Pulse oximeters measure peripheral oxygen saturation (SpO2), which reflects the efficiency of the cardiopulmonary system in delivering oxygen-rich blood to the body. Athletes use SpO2 measurements to measure how hard they are working out, but this measurement has become even more important during the COVID-19 pandemic. Healthcare workers are concerned about the drop in SpO2 because it is an early warning sign of damage to lung tissue from the SARS-CoV-2 virus that causes COVID-19.
For people with mild symptoms who have been asked to isolate at home, having a low-cost pulse oximeter readily available would help diagnose the condition and get the warnings needed to seek medical care promptly.
This article briefly discusses the symptoms of COVID-19 and the need to monitor SpO2. It then shows developers how to use a Microchip Technology digital signal controller (DSC) and a few additional components to design a low-cost pulse oximeter to provide early warning to home users of late-stage symptoms of COVID-19 infection.
COVID-19 and the need to measure blood oxygen saturation
Due to the destructive effects of the SARS-CoV-2 virus, COVID-19 presents with various symptoms. For healthcare workers, a particularly worrying symptom is related to lung tissue damage, which can lead to compromised respiratory systems and reduced oxygen uptake. Although doctors will use chest X-rays and computed tomography (CT) scans to confirm this stage of COVID-19, they tend to use SpO2 measurements as an early indicator.
Arterial oxygen saturation (SaO2) can be directly determined by drawing a sample from a patient's artery and analyzing blood gas levels, while SpO2 measurement is a non-invasive alternative. Although direct measurement of arterial blood gases may be required in some cases, SpO2 measurements have been shown to provide reliable estimates of SaO2. Perhaps most importantly, optical pulse oximeters can be used at home to measure as reliable as in a clinical setting.
Optical pulse oximeters use the difference in light absorption between deoxyhemoglobin (Hb) and oxyhemoglobin (HbO2) to measure SpO2. In oxygen-enriched lungs, hemoglobin in red blood cells can rapidly reversibly bind up to four oxygen molecules to generate HbO2, which absorbs light at a wavelength of 940 nm more than light at a wavelength of 660 nm.
When red blood cells carrying HbO2 pass through peripheral blood vessels, the partial pressure of blood oxygen (the pressure of a single gas component in the mixed gas) is low, the affinity of hemoglobin for oxygen decreases, and HbO2 begins to release oxygen molecules, which eventually become Hb. In this deoxygenated state, the molecule's absorption spectrum changes, with a higher absorption rate for light at a wavelength of 660 nm than for light at a wavelength of 940 nm.
Because HbO2 becomes Hb when the partial pressure of oxygen in the blood is low, SpO2 can be determined with a simple formula:
SpO2 = HbO2 / (HbO2 + Hb)
The relative concentrations of Hb and HbO2 in blood can be determined by measuring the absorption of light at wavelengths of 660 nm and 940 nm.
Pulse oximeters use the relationship between partial pressure of oxygen, hemoglobin oxygen binding state, and light absorption differences to provide reliable SpO2 measurements.
Main subsystems of a typical pulse oximeter
A typical pulse oximeter design consists of three main subsystems:
Light delivery subsystem, including analog switches and drivers, and light emitting diodes (LEDs) that emit red (660 nm wavelength) and infrared (IR) (950 nm wavelength) light. Some systems also include a green (530 nm wavelength) light source, which can be used with photoplethysmography (PPG) to determine heart rate by monitoring changes in the volume of blood vessels in the skin.
Light detection subsystem, including photodiode, signal conditioning chain, and analog-to-digital converter (ADC).
· DSC or microcontroller to coordinate light transmission and light detection subsystems and calculate SpO2 from measured data.
While these basic subsystems are present in any pulse oximeter, their implementation can vary widely. For transmission pulse oximeters, photodiodes and LEDs are placed on either side of the user's finger or earlobe, respectively. A common finger clip device has red, infrared, and green (optional) LEDs on one side and a photodiode on the other. For reflective pulse oximeters, both the photodiode and LED are placed on the same side of the skin, with an optical barrier between them to reduce artifacts. For example, OSRAM's SFH7060 is a ready-to-use reflective measurement device that contains an LED and photodiode in a single package measuring only 7.2 x 2.5 x 0.9 mm.
Whether using these optically packaged devices for transmissive or reflective measurements, designers can add relatively few components to a low-cost pulse oximeter design that can be used for home users. Provide relevant information and advise on the need for further evaluation by a healthcare professional. In a design example built on Microchip Technology's DSPIC33FJ128GP802 DSC, the microcontroller's integrated peripherals are used to control the illumination of the skin by red and infrared LEDs, and the conditioned photodiode output signal is then digitized.
Regardless of the light source type, pulse oximeter designs typically rely on a single photodiode with a broad spectral response range to capture transmitted or reflected signals. To ensure that the received signal corresponds only to red or infrared wavelengths, the hardware or software control logic turns on only the red or infrared light source for a given time, and the two light sources alternately emit light to complete the measurement sequence.
Implementing low-cost pulse oximeter hardware design
In this design, the DSC uses an external Microchip Technology MCP4728 digital-to-analog converter (DAC) to put individual MBT2222 transistors at the desired level to drive each LED at the desired brightness. To precisely time the "on" sequence of each LED, the DSC uses two of the pulse width modulation (PWM) outputs to control Analog Devices' ADG884 analog switches.
To handle the photodiode output, a single Microchip Technology MCP6002 device provides a pair of op amps to implement a basic two-stage signal conditioning chain. Here, the first stage uses one of the MCP6002's op amps, configured as a transimpedance amplifier, to convert the photodiode's current output to a voltage signal. After a high pass filter to reduce noise, a second op amp in the MCP6002 provides gain and DC offset adjustments to optimize the swing of the conditioned signal over the full range of the DSC's integrated ADC.
In operation, the DSC uses its PWM output and ADC input to synchronize the LED illumination and ADC digitization of the conditioned photodiode output signal. In this way, each alternating cycle of red and infrared light illumination is coordinated with signal acquisition and conversion. With both LEDs off, an ADC sample is also taken to measure ambient light, which can be used to optimize LED brightness and SpO2 measurements. The end result is a precisely controlled sequence of events, coordinating LED illumination with ADC digitization to capture red wavelength results representing Hb, then ambient light, and finally infrared wavelength results representing HbO2.
Implementing an interrupt-driven software solution
The pulse oximeter firmware package provided by Microchip comes with a sample program that demonstrates how to use the DSC to perform these irradiation control and data conversion sequences. Here, the program implements the interrupt-driven approach using a pair of DSC timers (Timer2 and Timer3) for the respective "on" sequences of the IR LED and the red LED. In this way, each timer in turn provides the time base for the DSC's two output compare (OC) blocks (OC1 and OC2), which are used to control the analog switches for the infrared and red LEDs, respectively.
