Spectral Analysis of Electrical Impedance Measurements on the Lower Limbs

Electrical impedance signals from the lower limbs of normal individuals and patients with atherosclerosis have been studied in the frequency domain. A current of 1 mA at 40 kHz was introduced with strip electrodes around the ankles, and voltage differences were measured with electrodes above the knee and above the current electrode of the same limb. The impedance signal AZ and the ECG were recorded on magnetic tape and transferred to a digital computer. The first derivative of the impedance signal, dZ/dt, was calculated by numerical methods from the coherent average of at least 20 AZ signals, and the spectra were obtained by an FFT algorithm. For young normal individuals, only 1 percent of the dZ/dt signal energy lies above 12 Hz. Patients with arterial obstructions present a greater attenuation in the lower harmonics, stretching the 99 percent energy bandwidth to 32 Hz. Although the amplitude of some spectral components can be used for noninvasive evaluation of arterial disease with a precision of up to 90 percent, it was found that the phase spectra are very little influenced by atherosclerosis.