Advances in semiconductor technology have made it possible for nearly all human body signals—even those traditionally monitored in a clinical environment—to be measured and recorded by a wearable device (Figure 1). Some of the most important signals to be measured are those of the cardiorespiratory (heart and lungs) system. Designers of wearables are challenged by the need to reduce size and power consumption as much as possible to extend battery life. While there are several ways in which heart and respiration measurements can be made, two of the lowest power techniques measure the heart’s electrical signals (biopotential) and the resistance of the thoracic cavity (bioimpedance).
In this design solution, we review the concepts of biopotential and bioimpedance and the challenges encountered when measuring both in a wearable device. We then introduce an innovative approach that significantly simplifies how these measurements are made while reducing both device size and power consumption.
Medical Monitoring Using a Wearable Device
Read more about wearables here: Wearables for a healthier world: Technology in action
Biopotential and Bioimpedance
Biopotential measurements require placing two or more electrodes in contact with the skin of a patient’s body to detect the small electrical signals generated by the heart. The signals are then conditioned and sent to a microprocessor for storage, calculation and/or display. Three important cardiac parameters are recorded using this technique: ECG, R-R intervals and PACE detection. An electrocardiogram (ECG or EKG) is the measurement and graphical representation, with respect to time, of the electrical signals associated with the heart muscles. The R-R interval is the time between the peak amplitudes of the heart’s periodic electrical signal, also known as R peaks (Figure 2).
R-R Interval in a Typical ECG Waveform
ECG and R-R measurement can be used for heart-rate monitoring to assist in the diagnosis of specific heart conditions, such as arrhythmias. However, these conditions can be difficult to diagnose because they do not always present themselves in a clinical environment. Wearable devices provide medical professionals with the ability to monitor patients over an extended period, outside the hospital environment. This provides them with more information to assist detection and diagnosis.
In addition to the biological signal from the heart itself, an ECG must also detect a separate man-made signal in patients with implanted pacemakers. This signal is referred to simply as "PACE." The PACE signal is relatively short—tens of microseconds to a couple of milliseconds—with an amplitude ranging from a few millivolts to nearly a volt. An ECG must detect the presence of a PACE signal while simultaneously preventing it from distorting the signals from the heart.
Bioimpedance (also referred to as BioZ) is the response of a living organism to an externally applied electric current. It is a measure of the opposition to the flow of that electric current through the tissues via two electrodes placed in contact with the skin. It is used in the measurement of respiration rate and hydration levels in a patient.