Test & measurement
patient SpO2 level efficiently. Finally, digital integrator options allow users to enter an
extremely efficient power consumption mode to enable longer run times in portable PPG solutions by disabling analogue blocks in the optical signal path.
What Is Oxygen saturatIOn?
Oxygen saturation is the percentage of oxygen saturated hemoglobin within the blood with respect to the total available hemoglobin. The gold standard for measuring oxygen saturation is
the atrial blood oxygenation measurement, SaO2. However, this method requires laboratory-based blood gas analysis of a blood sample. The calibration section covers this in greater depth. SpO2 is an estimate of the oxygen saturation
levels measured at the peripherals of the body, using a pulse oximeter. Until recently, the most common way to measure oxygen saturation has been to use a pulse oximeter positioned on the finger.
hOW DOes a Pulse OxImeter WOrk?
A pulse oximeter works on the principal that absorption of light in oxygenated haemoglobin
(HbO2) and deoxygenated haemoglobin (RHb) differ significantly at specific light wavelengths. Figure 1 shows the extinction coefficient of
HbO2, Hb, and methemoglobin (MetHb) across the visible and infrared light spectrum. The extinction coefficient is a measurement of how strongly a chemical substance absorbs light at a given wavelength. From Figure 1, it can be seen
that HbO2 absorbs more red light (600nm) and allows more infrared light (940nm) to pass through. RHb absorbs more light at infrared wavelengths, which allows more red light to pass
through than in HbO2. The most basic pulse oximeter consists of two
LED (one red 660nm LED and one infrared (IR) 940nm LED) and a single photodiode (PD) in a reflective or transmissive configuration (see Figure 4). The pulse oximeter will pulse the red LED and measure the resulting signal on the PD. Repeat this for the IR LED and finally with both LEDs off to get a baseline for any ambient external light sources. This generates a photoplethysmography (PPG) signal for both wavelengths. The signal contains DC and AC components.
The DC component is due to constant reflective matter such as skin, muscle and bone, and venous blood. When a body is at rest and motion is less of a factor, the AC component comprises mainly of reflected light from the pulsation of artery blood. The AC component depends on heart rate and artery thickness, with more reflected or transmitted light in systolic (pump) than the dystopic (relaxation). During the systolic phase, blood is pumped from the heart, which increases atrial blood pressure. The increase in blood pressure expands the arteries and leads to an increase in atrial blood volume. This increase in blood causes an increase in light absorption. Blood pressure drops during the diastolic phase and therefore so does the absorption of light. Figure 3 shows the diastolic trough and systolic peaks
Instrumentation Monthly November 2021
caused by the beating heart. The Beer-Lambert law explains that light decays
exponentially when travelling though absorptive material. This can be used to determine the level of oxygenated hemoglobin to total hemoglobin. The intensity of light absorbed at the diastole
and systole are related by: Where α measures the absorption rate of light
in atrial blood and d2 is the AC amplitude of the PPG signal (see Figure 3). Idiastole is equal to the
in every optical design that cause variations to the
RoR to SpO2 relationship. These include mechanical baffle design, LED to PD spacing, electronic and mechanical ambient light rejection, PD gain errors, and many more. To obtain clinical grade accuracy from a PPG-
based SpO2 pulse oximeter, a lookup table or algorithm must be developed for the correlation
between RoR and SpO2.
CalIbratIOn Calibration of the measurement system is required
to develop a high accuracy SpO2 algorithm. To calibrate an SpO2 system, a study must be completed where a participant’s blood oxygen
levels are medically reduced, monitored, and overseen by a medical professional. This is known as a hypoxia study. The SpO2 measurement system can only be
as accurate as the reference. Reference options include medical grade finger clip pulse oximeters and the gold standard co-oximeter. The co-oximeter is an invasive method of measuring the oxygen saturation of blood that yields high accuracy, but in most cases is not convenient to administer. The calibration process is used to generate a best fit curve of RoR value calculated from the
optical SpO2 device to the co-oximeter SaO2 measurement. This curve is used to generate a
lookup table or equation for calculating SpO2. Calibration will be required for all SpO2 designs
Figure 3. Light attenuation through tissue.
DC component labelled d1. By computing AC and DC from a PPG signal,
we are able to determine the change in absorption of light in atrial blood –α.d2 caused by blood pumping from the heart, with no contribution from other tissue. The ratio of the AC component to the DC
component is known as the perfusion index, which is the ratio of the pulsating blood flow to the nonpulsatile static blood flow. The goal of a PPG-
based heart rate or SpO2 measurement system is to increase the AC to DC signal ratio. PI = AC/DC The perfusion index for infrared and red
wavelengths can be used to calculate the ratio of
ratios (RoR), which is the ratio of PIred to PLir. As the absorption of the light at a given wavelength is
proportional to the: In theory, the RoR can be substituted into the
following formula to compute SpO2: Where: EHbO2,red = extinction coefficient of
HbO2 at 600nm, EHbO2,ired = extinction coefficient of HbO2 at 940nm ERHb,ired = extinction coefficient of RHb at 940nm, ERHb,red = extinction coefficient of RHb at 600nm However, the Beer-Lambert law cannot be used directly as there are a number of variable factors
as RoR is dependent on a number of variables such as LED wavelength and intensity, PD response, body placement, and ambient light rejection, which will differ with each design. An increased perfusion index and, in turn, a
high AC dynamic range on the red and IR wavelengths will increase the sensitivity of the RoR calculation and, in turn, return a more
accurate SpO2 measurement. During a hypoxia study, 200 measurements
equally spaced between 100 per cent and 70 per cent blood oxygen saturation need to be recorded. Subjects are chosen with a variety of coloured skin tone, and an equal spread of age and gender. This variation in skin tone, age, and gender accounts for differing perfusion index results from a spread of individuals. The overall error for transmissive pulse
oximeters must be ≤3.0 per cent and ≤3.5 per cent for reflective configuration.
DesIgn COnsIDeratIOns: transmissive vs. reflective
A PPG signal can be obtained using a transmissive or reflective LED and PD configuration. A transmissive configuration measures the nonabsorbed light passed through a part of the body. This configuration is best suited to areas such as the finger and earlobe where measurement benefits from the capillary density of these body locations, which make the measurements more stable, repetitive, and less sensitive to variations in placement. Transmissive configurations achieve a 40dB to 60dB increase in the perfusion index.
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