README.md



Pulse Oximetry
Pulse oximetry devices use several LEDs to measure pulse rate and blood oxygen content. The LEDs are tuned to specific wavelengths corresponding to the absorbance bands 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.
This project examines the practicality of a fabricatable pulse oximeter. A variety of approaches will be considered, but preference is given to designs that are not dependent on manufacturer-specific sensors and proprietary algorithms.

Status
An initial prototype apparatus has been fabricated and compared with a commercial version:

The apparatus uses an OpenMV board to watch the commercial device's display and exports the data over UART to the Teensy datalogger. Here is an SpO₂ log while holding my breath, showing a dip from 99% down to 91% over the course of a ~minute:

The apparatus also records raw analog sensor data from the commercial pulse oximeter, and compares it to data from the fabricated sensor. Here is a close-up of these two curves:

The blue plot, representing the commercial (Zacurate-brand) device uses more of the A/D converter's 0-3.3 VDC range, and doesn't have the slow rise time of the fabricated sensor (orange plot). In both lines, the fast pair of pulses show the photodiode response to the IR and red LED signal. Here is an early pass at an algorithm to calculate SpO₂ from the commercial device's signal (blue) compared to its displayed output (orange); the fabricated sensor is still a work-in-progress:

See the Background section for more information on how these sensors work, the Apparatus section for more information and code related to the first prototype, and my NMM page for a look at data processing.

Next Steps

iterate the sensor prototype

solve analog front-end issues: rise time, A/D bandwidth
improve mechanical design: stabilize finger clip and isolate from commercial device


improve algorithm

filtering, curve fitting, etc


Questions
Is there a need for a crude, distributed-fab coronavirus early warning device? Something that, rather than giving an absolute SpO₂ reading, provides a go/no-go indication relative to an initial condition. The standard for commercial pulse-ox devices is master calibration in a clinical trial using human subjects that deliberately enter a hypoxic state. Instead, what about a cheap indicator that simply indicates the change in SpO₂ since its first measurement, using a green-yellow-red indication rather than a digital display? Such a device could be provided to patients to wear continuously upon hospital discharge (or an asymptomatic positive covid-19 test result), and would give them an indication when they need to call their doctor due to plunging blood oxygen content. Crucially, the device would be cheap and would not require specialized components, so it could be made easily anywhere.

References

overview of pulse oximetry 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
the other two hemoglobins (MetHb and COHb)
signal artifacts: physical movement, signal:noise ratio, ambient light
calibration curve accuracy


lots of IP from Masimo Corp


https://patents.google.com/patent/US7280858B2/en (active thru 2025)

https://patents.google.com/patent/US6697656B1/en (exp 6/2020)

https://patents.google.com/patent/US6684090B2/en (exp)


earlier overview: Yelderman, Mark, and William New. "Evaluation of pulse oximetry." Anesthesiology: The Journal of the American Society of Anesthesiologists 59.4 (1983): 349-351.
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).