CARDIAC OUTPUT IS commonly monitored by methods like invasive intra-cardiac catheterization and non-invasive Doppler ultrasound anatomy. Yet catheter insertion invites both risk and side effects, while echocardiography requires equipment that is typically bulky and controlled by skilled operatorsthereby increasing cost. Such issues, coupled with advances in wireless communications, have prompted the use of wearable healthcare sensor systems. At the Korea Advanced Institute of Science and Technology, a low-power, sensitive, Thoracic Impedance Variance (TIV) and electrocardiogram (ECG) monitoring system-on-a-chip (SoC) has been developed by Long Yan, Joonsung Bae, Seulki Lee, Taehwan Roh, Kiseok Song, and Hoi-Jun Yoo.
The team implemented this SoC in a poultice-like plaster sensor. With the help of a balanced, sinusoidal current source with a quality factor above 30 and low-noise, reconfigurable readout electronics, they found 0.1-Ω TIV detection to be possible with sensitivity of 3.17 V/O and a signal-to-noise ratio beyond 40 dB. To remotely start and stop the SoC, they used centimeter-range 13.56- MHz fabric inductor coupling. In order to achieve 0.2 nJ/b, 1-Mb/s energy-efficient external data communications, they relied on 5% duty-cycled body-channel communication.
This SoC comprises five functional blocks: a system startup module; four reconfigurable electrode-sensor front ends; a differential sinusoidal current generator; a digital module; and a duty-cycled body-channel transceiver. The SoC forms a compact, wearable sensor because it is integrated on a fabric circuit board along with a flexible battery. A total of 25 adhesive, screen-printed fabric electrodes allow TIV and ECG detection at 16 different sites around the heart. The base station can be located in any position on the body. In body-channel receiver mode, the CMOS-based SoC dissipates peak power of 3.9 mW. It consumes 2.4 mW when operating in TIV and ECG detection mode. See "A 3.9 mW 25- Electrode Reconfigured Sensor for Wearable Cardiac Monitoring System," IEEE Journal Of Solid-State Circuits, Jan. 2011, p. 353.