@@ -16,6 +16,7 @@ Pulse oximetry devices use several LEDs to measure pulse rate and blood oxygen c
- 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."
- detailed discussion of pulse-ox machine design: Pologe, Jonas A. "Pulse oximetry: technical aspects of machine design." International anesthesiology clinics 25.3 (1987): 137-153.
- a design study weighing the relative merits of different pulse-ox probe types for a low-cost device: Parlato, Matthew Brian, et al. "Low cost pulse oximeter probe." Conjunction with Engineering, World Health and the MEdCal Project (2009).
### 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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@@ -45,14 +46,25 @@ Typical commercial pulse oximeters use a red LED (660 nm) and an IR LED (940 nm)
_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 blood oxygen concentration from errors related to skin absorbance and venous blood (whose oxygen has already been taken up by cells), the signal processing algorithm isolates 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. This _pulsatile arterial blood_ increases the optical path length of the measurement as blood pressure swells the arteries, producing periodic oscillations in the absorption signal. The other contributors to absorption, such as tissue and venous/capillary blood, are effectively constant in this frequency regime. By calculating the ratio of the AC and DC signals at each wavelength, then taking the ratio of these two absorption ratios, a value $`R`$ can be determined which is only related to the relative concentration of oxyhemoglobin (O2Hb) and reduced hemoglobin (Hb):
In order to differentiate the slight intensity change caused by varying blood oxygen concentration from errors related to skin absorbance and venous blood (whose oxygen has already been taken up by cells), the signal processing algorithm isolates 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. This _pulsatile arterial blood_ increases the optical path length of the measurement as blood pressure swells the arteries, producing periodic oscillations in the absorption signal. The other contributors to absorption, such as tissue and venous/capillary blood, are effectively constant in this frequency regime. By calculating the ratio of the AC and DC signals at each wavelength, then taking the ratio of these two absorption ratios, a value $`R`$ can be determined which is only related to the relative concentration of oxyhemoglobin (O<sub>2</sub>Hb) and reduced hemoglobin (Hb):
As the photodiode sensor does not differentiate by wavelength, the device rapidly cycles between red, IR, and no LED, allowing the system to compensate for ambient light as well. The cycling speed must be substantially faster than the heart rate, since the ratio $`R`$ assumes absorption at all wavelengths is carried out simultaneously in order to cancel out path length. $`R`$ is then related to SpO2 using an empirically determined chart:
As the photodiode sensor does not differentiate by wavelength, the device rapidly cycles between red, IR, and no LED, allowing the system to compensate for ambient light variation as well. The cycling speed must be substantially faster than the heart rate, since the ratio $`R`$ assumes absorption at all wavelengths is carried out simultaneously in order to cancel out path length. $`R`$ is then related to SpO2 using an empirically determined curve:
_Figure source, via Ohmeda Corp: Pologe, Jonas A. "Pulse oximetry: technical aspects of machine design." International anesthesiology clinics 25.3 (1987): 137-153._
Note that methemoglobin (MetHb) and carboxyhemoglobin (CoHb) are not factored in with this method and will thus cause systematic errors; the above calculation assumes these two compounds are minimally present. Additional wavelengths are needed to quantify all four hemoglobin species.
Note that methemoglobin (MetHb) and carboxyhemoglobin (COHb) are not factored in with this method and will thus cause systematic errors; the above calculation assumes these two compounds are minimally present. Additional wavelengths are needed to quantify all four hemoglobin species.
### Practical Considerations
Commercial pulse oximeters trace their calibrations back to empirical studies on human volunteers whose blood oxygenation is simultaneously observed using an invasive measurement device. To avoid needing to repeat this process for every device that is manufactured, designers rely on pre-assembly binning or per-unit spectroscopy testing to compensate for LED wavelength variation, and likely perform extensive electrical testing to ensure photodiode and amplifier differences are accounted for. The spirit of this exercise, open, low-cost devices that can be made anywhere and remain useful, means these techniques aren't particularly useful.
A few paths exist that may be worth pursuing, given the aforementioned concerns:
- Build an uncalibrated device that allows users to track _changes_ in their blood oxygenation over time. Even without an absolute reference in terms of SpO<sub>2</sub>, this data could be used as an early warning for respiratory ailments. This fits with the use case, too; clinical devices need to be usable as spot-check instruments, where as a personal device could be used for weeks or months by one person.
- Develop a calibration system that can be easily manufactured and deployed based on fundamental principles, i.e. one that does not need to be _itself_ calibrated. One could build a spinning hollow clear plastic wheel with two chambers and controlled thickness, with the chambers filled with various concentrations of a solution whose absorption spectrum closely matches that of blood at a given oxygenation level. The wheel would be spun to simulate the heartbeat, and different wheels would represent different SpO<sub>2</sub> values. The solution could be accurately mixed using basic laboratory equipment, such as a scale or a pipette.
- Design an automated calibration system that uses a camera and optical character recognition to gather SpO<sub>2</sub> values from a commercial or clinical instrument and build a calibration table for the low-cost device while it is simultaneously clipped to the patient. Caregivers could "train" the low-cost device prior to patient discharge so they can self-monitor for flare-ups or subsequent respiratory ailments.
- Develop a methodology for cheaply and accurately characterizing LEDs and other components in the low-cost sensor, so that a master calibration file from a clinical study can be propagated to other devices as is done by traditional manufacturers.
