Coherent ensemble averaging techniques for impedance cardiography

Abstract

EKG synchronized ensemble averaging of the impedance cardiogram tends to blur or suppress signal events due to signal jitter or event latency variability. Although ensemble averaging provides some improvement in the stability of the signal and signal to noise ratio under conditions of nonperiodic influences of respiration and motion, coherent averaging techniques were developed to determine whether further enhancement of the impedance cardiogram could be obtained. Physiological signals were obtained from sixteen male and female subjects during resting conditions, while delivering a speech and while undergoing submaximal bicycle exercise. Results indicated that improved resolution of dZ/dt signal events could be obtained using coherent ensemble averaging. Although some improvement in precision of event location was obtained, most enhancement of the impedance cardiogram occurred in measurement of the amplitude of the dZ/dt maximum (ejection velocity) during speaking and exercise conditions. Validated increases in dZ/dt maximum exceeding 20% were obtained in some subjects with coherent averaging, suggesting that the diagnostic utility of impedance cardiography can be improved by using this technique.