Microphonics in biopotential measurements with capacitive electrodes

Abstract

Abstract-Biopotential measurements with capacitive electrodes do not need any direct contact between electrode and skin, which saves the time devoted to expose and prepare the contact area when measuring with conductive electrodes. However, mechanical vibrations resulting from physiological functions such as respiration and cardiac contraction can change the capacitance of the electrode and affect the recordings. This transformation of mechanical vibrations into undesired electric signals is termed microphonics. We have evaluated microphonics in capacitive ECG recordings obtained from a dressed subject seated on a common chair with electrodes placed on the front side of the backrest of the chair. Depending on the softness of the backrest, the recordings may be clearly affected by the displacement of the thorax back wall due to the respiration and to the heart's mechanical activity. I. INTRODUCTIONONVENTIONAL surface biopotential measurements require the exposure and preparation of the skin areas where electrodes are to be placed, and/or the use of an electrolyte. Dry-electrodes do not need any electrolyte nor skin preparation but still need the exposure of the body contact areas. Off-body biopotential measurements have been tried [1], [2] that do not need any contact with the body, but the actual origin of the recorded signals is disputed because electrostatic charges and distance variations between the body and the electrodes can result in signals larger than surface biopotentials, hence not attributable to them [3], [4].Strapping electrically insulated metal electrodes to the body implies a mechanical contact that establishes a fixed distance between the body and the metal part of the electrode, thus offering more reliable signals [5]. An alternative technique is to detect bioelectric signals through the subject's clothes by using electrodes placed on an object that comes into contact with his/her body [6], [7], [8]. A capacitor is formed between the skin and the electrode metal, and the electrically-insulating clothes become the capacitor's dielectric. This approach frees the user from servitudes such as battery replacement in portable devices and avoids any cumbersome electrodes or wires. However, distance variations between capacitor plates (the skin and the electrode) can result in artifacts [9]. Those distance variations can result from relatively large voluntary movements but also from physiological movements such as those related to the breathing and heart activity. In this work we analyze the origin, relevance and reduction of those physiologically-induced movement artifacts. II. MICROPHONICS IN MEASUREMENTS WITH CAPACITIVE ELECTRODESMicrophonics is any electrical interference caused by mechanical vibration. When a metal electrode is placed on a seat's backrest and a dressed person comes into mechanical contact with it, thoracic movements due to respiration and heart contractions may change the actual distance between the body and the electrode (see Fig. 1). In each cardiac cycle, atrial and ventricula...

A differential capacitive electrical field sensor array for contactless measurement of respiratory rate

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Motion Artifacts in Capacitive ECG Measurements: Reducing the Combined Effect of DC Voltages and Capacitance Changes Using an Injection Signal

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Assessment of trends in the cardiovascular system from time interval measurements using physiological signals obtained at the limbs

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