Browsing by Author "Lu, Chih-Cheng"

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Open Access
Influence of signal fidelity on impedance cardiographically derived values at resting and accelerated heart rates
(1991) Hurwitz, Barry E.; Shyu, Liang-Yu; Lu, Chih-Cheng; Reddy, Sridhar P.; Schneiderman, Neil; Nagel, Joachim H.
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.
Open Access
Signal fidelity requirements for deriving impedance cardiographic measures of cardiac function over a broad heart rate range
(1993) Hurwitz, Barry E.; Shyu, Liang-Yu; Lu, Chih-Cheng; Reddy, Sridhar P.; Schneiderman, Neil; Nagel, Joachim H.
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 dZ/dtmax, stroke volume, and systolic time intervals (LVET, PEP, QZ, QX). Simultaneously measured ΔZ and dZ/dt signals from two impedance cardiographs, with corner frequencies of 120 and 60 Hz for the ΔZ and 50 and 15 Hz for dZ/dt channels, were contrasted over a broad range of heart rate (70–150 bpm). In addition to the analog dZ/dt signals obtained from the instruments, the ΔZ signals were digitally converted to dZ/dt 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 dZ/dtmax amplitude and stroke volume measurements as heart rate increased. The attenuation of dZ/dtmax 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 dZ/dt 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.