in the body can have a hard time working properly.
• The blood oxygen level measured is called oxygen saturation level (abbreviated O2sat or SaO2). This is a percentage of how much oxygen blood is carrying compared to the maximum it is capable of carrying
• Most people need an oxygen saturation level of at least 89% to keep their cells healthy. Having an oxygen level lower than this for a short time is not believed to cause damage. However, your cells can be strained or damaged if low oxygen levels happen many times.
• An oximeter indirectly measures the amount of oxygen that is carried by your blood. An arterial blood gas (ABG) directly measures both the amount of oxygen carried by your blood and the actual amount of gases (oxygen and carbon dioxide) that are in your blood. To get an ABG, blood is taken directly out of your artery (usually from the wrist) and can be painful. Oximetry is painless but is not as accurate as an ABG. Also, a pulse oximeter does not measure your carbon dioxide level.
Pulse oximeters are in common use because they are:
• non invasive
• cheap to buy and use
• can be very compact
• detects hypoxaemia earlier than you using your eyes to see cyanosis.
How accurate is the pulse oximeter?
The oxygen level from a pulse oximeter is reasonably accurate. Most oximeters give a reading 2% over or 2% under what your saturation would be if obtained by an arterial blood gas. For example, if your oxygen saturation reads 92% on the pulse oximeter, it may be actually anywhere from 90 to 94%. The oximeter reading may be less accurate if a person is wearing nail polish, artificial nails, has cold hands, or has poor circulation. A pulse oximeter may also be less accurate with very low oxygen saturation levels (below 80%) or with very dark skin.
Limitations
To get the best reading from oximeter, you need to make sure enough blood is flowing to the hand and finger wearing the device. The best reading, therefore, is achieved when your hand is warm, relaxed, and held below the level of your heart.
If you smoke, unfortunately, the reading on your oximeter may be higher than your actual oxygen saturation. This is because smoking increases carbon monoxide levels in the blood, the oximeter cannot tell the difference between the gas carbon monoxide from oxygen and will register falsely high on oxyhemoglobin as the blood will be very red.
Limitations
Readings will be inaccurate due to
1. Shivering (Motion artifacts)
2. low flow,
3. very thick skin
4. and poor placement of the sensors.
5. As the light source be placed on the nail and the detector on the soft tissue of the finger so the patient should not have nail polish on.
Design
The amount of light that is absorbed by the finger depends on many physical properties and these properties are used by the pulse oximeter to calculate the oxygen saturation.
The amount of light absorbed depends on the following:
1. concentration of the light absorbing substance.
2. length of the light path in the absorbing substance
3. oxyhemoglobin and deoxyhemoglobin absorbs red and infrared light differently
Design
Physical property No.1
Amount of light absorbed is proportional to the concentration of the light absorbing substance
Hemoglobin (Hb) absorbs light. The amount of light absorbed is proportional to the concentration of Hb in the blood vessel. In the diagram, the blood vessels in both fingers have the same diameter. However, one blood vessel has a low Hbconcentration ( i.e. low number of Hb in each unit volume of blood) and the other blood vessel has a high Hb concentration ( i.e. high number of Hb in each unit volume of blood). Each single Hb absorbs some of the light, so more the Hb per unit area, more is the light is absorbed. This property is described in a law in physics called “Beer’s Law”.
• Beer’s Law: Amount of light absorbed is proportional to the concentration of the light absorbing substance
By measuring how much light reaches the light detector, the pulse oximeter knows how much light has been absorbed. More the Hb in the finger , more is the light absorbed.
Physical property No. 2
Amount of light absorbed is proportional to the length of the light path.
The light emitted from the source has to travel through the
artery. The light travels in a shorter path in the narrow artery
and travels through a longer path in the wider artery ( paths
are shown as green lines). Though the concentration of Hb
is the same in both arteries, the light meets more Hb in the
wider artery, since it travels in a longer path. Therefore,
longer the path the light has to travel, more is the light
absorbed. This property is described in a law in physics
called “Lambert’s Law”.
Lambert’s Law: Amount of light absorbed is proportional to
the length of the path that the light has to travel in the
absorbing substance.
Physical property No. 3
Oxyhemoglobin absorbs more infrared light than red light & deoxyhemoglobin absorbs more red light than infrared light
The pulse oximeter works out the oxygen saturation by
comparing how much red light and infra red light is
absorbed by the blood. Depending on the amounts of oxy
Hb and deoxy Hb present, the ratio of the amount of red
light absorbed compared to the amount of infrared light
absorbed changes.
