Test & measurement How to design a better pulse oximeter
It is more important than ever to design medical devices that are more convenient and less power hungry. Here, Robert Finnerty, systems applications engineer at
Analog Devices, 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.
the finger or ear, most commonly with a clip device to determine the ratio of oxygen saturated haemoglobin to total haemoglobin. This measurement is used to tell how well red blood cells are transporting oxygen from the lungs to other parts of the body. Normal
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SpO2 levels vary from 95 per cent to 100 per cent in a healthy adult. Levels below this range indicate a condition known as hypoxemia. This means that the body is not transporting enough oxygen to maintain healthy organs and cognitive function. A person suffering from hypoxemia may
experience dizziness, confusion, shortness of breath, and headaches. Several medical conditions can cause poor blood oxygenation and may require continuous or intermittent monitoring at home or in a clinical setting. SpO2 is one of the most common vital signs
recorded within a clinical setting. Some
conditions that require continuous SpO2 monitoring include asthma, heart disease, COPD, lung disease, pneumonia, and COVID- 19 induced hypoxia. One of the ways to determine whether symptomatic COVID-19 patients need
hospitalisation is by monitoring their SpO2 levels. If those levels fall below the baseline number (usually under 92 per cent), they need to be checked into an emergency room.
THE REcEnT LInk BETwEEn cOVID-19 AnD HypOxIA Very recently, COVID-19 patients have been diagnosed with a particularly insidious condition known as silent hypoxia. Silent hypoxia can do severe damage to the body before any of the typical COVID-19 respiratory symptoms, like shortness of breath, occur. An article on the National Center for Biotechnology Information website states “the ability to detect this silent form of hypoxia in COVID-19 patients before they begin to experience shortness of breath is critical for preventing the pneumonia from progressing to a dangerous level.” SpO2
monitoring is 18 November 2021 Instrumentation Monthly raditionally, peripheral blood oxygen
saturation (SpO2) is a measurement taken at the peripherals of the body on
also a key indicator in diagnosing sleep apnoea. Obstructive sleep apnoea causes the airways to become partially or fully blocked during sleep. This can be observed as long pauses in breathing or periods of shallow breathing causing temporary hypoxia. If untreated over time, sleep apnoea can increase the likelihood of heart attack, stroke, and obesity. It is estimated that sleep apnoea affects between one to six per cent of the total adult population.
THE URgEnT nEED FOR A BETTER pULSE OxImETER nOw AnD In THE FUTURE As patient care trends toward ambulatory and in-home monitoring, there is a need to develop vital sign monitoring devices that will not impede users from completing daily tasks. In the
case of SpO2, monitoring areas other than the finger and ear will present a host of design challenges. The recent emergence of silent hypoxia makes the case for development of more portable clinical-grade pulse oximeter units even more compelling. This article will explain some of the
fundamental principles of SpO2 measurement and introduce ADI’s latest generation of optical
AFEs, the ADPD4100 and ADPD4101, which reduce design complexities for medical grade
SpO2 devices. Built-in high performance automatic ambient light rejection reduces the burden on mechanical and electronic design. The high dynamic range in the ADPD4100 at lower power consumption reduces the number of photodiodes or LED current in a design to determine slight variations in
Figure 2. Basic pulse oximeter circuit.
Figure 1. Extinction factor of light through haemoglobin.
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