Low-Frequency Response and the Skin-Electrode Interface in Dry-Electrode Electrocardiography

In recent years, there has been a growing interest in the area of ambulatory electrocardiogram (ECG) recording using dry or unjelled electrodes for long-term physiological monitoring. The key advantage of dry electrodes is the elimination of allergic reactions or other forms of skin irritation, commonly associated with electrolyte gels. It results in the improvement of patient comfort and compliance, allowing the recording technique to cater for a wider range of users such as elderly, the long-term ill, cardiac rehabilitation patients, paediatrics and neonates. Furthermore, dry-electrode recording does not require preparation of the electrodes before or after application apart from cleaning and they can be re-used almost indefinitely. The durability of dry electrodes over gel-based ones permits their shelf-life to be extended and considerably increases the length of time for which they can be worn, allowing long-term ambulatory ECG recording at much lower cost. Embedded in remote telemetry systems, dry-electrode ECG recording can thus contribute to the improvement of health care delivery. The investigation of the use of dry electrodes for ECG monitoring has led to the development of several pasteless electrode systems which overcome the disadvantages associated with traditional approaches employing wet electrodes. The following question however was immediately raised: how should the recording amplifier be adapted to the high source impedance commonly associated with dry electrodes? Optimised designs of the amplifier front-end have usually involved measuring the impedance of the skin-electrode interface (Burke & Gleeson, 2000; Chang et al., 2010; Ko et al., 1970; Muhlsteff & Such, 2004; Valverde et al., 2004). Some solutions have then inserted resistors in series with unbalanced electrodes to match the effective impedance seen at each input of the recording amplifier (Lee et al., 2006). Others have fabricated dry electrodes having impedances lower in magnitude than those of conventional Ag/AgCl wet electrodes (Chang et al., 2010; Wolfe & Reinhold, 1974). Commercial dry-electrode Holter monitors providing diagnostic quality ECGs are however not available to date. The recent development in 2009 of a wearable two-channel dry-electrode ECG system called care.mon has shown some prospects in the realisation of long-term telemetric application in the near future (Fuhrhop et al., 2009). The designers have admitted, however, that their prototype cannot get a signal of the same quality as that of a standard electrode Holter system. A critical source of error was soon identified as low-frequency distortion introduced at the amplifier’s front-end. In this chapter, the authors show how high-pass filtering can affect the quality of the recorded ECG waveform and demonstrate that the risk of distortion is 2