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The authors measured transthoracic impedance in nine presumed healthy adult subjects with a two-frequency plethysmograph at 57 kHz and 185 kHz. The measurement protocol included periods of normal breathing without motion and periods of motion without breathing. The authors analyzed the cross-correlation and the ratio between the signals at both frequencies for all the different maneuvers. The correlation coefficient was between 0.97 and 1 for breathing, the minimal cross-correlation (0.81) was for simulated obstructive apnea. The authors found that the amplitude ratio between the two-frequency signals was different for normal breathing and for motion. Based on these results, the authors designed and tested an adaptive filter to increase the signal-to-artifact ratio (SAR). The increase in SAR (mean±standard deviation) compared with the signal at 57 kHz was: 183%±117% for arm movement, 133%±93% for leg movement, and 34%±62% for simulated obstructive apnea.