SIGNAL FIDELITY REQUIREMENTS FOR DERIVING IMPEDANCE CARDIOGRAPHIC MEASURES OF CARDIAC-FUNCTION OVER A BROAD HEART-RATE RANGE

Our findings indicate that the impedance cardiogram spectrum extends f rom DC to 50 Hz. Any amplifier with an upper band limit less than 50 H z can be expected to produce attenuation and distortion of the impedan ce cardiogram. This signal attenuation may be systematically enhanced under conditions of high heart rate when a greater proportion of signa l energy will be in the upper frequency range of the impedance cardiog ram spectrum. Therefore, the present study was designed to assess the influence of amplifier bandwidth on dZ/dt(max), stroke volume, and sys tolic time intervals (LVET, PEP, QZ, QX). Simultaneously measured DELT AZ and dZ/dt signals from two impedance cardiographs, with corner freq uencies of 120 and 60 Hz for the DELTAZ and 50 and 15 Hz for dZ/dt cha nnels, 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 DELTAZ signals were digitally converted to dZ/dt by off-line digi tal differentiation with a 50 Hz corner frequency. The results demonst rated that the measurements with the 15 Hz corner frequency, when comp ared with the 50 Hz corner frequency measurements, systematically atte nuated the dZ/dt(max) amplitude and stroke volume measurements as hear t rate increased. The attenuation of dZ/dt(max) and stroke volume rang ed from about 13% to 26% as heart rate increased from 70 to 150 bpm. W hen the upper bandlimit was 50 Hz, the dZ/dt signal had greater resolu tion of waveform events and produced less prolonged systolic time inte rvals. The 15 Hz amplifier differentially influenced the B point, Z-pe ak and X minimum, having no apparent effect on the temporal location o f 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 wit h insufficient upper bandlimits will differentially influence ICG-deri ved measurements as heart rate varies.