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The authors measured transthoracic impedance between 12.5 and 185 kHz in nine adults. They used a system with two impedance channels, both simultaneously detecting the real part of impedance at two different frequencies. They used only two electrodes in the midaxillary line, connecting both channels in parallel. The amplitude relation between the two channels was measured for different maneuvers and frequencies. Results show for normal breathing an increase of the signal of 20% and a decrease in motion artifacts from 12.5 to 185 kHz. It is concluded that, for the maneuvers studied, it is better to work at higher frequencies than the ones commonly used. Also, the authors suggest a method to further increase the signal-to-motion artifact ratio based on measurement at two frequencies.