Questionable Wyze Watch 47 SpO2 Readings
April 8, 2021
Easy Test – Simply hold your breath
There is no obvious difference in the readings wearing the watch on top of the wrist or the underside.
Pulse Oximetry
Pulse oximetry is a tool used for the noninvasive measurement of blood
oxygenation (i.e., SpO2. Pulse oximetry is based on
two principles: modulation of transmitted light by absorption of pulsatile
arterial blood and different absorption characteristics of HbO2 and
RHb for different wavelengths.
Pulse oximetry can be classified as transmissive and reflective:
- Transmissive pulse oximetry is when the photodiode and the LED are placed on opposite sides of the human body (e.g., finger). The body tissue absorbs some of the light, and the photodiode collects the residual light that passes through the body.
- Reflective pulse oximetry is when the photodiode and the LED are on the same side. The photodiode collects the light reflected from various depths underneath the skin. Wrist based SpO2 measurement solutions are classified as reflective pulse oximetry.
Figure 1 shows pulsatile arterial blood and other blood and tissue components.
Figure 1. A schematic of pulsatile arterial blood and other blood and tissue components.
The pulsatile arterial blood absorbs and modulates the incident light passing through the tissue and forms the photoplethysmographic (PPG) signal, as shown in Figure 2. The AC component of the PPG signals represents the light absorbed by the pulsatile arterial blood. This AC component is superimposed on a DC signal that captures the effects of light absorbed by other blood and tissue components (e.g., venous and capillary blood, bone, water, etc.). The ratio of the AC signal to the DC level is called the perfusion index (PI).
Note that the DC and AC components of the received PPG signals are different for different LED wavelengths. This is due to the different absorption characteristics of HbO2, RHb, and other tissue components for different wavelengths.
Figure 2. Photoplethysmographic (PPG) signals received by a photodiode from red and infrared LEDs.
Figure 3 shows the molar absorption coefficients of HbO2 and RHb. To measure SpO2, two LEDs with different wavelengths are required. In addition, these two wavelengths should be selected such that the molar absorption coefficients of HbO2 and RHb are well separated. A red LED at 660nm and an infrared LED at 880nm are commonly used in pulse oximetry.
Figure 3. Molar absorption coefficients of HbO2 and RHb.
For more information, a detailed theory of pulse oximetry and noninvasive SpO2 measurement can be found in Development of a fractional multi-wavelength pulse oximetry algorithm[1].
Calibrating the SpO2 Algorithm
SpO2 measurement is achieved by the following equation:
where R is determined by the following equation:
and a, b, and c are calibration coefficients. This section describes how to
obtain these coefficients.
Why is Calibration Required?
The SpO2 measurement performance of a device must be verified
before the device is released to the market. The U.S. Food and Drug
Administration (FDA) suggest using standards presented in the following:
- ISO 80601-2-61:2017 – Medical electrical equipment -- Part 2-61: Particular requirements for basic safety and essential performance of pulse oximeter equipment
- Pulse Oximeters – Premarket Notification Submissions [510(k)s] Guidance for Industry and Food and Drug Administration Staff
According to these regulations, manufacturers need to declare the calibration range, reference, accuracy, methods of calibration and range of displayed saturation level. Furthermore, for the performance assessment, the FDA requires at least 200 data points equally spaced over a saturation range of 70% to 100%. Test subjects should have different ages, gender, and skin tones. For instance, the FDA requires that at least 30% of the volunteers must have dark skin pigmentation. The overall error or the root mean square error (RMSE) must be below 3.0% for transmissive pulse oximetry and below 3.5% for reflective pulse oximetry.