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pulseox.md 2.91 KiB

Pulse Oxygenation

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

  • 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
      • 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
  • 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."

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 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.

pulseox1

pulseox2

pulseox3

Operational Theory

Pulse oximetry is based on the Beer-Lambert law, a principle that relates the concentration of a species to the attenuation of light through a sample:

I=Iin*e^-(D*C*E)

where I is the intensity of light transmitted through the sample; Iin is the intensity of the light prior to absorption by the sample; D is the optical path length; C is the solute concentration; and E is the extinction coefficient, the sample's absorption at a given wavelength of light.