Abstract
Our findings indicate that the impedance cardiogram spectrum extends from DC to 50 Hz. Any amplifier with an upper band limit less than 50 Hz can be expected to produce attenuation and distortion of the impedance cardiogram. This signal attenuation may be systematically enhanced under conditions of high heart rate when a greater proportion of signal energy will be in the upper frequency range of the impedance cardiogram spectrum. Therefore, the present study was designed to assess the influence of amplifier bandwidth on d Z/d t max, stroke volume, and systolic time intervals (LVET, PEP, QZ, QX). Simultaneously measured Δ Z and d Z/d t signals from two impedance cardiographs, with corner frequencies of 120 and 60 Hz for the Δ Z and 50 and 15 Hz for d Z/d t channels, were contrasted over a broad range of heart rate (70–150 bpm). In addition to the analog d Z/d t signals obtained from the instruments, the Δ Z signals were digitally converted to d Z/d t by off-line digital differentiation with a 50 Hz corner frequency. The results demonstrated that the measurements with the 15 Hz corner frequency, when compared with the 50 Hz corner frequency measurements, systematically attenuated the d Z/d t max amplitude and stroke volume measurements as heart rate increased. The attenuation of d Z/d t max and stroke volume ranged from about 13% to 26% as heart rate increased from 70 to 150 bpm. When the upper bandlimit was 50 Hz, the d Z/d t signal had greater resolution of waveform events and produced less prolonged systolic time intervals. The 15 Hz amplifier differentially influenced the B point, Z-peak and X minimum, having no apparent effect on the temporal location of the B point, but delaying the Z-peak about 21.7 ms and the X minimum about 7.4 ms. These findings indicate that impedance cardiographs with insufficient upper bandlimits will differentially influence ICG-derived measurements as heart rate varies.
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