Influence of signal fidelity on impedance cardiographically derived values at resting and accelerated heart rates

Abstract

Since the spectrum of the impedance cardiogram (ICG) 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 ICG. This signal attenuation may be systematically enhanced under conditions of high heart rates (HR) when a greater proportion of signal energy will be in the upper frequency range of the ICG spectrum. Therefore, the present study was designed to assess the influence of amplifier bandwidth and signal fidelity on dZ/dtmax, stroke volume (SV), and systolic time intervals (LVET, PEP, OZ, HI). The performance of commonly available commercial systems was tested over a broad range of HRs. The results demonstrated that a digitally differentiated dZ/dt signal using a differentiator with a corner frequency of 50 Hz, when compared with the 15 Hz corner frequency used In the commercial impedance cardiograph, systematically enhanced the dZ/dtmax amplitude and SV measurements as HR increased. For SV the increase ranged from 17 to 30% as HR increased from 70 to 150 bpm. Moreover, the digitally filtered signal had greater resolution and produced less prolonged PEP and QZ intervals and greater HI with increasing HR. These findings indicate that impedance cardiographs with insufficient upper band limits will differentially influence ICG-derived measurements as HR varies.