Vital Care - 02.a SPO2 basics [PPG]

Title: Vital Care

By: Md. Kamrul Hussain

Project Category: Design Challenge

Project Name: Design For A Cause 2021

Blog post: 02

SpO2:

SpO2 is the measurement of percentage of oxygen saturation in blood. The basic principal is Photoplethysmography [PPG].

In this blog, I’ll share my experience on PPG experiment following traditional method. And then I’ll share how we can use the MAX30102 for this purpose in my next blog.

I worked with PPG before and posted a blog in the 'project 14' group. That time, I used RED / IR LEDs and Photodiodes for reflectance PPG and constructed the Analog Front End using general purpose op-amp as transimpedance amplifier and filter circuitry.

Before getting into the experiment, I would like to share some background study [theories] for those who wants to work on PPG from scratch.

Photoplethysmography:

"Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system."

-source: https://www.ncbi.nlm.nih.gov/pubmed/17322588

The working principle is pretty simple. A red LED is used to pass light through finger tip and a photodiode is used to receive the reflected light. This contains both the DC and pulse AC components in terms of photo-current. A transimpedance amplifier converts the signal into voltage and amplifies it. Filters are used to get rid of the DC component and the remaining AC component is further amplified to observe using oscilloscope or send to PC using a DAQ system. The PPG signal represents the heart beat, which can be captured at finger tip and takes a small time gap. A simultaneous comparison of ECG and PPG explains the relation and time gap between arrival.

**images are collected from internet [https://www.comm.utoronto.ca/~biometrics/PPG_Dataset/ ]

Block Diagram -

Key Analog Modules -

• Light source
• Transimpedance amplifier
• Basic High and Low Pass filters

Sensor-

The sensor module consists of a RED LED and photodiode. LED intensity can be varied using an onboard potentiometer.

Figure: sensor module with AFE

Transimpedance Amplifier and signal conditioning-

The TIA converts the photodiode’s current output signal to a usable voltage level. The implementation of this current-to-voltage conversion consists of a photodiode, an amplifier and a resistor/capacitor feedback pair.

The output signal is passed through a High Pass filter [0.03Hz] and then to a Low Pass filter [cut off ~15Hz] to produce a clean PPG signal which can be observed using oscilloscope or DAQ. But an opto-isolator can be very useful to get a noise free output. Without an opto-isolator, oscilloscope / DAQ ground connects to the AFE supply and produces distorted signal. I have used batteries to power the AFE, so that i can avoid noise at the input side.

Output:

using oscilloscope-                                                                      using USB DAQ developed by BMPT, University of Dhaka

Pulse Oximetry using MAX30102 :

Pulse oximetry is a noninvasive method of measuring an individual’s blood oxygen saturation levels. Oxygen saturation levels, referring to the ratio of oxygenated hemoglobin to total hemoglobin in the blood, can aid in detection of hypoxemia. There is an inherent heart-rate signal associated with pulse-oximetry measurement. Usually two different light source of RED and IR spectrum are used and light absorption is measured with a photodiode. The change in absorbance of each wavelength determines oxygen saturation levels following Beer-Lamber law.

The MAX30102 is an integrated pulse oximetry and heart-rate monitor module. It includes internal LEDs, photodetectors, optical elements, and low-noise electronics with ambient light rejection.

It follows reflective pulse oximetry where the photodiodes are placed on the same side of the test site as the LEDs. The LED light illuminates the skin while the reflected signal is monitored for changes in light absorption.

Red and IR LEDs are used to determine separate PPG signals. As the DC components and AC components of the two LEDS have different amplitudes, they must be normalized to make useful comparisons. For this comparison, a ratio ‘R’ is determined, the ratio of oxygenated hemoglobin to the deoxygenated hemoglobin. which is directly proportional to SpO2.

In my next blog, I'll share the configuration of the MAX30102 and it's PPG output. Arduino NANO 33 IOT will be used to capture the PPG signals and calcualte Heart Rate and SPO2 from this module.