The method exploits the fact that Hb has a higher optical
absorption coefficient in the red region of the spectrum
around 660nm compared with HbO2. On the other hand,
in the near-infrared region of the spectrum around 940nm,
the optical absorption by Hb is lower compared to HbO2.
As the amount of oxy Hb and deoxy Hb changes,
the light ratio comparing red and infrared light also
changes. The pulse oximeter uses the ratio to
work out the oxygen saturation.
pulse oximeter uses Beer’s and Lambert’s Law ( absorbance depends
on concentration and path length) as part of its factors that it uses to
compute oxygen saturation. Unfortunately, there is a problem. Beer
and Lambert law have very strict criteria to be accurate. For an
example, the light that goes through the sample should go straight.
Blood is not a neat red liquid. Instead, it is full of various irregular
objects such as red cells etc. This makes the light scatter, instead of
going in a straight line. Therefore Beer and Lamberts Law cannot be
applied strictly.
Because Beer and Lamberts law cannot be applied strictly, there would be errors if they were used to directly calculate oxygen saturation. A solution to this is to use a “calibration graph” to correct for errors. A test pulse oximeter is first calibrated using human volunteers. The test pulse oximeter is attached to the volunteer and then the volunteer is asked to breath. At intervals, arterial blood samples are taken. As the volunteers blood desaturates, direct measurements made on the arterial blood are compared simultaneously with the readings shown by the test pulse oximeter. In this way, the errors due to the inability of applying Beers and Lamberts law strictly are noted and a correction calibration graph is made. However, in order to not harm the volunteers, the oxygen saturation is not allowed to drop below about 75 – 80 %.
Pulsating Blood
In a body part such as a finger, arterial blood is not the only thing that absorbs light. Skin and other tissues also absorb some light. This poses a problem , because the pulse oximeter should only analyse arterial blood while ignoring the absorbance of light by surrounding tissues. For an example of how tissues can interfere, take the two situations shown below. One is a thin finger and the other is a fat finger. The tissues in the thin finger absorbs only a little extra light, while the fatter finger shown on the right absorbs much more light. However, the pulse oximeter has no way to measure if the finger is fat or thin, and therefore has the potential to get confused because it doesn’t know how much light is absorbed by blood and how much is absorbed by the tissues surrounding blood.
How to solve this problem??
The pulse oximeter wants to only analyse arterial blood, ignoring
the other tissues around the blood. Arterial blood is the only thing
pulsating in the finger. Everything else is non pulsating. Any
“changing absorbance” must therefore be due to arterial blood.
On the other hand, any absorbance that is not changing , must be
due to non pulsatile things such as skin and other “non arterial”
tissues.
Some clever mathematics can extract the “changing absorbance”
signal from the total signal. The computer subtracts the non changing
part of the absorbance signal from the total signal. After the
subtraction, only the “changing absorbance signal” is left, and this
corresponds to the pulsatile arterial blood. In this way, the pulse
oximeter is able to calculate the oxygen saturation in arterial blood
while ignoring the effects of the surrounding tissues.
The pulsatile signal is very small. Typically , only about 2 % of the total signal is
pulsatile ! Drawn to scale, 2 % of the total signal will look like the diagram below. The
orange part represents the “non changing” light absorbed by the tissues. The red
shows the changing absorbance due to pulsatile arterial blood. See how small this
pulsatile signal is. Off all the light that passes through the finger, it is only the small
pulsatile part that the pulse oximeter analyses. Because it is such a small amount of
the total light, the pulse oximeter is very susceptible to errors if for an example, the probe is not placed properly or if the patient moves the probe.
The time invariant absorbance due to venous blood or surrounding tissues does not have any effect on the measurement. This normalization is carried out for both the red (R) and the infrared (IR) wavelengths
The pulsatile part of the PPG signal is considered as the “AC” component, and the non- pulsatile part, resulting mainly from the venous blood, skin and tissue, is referred to as the “DC” component. A deviation in the LED brightness or detector sensitivity can change the intensity ofthe light detected by the sensor. This dependence on transmitted or backscattered light intensity can be compensated by using a normalization technique where the AC component is divided by the DC component.
Pulse oximeters often show the pulsatile change in absorbance in a graphical form. This is called the “plethysmographictrace ” or more conveniently, as “pleth”.
