Pulse oxygenation devices use several LEDs to measure pulse rate and blood oxygen content. The LEDs are tuned to specific wavelengths corresponding to the absorbance band of oxygenated and reduced hemoglobin; by cycling through the LEDs rapidly the device compensates for skin differences and ambient light, returning saturation and pulse rate.
## Background
### Background
- overview of pulse oxymetry physics and engineering challenges from 1989: Tremper, Kevin K., and Steven J. Barker. "Pulse oximetry." Anesthesiology: The Journal of the American Society of Anesthesiologists 70.1 (1989): 98-108.
- engineering challenges identified
- LED center wavelength consistency
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@@ -15,8 +16,8 @@ Pulse oxygenation devices use several LEDs to measure pulse rate and blood oxyge
- changing LED wavelengths with temp: ~0.1 nm/C: Reynolds, K. J., et al. "Temperature dependence of LED and its theoretical effect on pulse oximetry." British journal of anaesthesia 67.5 (1991): 638-643.
- "... equation (2) is only an approximation and pulse oximeters are usually calibrated empirically using data obtained by inducing hypoxia in healthy volunteers."
## Commercial example
A quick teardown of a ~$20 500BL from Walgreens revealed no integrated photonics package or ASIC; instead, the device uses a bi-color IR/red LED on one side of a spring-loaded plastic clam-shell and a PCB with a decent sized photodiode on the other, paired with an [SGM8634](www.sg-micro.com/uploads/soft/20190626/1561538475.pdf) op-amp and an STM32F100-series 32-bit Arm Cortex M3 microcontroller. The display is a custom multi-segment LED device, but the PCB labels suggest an OLED is used for an alternate model. TX/RX test points were spotted that could be investigated further; with any luck, these could be used to pull live data out of the instrument.
### Commercial Example
A quick teardown of a ~$20 500BL from Walgreens revealed no [integrated photonics package](https://www.maximintegrated.com/en/products/interface/sensor-interface/MAX30101.html) or [signal processing ASIC](https://www.maximintegrated.com/en/products/interface/sensor-interface/MAX32664.html); instead, the device uses a bi-color IR/red LED on one side of a spring-loaded plastic clam-shell and a PCB with a decent sized photodiode on the other, paired with an [SGM8634](www.sg-micro.com/uploads/soft/20190626/1561538475.pdf) op-amp and an [STM32F100](https://www.st.com/en/microcontrollers-microprocessors/stm32f100-value-line.html)-series 32-bit Arm Cortex M3 microcontroller. The display is a custom multi-segment LED device, but the PCB labels suggest an OLED is used for an alternate model. TX/RX test points were spotted that could be investigated further; with any luck, these could be used to pull live data out of the instrument.
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@@ -24,9 +25,14 @@ A quick teardown of a ~$20 500BL from Walgreens revealed no integrated photonics
## Operational Theory
### Operational Theory
Pulse oximetry is based on the [Beer-Lambert law](https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law), a principle that relates the concentration of a species to the attenuation of light through a sample:
```math
I=I_{in}e^{-(DC\epsilon)}
```
where $`I`$ is the intensity of light transmitted through the sample; $`I_in`$ is the intensity of the light prior to absorption by the sample; $`D`$ is the optical path length; $`C`$ is the solute concentration; and $`\epsilon`$ is the extinction coefficient, the sample's absorption at a given wavelength of light.
where $`I`$ is the intensity of light transmitted through the sample; $`I_{in}`$ is the intensity of the light prior to absorption by the sample; $`D`$ is the optical path length; $`C`$ is the solute concentration; and $`\epsilon`$ is the extinction coefficient, the sample's absorption at a given wavelength of light. Typical commercial pulse oximeters use a red LED (660 nm) and an IR LED (940 nm) to quantify the relative concentration of reduced and oxygen-rich hemoglobin in a person's bloodstream based on the following absorbance curves:
_Figure source: Bülbül, Ali & Küçük, Serdar. (2016). Pulse Oximeter Manufacturing & Wireless Telemetry for Ventilation Oxygen Support. International Journal of Applied Mathematics, Electronics and Computers. 211-211. 10.18100/ijamec.270309._
In order to differentiate the slight intensity change caused by varying oxygen concentration from errors related to skin absorbance, the signal processing algorithm only looks at the AC portion of the signal, since within a reasonable range (~0.5 - 3 Hz) this corresponds to blood rushing through arteries with each heartbeat. As the photodiode sensor does not differentiate by wavelength, the device rapidly cycles between red, IR, and no LED, thus compensating for ambient light as well. Using two wavelengths to quantify two species results in a system of two equations. Note that, as mentioned above, methemoglobin (MetHb) and carboxyhemoglobin (CoHb) are not factored in with this method and can thus cause systematic errors; additional wavelengths are needed to quantify all four hemoglobin species.
