Medical Electronics


Implementing heart rate variation measurement in a wristband: a small, robust and accurate optical solution


Peter Trattler and Max Rosengarden, ams, describes the technical challenges involved in reliably capturing continuous heart rate variation measurements with a wristband, and compares its performance with a reference standard set by an ECG chest strap


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onitoring the behaviour of a person’s heart can reveal immensely valuable information about their health, lifestyle and even emotional state, as well as the early onset of heart disease. In a medical setting, this monitoring can readily be performed with equipment dedicated to the task: heart patients in a hospital, for instance, are routinely fitted with an electro-cardiogram (ECG) chest strap. This device accurately records the electrical pulses emitted by the heart muscle as it contracts with each heart beat.


An ECG chest strap, however, is cumbersome, uncomfortable and expensive, which restricts its use to medical settings and high-end sports equipment. This means that continuous, 24/7 heart monitoring is not today readily available to consumers outside the confines of a hospital. Such 24/7 heart monitoring could be of benefit, of course, to patients with a chronic heart condition, but equally to anyone who might want to use the


information that their heart rate, and particularly the variations in their heart rate, could tell them about their state of physical and mental health. But how might it be possible to capture ECG-like data about heart rate variability in a convenient, easily wearable device?


This was the question that ams set out to answer with its new AS7000 bio-sensor, an optical module which is small and thin enough to fit in popular wearable form factors such as smart watches and fitness bands. Highly integrated, the AS7000 enables designers to realise a simpler, more comfortable and cheaper implementation of HRM in a wristband than previous systems using multiple discrete components.


Medical value of HRV measurement


In heart rate monitoring, the primary measurement is of the average heart rate, normally expressed in beats per minute. This is known as heart rate measurement,


Figure 2: The peak of the PPG signal at each heart beat is flatter and less well defined than that of the ECG signal. The x axis shows the time in seconds


good thing). In fact, the pattern of a person’s HRV is strongly affected by factors such as their emotional state, heart health, and sleeping state, so HRV measurements are valuable to those interested in long- term lifestyle and health monitoring, as well as to clinical applications.


The ECG method of measuring heart activity via electrodes on the body senses the electrical stimulus generated from the central nervous system to make the muscles inside the heart contract. By measuring the interval between each electrical pulse, an HRV reading can be captured.


Figure 1: The ECG signal shows a sharp spike at each heart beat. The x axis shows the time in seconds


26 May 2017 Components in Electronics


As the heart beats, a pressure wave ripples through the blood vessels. This wave slightly changes the diameter of the blood vessel. This enables the implementation of an alternative method of HRM, called photoplethysmography (PPG). The principle of PPG is that the contraction and dilation of blood vessels at each heart beat affect the transmission of


have a very close correlation ( I > 0.97). Reflective PPG, then, seems to enable the implementation of pulse rate and pulse rate variation measurements in a wristband. This is the application for which the AS7000 was developed: it is a module combining two LED light sources, a highly sensitive photo-diode (light sensor), an analogue front end (AFE) for signal conditioning and amplification, and a small embedded processor which converts the raw reflected light signals into digital pulse rate measurements. The nature of the signal produced by


reflective PPG, however, makes the measurement of pulse rate variation particularly difficult. Figure 1 shows an ECG signal obtained


using three limb-clamp electrodes; Figure 2 shows an optical PPG signal obtained on the wrist. The spike in an ECG signal is called an R-wave. To measure heart rate variability, the sensor measures the time between the peaks of the R-waves (called


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or HRM. An important secondary measurement, however, is the variation of this rate within a sampling interval, known as heart rate variation (HRV). Just as an acoustic engineer measures the purity of a test tone through a speaker, cardiologists too are interested in the purity and uniformity of a heart rate (although it should be said that, in the case of the heart beat, a uniform rate is not necessarily a


light (for instance through the fingertip in transmissive PPG) or the reflections of light (for instance from near the surface of the wrist in reflective PPG). Of course, PPG measures the pulse rate and pulse rate variation (PRV), rather than the electrical activity directly at the heart. Nevertheless, both PRV and HRV quantify the fluctuations of a signal over time, and medical research shows that HRV and PRV


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