By Bryce Beamer
Electrocardiography, often abbreviated as ECG or EKG, debuted in the early 1900s and remains critical technology for diagnosing and monitoring heart conditions. Hardware innovation has reduced the size and cost of ECG technology, opening new opportunities to unlock health and wellness insights based on ECG waveforms and heart rate variability. When combined with wearable IOT infrastructure and AI, ECG heart monitoring may be the key to unlocking new insights that support lifelong cardiac health.
How many electrodes are used for an ECG?
ECG creates a current sensing circuit, measuring the differential charge between two or more electrodes placed on the body. This illustration shows how ECG electrode placement gives three different ECG waves based on the directionality of the lead.
A simple single lead (two electrode) ECG is adequate for measuring heart rate and heart rate variability in sports and wellness applications. Placing additional electrodes in the proper locations gives a more thorough view of the heart's function. In advanced medical diagnostics, ECG systems have up to 10 electrodes that can provide 12-18 leads that help doctors assess the heart’s function and structure to diagnose complex arrhythmias.
How do ECG electrodes work?
Signal quality in a current sensing circuit is essential in any cardiac monitoring device to capture clean data that can lead to actionable insights. In the case of ECG, this all starts with the electrode design. Electrode coupling is critical to reducing noise and preserving the ECG wave.
As ions from the body's ionic current reach the electrolyte on an electrode, they are involved in redox reactions at the electrode surface. In these reactions, electrons transfer from the ions to the electrode, converting the ionic current into an electrical current. This is enabled by a phenomenon known as the Helmholtz double layer.
An ECG signal is typically only 0.5mV to 1mV in amplitude, making it very susceptible to different types of noise or distortion. Motion artifacts are the most common cause of noise, stemming from a sensor moving or sliding on the surface of the skin, the skin stretching or changing interface area, or a change in the distance or total disconnection between the electrode and skin. A patient simply changing position can potentially cause motion artifacts.
To mitigate these challenges, electrode design must be carefully considered. Materials, shape, size, and adhesion methods all play a role in minimizing motion artifacts and other noise sources. In medical applications, sticky electrodes are used to prevent electrode motion; however, wearable ECG monitors and garment-based electrodes traditionally used in sports and fitness applications may improve placement accuracy and enhance comfort, resulting in higher compliance.
Types of ECG electrodes
The two main classes of electrodes are “wet” and “dry” electrodes. Both types functionally perform the same; however, a wet electrode includes an electrolyte that interfaces with the skin. In contrast, a dry electrode uses the skin’s moisture and sweat to create the required ionic conductor.
The ionic conductor is essential for forming the Helmholtz layer described above with either type. Without an adequate electrolyte layer, signal noise and impedance issues will dramatically decrease the signal-to-noise ratio. Variabilities in human skin, such as body hair, lotion, skin treatments, and dead skin, can also introduce contaminants, increase impedance, or impact electrolyte function.
Using a wet electrode is one way to ensure a good electrolyte layer and quickly create a low-impedance skin-electrode interface. However, over time, the electrode can dry out, causing unpredictable signal amplitude decay. A sticky electrode — where a foam adhesive encapsulates an electrolyte gel between a metallic snap (Ag AgCl) and the skin — is the most common form of a wet electrode.
Dry electrodes offer convenience and comfort to the user. They come in many form factors, such as carbon-doped polymer films, metalized fabrics, or screen-printed conductive links. However, in many cases, they require some time to build up a suitable electrolyte interface between the skin and electrode. This process leads to a phenomenon called startup noise, where signal noise masks the ECG wave.
In wet or dry electrode applications, the electrolyte layers can dry out and create dynamic changes in impedance that dramatically impact the amplitude of the signal but not the period. For this reason, if the signal-to-noise ratio is low enough to detect the necessary features, the amplitude can be dynamically compensated for in many applications.
ECG system design takeaways
Choosing the appropriate electrode and embodiment is crucial in designing ECG systems. However, it is best to outline the system's requirements for any new applications by first identifying the data insights required in the product's value proposition. Combining these baseline technological requirements with the needs of the users and care providers creates a holistic view of the product requirements.
Developing the next generation of ECG products will help unlock new insights into cardiac health, but it will require finding the balance between usability and technical performance.
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