A Smart Idea to Reject Motion Artefacts from ECG Measurements Due to Sensor-Body Impedance

Abstract

Electrocardiogram (ECG) records the electrical activity of the heart and is the most significant diagnostic indicator of health. The electrical activity propagates towards the skin and is typically measured using gel silver/silver chloride (Ag/AgCl) electrodes. The quality of the measured signal depends on the amplitude of the electrical activity, the conductivity of the tissues and skin, and that of the measuring electrode. Gel electrodes can cause skin irritability and are not suitable for long-term measurements requiring the diagnosis of paroxysmal arrhythmias. Dry electrodes are comparatively comfortable to wear but are highly dependent on body hair, sweating, and movement. Non-contact electrodes on the other hand avoid ohmic contact with the skin by providing a capacitive coupling between the skin and the electrode. They can be easily worn over clothing or can be measured from a distance. However, motion artefact remains the biggest challenge in such systems. This study proposes a novel two-channel circuitry for ECG acquisition along with a mathematical model to mitigate the effect of changing sensor-electrode capacitance in non-contact sensors. This method was modeled and validated at different sensor-electrode positions or air-gaps. The effect of change in mutual and parasitic capacitance between the sensor’s channels, body tissue impedance, and the ratio of the areas of the two channels was also analysed in terms of Signal to Noise Ratio (SNR). The highest SNRs were achieved with smaller air-gaps and higher ratios of channels' areas.