We have developed a monitoring system. The system comprises of an 8-electrode belt, designed to be worn on a transverse plane around the thorax, with an additional reference electrode to be attached on the waist for minimizing baseline drifts. A current source circuit generates a 3mA, 20kHz current, which is injected through a switch matrix to the body in an opposite configuration. For each injection, five differential voltages are measured for a period of 6msec in the 4-electrode method and amplified. These bio-impedance measurements are filtered using a BPF with a bandwidth of 1kHz around a central frequency of 20kHz, and sampled in a frequency of 200kHz with a resolution of 12 bits using an A/D card (National Instruments 6025E). The system also measures the ECG signal, using two of the eight electrodes, for a period of 5 seconds. The ECG signal is filtered using a BPF in the range of 5-30Hz and sampled in 1kHz. The measurement sequencing, which is controlled by a microprocessor, is performed in the following order: a 5 seconds ECG signal is first measured and analyzed in real-time for extracting the mean R-R interval. A constant delay of 300msec from the last detected R-wave ensures that the sequential bio-impedance measurements are performed during the iso-potential interval of the electrical activity of the heart to ensure an approximately fixed geometrical state of the heart. In addition, all measurement are performed during tidal respiration so that lungs' conductivity changes would be as little as possible affected by breathing. Four opposite current injections, each accompanied with five voltage measurements, are performed for a total measuring period of approximately 150msec, yielding a total of 20 independent measurements. The sampled measurements are transmitted to a portable PC through an interface unit to be stored and off-line processed. The entire system is power supplied from a rechargeable battery for complying with patient safety limitations.
Current results: 18 healthy volunteers and 34 CHF patients were measured with the system during tidal respiration, and the ability to monitor the respective lung conductivities was assessed. A mean left and right lung conductivities of [0.09+0.01,0.09+0.01]S/m for the control group and [0.12+0.03,0.11+0.02]S/m for the CHF group was found, indicating a significant separation (p<0.0003 for both lungs) between the two groups. The system reproducibility was better than 3% for both within and between tests measurements, and a correlation with long-term monitoring of two patients during medical treatment was also shown