SaO2 calculation
Most pulse oximeters measure absorbance at two different wavelengths and are calibrated using
data collected from CO-oximeters by empirically looking up a value for SpO2, giving an
estimation of SaO2 using the empirical relationship given by
where 𝑅/𝐼𝑅 is based on a normalization where the pulsatile (AC) component is divided by the
corresponding non-pulsatile (DC) component for each wavelength, and 𝐴 and 𝐵 are linear
regression coefficients which are related to the specific absorptions coefficients of Hb and HbO2.
The constants 𝐴 and 𝐵 are derived empirically during in-vivo calibration by correlating the ratio
calculated by the pulse oximeter against SaO2 from arterial blood samples by an in vitro
oximeter for a large group of subjects. Pulse oximeters read the SaO2 of the blood accurately
enough for clinical use under normal circumstances because they use a calibration curve based
on empirical data
Hardware design
Design Considerations
Accounting for ambient (room) light
In addition to the red and infra red LED light sources, there is also light in the room (ambient light) that the pulse oximeter is working in. Some of this room light can also reach the detector. The pulse oximeter needs the red and infra red light to calculate oxygen saturation. On the other hand, the room light is unwanted “noise”, and needs to be taken account of.
While placing the figure inside the equipment
1. Switch on Only Red light and record signal
2. Switch on Only infra red light and record the signal
3. Switch off both lights and record the signal
Signal 3 is only due to ambient light, which can then be subtracted from the original signal.
Design Considerations
Problem of movement
When you think of problems associated with pulse oximeters it is important to remember that the signal that is analyzed is really tiny. As explained before, it is only about 2 % of the total light that is analyzed. Which such a small signal, it is easy to see how errors can occur. Pulse oximeters are very vulnerable to motion, such as a patient moving his hand. As the finger moves, the light levels change dramatically. Such a poor signal makes it difficult for the pulse oximeter to calculate oxygen saturation.
Design Considerations
Problem of optical shunting
The pulse oximeter operates best when all the light passes through arterial blood. However, if the probe is of the wrong size or has not being applied properly, some of the light , instead of going through the artery, goes by the side of the artery (shunting). This reduces the strength of the pulsatile signal making the pulse oximeter prone to errors. It is therefore important to select the correct sized probe and to place the finger correctly in the chosen probe for best results.
Problem of electromagnetic interference
Electrical equipment such as surgical diathermy emit strong electric waves which may be picked up by the wires of the pulse oximeter. These waves make small currents form in the wires, confusing the pulse oximeter which assumes these currents come from the light detector. During diathermy use, one should be cautious about interpreting pulse oximeter readings.
Design Considerations
Design
The red LED is on for 50 µsec, both LEDs are off for 450 µsec, the NIR LED is on for 50 µsec, and then both LEDs are off for 450 µsec. The system repeats this cycle continuously. The transimpedance amplifier, converts the photodiode current generated by the LEDs to a voltage at the output. The signal then travels through a bandpass filter and gain stage to the ADC. The microcontroller acquires the signals from the ADC, computes the ratio of the red- and NIR-LED signals, and compares the results with a look-up table. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate.
where AOL(jΩ) is the open-loop gain of the amplifier over frequency; β is the system-feedback factor, equaling 1/(1+ZF/ZIN); ZIN is the distributed input impedance, equaling RPD||jΩ(CPD+CCM+CDIFF); and ZF is the distributed feedback impedance, equaling RF||jΩ(CRF+CF).
Iin is the signal current generated by the photodiode in response to the incident light and Cin is the zero bias capacitance of the photodiode.
The current to voltage gain in the TIA is given by:𝑉𝑇𝐼𝐴 (𝑑𝑖𝑓𝑓) = 𝑉𝑇𝐼𝐴+ − 𝑉𝑇𝐼𝐴-= 𝐼𝑖𝑛 ∗ 𝑅𝑓
For example, for a photodiode current of Iin= 1 μA and a TIA gain setting of Rf = 100 kΩ, the differential output of the TIA is equal to 100 mV.
The Rf amplifier and the feedback capacitor (Cf) form a low-pass filter for the input signal current.
With the amplifierused in the inverting configuration,the light shining on a photodiode producesa small current that flows to theamplifier summingjunctionsand throughthe feedbackresistor. Giventhe very largefeedbackresistor value, this circuit is extremelysensitive to changes in light intensity.For example, an input light signal ofjust 0.001μW can produce a full-swingoutput.
Transimpedance Amplifier Requirements• Low input bias current over temperature range of
interest, this input bias current converted into error voltages with amplitude dependent upon Rf.
• Low input capacitance relative to photodiode capacitance
• High gain-bandwidth product• Low voltage noise• For maximum precision, low offset drift over
temperature
