Commit 56a4ddf1 authored by Zach Fredin's avatar Zach Fredin


parent 44812302
## 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 SpO<sub>2</sub> 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. 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](file:///home/zach/Documents/NMM/site/people/zach/final.html) 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 SpO<sub>2</sub> 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 SpO<sub>2</sub> 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
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