Page 32 - EngineerIt August 2021

MEASUREMENT

How to design a better pulse oximeter

Robert Finnerty, Systems Applications Engineer

It is more important than ever to design medical devices that are more convenient and less
power hungry. This article covers the fundamentals of SpO2 measurement and demonstrates
how a new generation of optical analogue front ends (AFEs) can help create a better oximeter.
The new devices can have reduced design complexity, a reduced burden on the mechanical
design and decreased power consumption.

Introduction The urgent need for a better pulse oximeter now
Traditionally, peripheral blood oxygen saturation (SpO 2) is a and in the future
measurement taken at the peripherals of the body on the finger As patient care trends towards ambulatory and in-home
or ear, most commonly with a clip device to determine the monitoring, there is a need to develop vital sign monitoring
ratio of oxygen saturated haemoglobin to total haemoglobin. devices that will not impede users from completing daily tasks. In
This measurement is used to tell how well red blood cells the case of SpO 2, monitoring areas other than the finger and ear
are transporting oxygen from the lungs to other parts of the will present a host of design challenges. The recent emergence of
body. Normal SpO 2 levels vary from 95% to 100% in a healthy silent hypoxia makes the case for development of more portable
adult. Levels below this range indicate a condition known as clinical-grade pulse oximeter units even more compelling.
hypoxemia. This means that the body is not transporting enough This article will explain some of the fundamental principles
oxygen to maintain healthy organs and cognitive function. of SpO 2 measurement and introduce ADI’s latest generation of
A person suffering from hypoxemia may experience optical AFEs, the ADPD4100 and ADPD4101, which reduce
dizziness, confusion, shortness of breath and headaches. design complexities for medical grade SpO 2 devices. Built-in
Several medical conditions can cause poor blood oxygenation high performance automatic ambient light rejection reduces
and may require continuous or intermittent monitoring at home the burden on mechanical and electronic design. The high
or in a clinical setting. dynamic range in the ADPD4100 at lower power consumption
SpO 2 is one of the most common vital signs recorded within reduces the number of photodiodes or LED current in a design
a clinical setting. Some conditions that require continuous SpO 2 to determine slight variations in patient SpO 2 level efficiently.
monitoring include asthma, heart disease, COPD, lung disease, Finally, digital integrator options allow users to enter an
pneumonia and COVID-19 induced hypoxia. extremely efficient power consumption mode to enable longer
One of the ways to determine whether symptomatic run times in portable PPG solutions by disabling analogue
COVID-19 patients need hospitalisation is by monitoring blocks in the optical signal path.
their SpO 2 levels. If those levels fall below the baseline
number (usually under 92%), they need to be checked into an What Is oxygen saturation?
emergency room. Oxygen saturation is the percentage of oxygen saturated
haemoglobin within the blood with respect to the total available
The recent link between COVID-19 and hypoxia haemoglobin. The gold standard for measuring oxygen
Very recently, COVID-19 patients have been diagnosed with a saturation is the atrial blood oxygenation measurement, SaO 2.
particularly insidious condition known as silent hypoxia. This However, this method requires laboratory-based blood gas
can do severe damage to the body before any of the typical analysis of a blood sample. The calibration section covers this in
COVID-19 respiratory symptoms, like shortness of breath, occur. greater depth.
An article on the National Centre for Biotechnology Information SpO 2 is an estimate of the oxygen saturation levels measured
website states: “The ability to detect this silent form of hypoxia in at the peripherals of the body, using a pulse oximeter. Until
COVID-19 patients before they begin to experience shortness of recently, the most common way to measure oxygen saturation
breath is critical for preventing the pneumonia from progressing has been to use a pulse oximeter positioned on the finger.
to a dangerous level.”
SpO 2 monitoring is also a key indicator in diagnosing How does a pulse oximeter work?
sleep apnoea. Obstructive sleep apnoea causes the airways A pulse oximeter works on the principal that absorption of light in
to become partially or fully blocked during sleep. This can be oxygenated haemoglobin (HbO 2) and deoxygenated haemoglobin
observed as long pauses in breathing or periods of shallow (RHb) differ significantly at specific light wavelengths. Figure 1
breathing causing temporary hypoxia. If untreated over time, shows the extinction coefficient of HbO 2, Hb, and methaemoglobin
sleep apnoea can increase the likelihood of heart attack, stroke (MetHb) across the visible and infrared light spectrum. The
and obesity. It is estimated that sleep apnoea affects between extinction coefficient is a measurement of how strongly a
1% to 6% of the total adult population. chemical substance absorbs light at a given wavelength.

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