A study of the physical properties of the ballistocardiograph

Abstract

Fourier analyses of the displacement ballistocardiograms of seven subjects recorded from four types of instruments have been made. The coefficients of these series were corrected for the distortion introduced by the ballistic bed and by transmission through the body. The coefficients representing the ballistic pattern from the four instruments were for each subject quite different in value in the corresponding terms. The effect of applying the corrections was to converge the coefficients of the corresponding terms of the series toward a common value. The common pattern thus predicted represents a basic acceleration pattern (or forcing function) and, by appropriate steps of integration, the associated velocity and displacement patterns. The coefficients of these basic series were compared with those of the original series obtained from the analyses of the patterns from the various instruments. This comparison was made in order to determine how closely the basic patterns are approximated by the records made by these instruments. The best agreement is between the pattern obtained from the low-frequency, critically damped ballistocardiograph and the basic velocity pattern. The next best correlation was between the high-frequency underdamped system and the basic acceleration pattern. It is also suggested from this research that the acceleration, velocity, and displacement can be represented with sufficient accuracy by Fourier series carried out to 11, 7, and 4 terms, respectively. This finding indicates, for example, that a ballistocardiograph adequate to represent the acceleration pattern must have a frequency response which is linear to 11 cycles per second and that adequate mechanical or electrical filtering must be present to eliminate the distortion introduced by the transmission of impulses through the body.