Evaluating a Capacitively Coupled, Noncontact Electrode for ECG Monitoring

December 1, 2005 By: Jaime M. Lee, Frederick Pearce, Craig Morrissette, Andrew D. Hibbs, Quasar Geophysical Technologies, Robert Matthews Sensors

The capacitively coupled noncontact electrode (CCNE), a new device that measures electrocardiogram (ECG) interbeat intervals through clothing, underwent a 40-person human trial at the Walter Reed Army Institute of Research (WRAIR) this past year. This sensor can detect ECG and respiratory signals through clothing and is being considered by the U.S. Army as a physiological monitoring detection sensor on the Future Force Warrior System uniform and for en route care to monitor wounded soldiers during evacuation. In this article, we compare three CCNE sensors to an FDA-approved monitor (using contact electrodes) to determine if the R-R interbeat intervals (the time between two consecutive R waves of the cardiac cycle and used to determine heart rate) measured by the two methods are the same or not. We present the results from determining heart rate based on interbeat intervals of the ECG while the test subject is at rest in a supine position.

Background and Rationale

Various attempts have been made to overcome the limitations of wet electrode technology for measuring bioelectric signals such as ECG and electroencephalogram (EEG) on the human body. Advances include a class of surface electrodes that doesn't use electrolytes [1]; these are referred to as active electrodes and use an impedance transformation at the sensing site via active electronics. Active electrodes are subdivided into two types: dry electrodes, which rely on a metallic surface in direct contact with the test subject and use a combination of resistive and capacitive coupling to the local skin potential; and insulated electrodes, which use only capacitive coupling [2].

In 2001, researchers at Quantum Applied Science and Research (QUASAR) developed a new class of sensor that measures the electric potential in free space, i.e., without physical contact to any object. These sensors were able to measure the ECG of a fully clothed person standing within a range of about 25 cm. In 2002, QUASAR developed a compact version of the sensor and named it the capacitively coupled noncontact electrode (CCNE), specifically to measure ECG through clothing (see Figure 1). This sensor could also be mounted in a litter configuration, eliminating the need for contact electrodes and providing early monitoring of wounded soldiers during evacuation.

For the first human trial of this new electrode technology, our goal was to compare the CCNE operating through regular clothing with a conventional 3-lead ECG using resistive electrodes to measure interbeat intervals.

The principal application envisioned for this technology is continuous monitoring of the ECG of military personnel as part of the Warfighter Physiological Status Monitor (WPSM) to be contained within the Future Force Warrior (FFW) soldier ensemble. FFW is the Army's flagship science and technology initiative to develop and demonstrate revolutionary capabilities for Future Force soldier systems. The many problems associated with contact electrodes for long-term ECG monitoring include loss of contact to the test subject due to drying of application glue or to environmental factors (e.g., rain) as well as test subject resistance to wearing the electrodes due to discomfort (e.g., skin irritation). Therefore, a noncontact ECG system would be of great benefit to the Army. Integrated into the FFW uniform, a noncontact system would provide not only simple heart-rate changes and interbeat interval variability, but with sufficient waveform fidelity, could also monitor arrhythmias and conduction abnormalities associated with hypotension and cardiac ischemia. A combat medic armed with the information from such a system could properly ascertain the level of injury and the survival potential of the injured and provide optimum care in the battlefield and during en route care from the battlefield.

 Figure 1. This photo shows the capacitively coupled noncontact electrode (CCNE) used in this study. This first version of the ECG electrode, including all amplification electronics, is ~1 in. square and 0.35 in. thick.
Figure 1. This photo shows the capacitively coupled noncontact electrode (CCNE) used in this study. This first version of the ECG electrode, including all amplification electronics, is ~1 in. square and 0.35 in. thick.

Methods and Materials

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