THE DESIGN OF THE BALLISTOCARDIOGRAPH

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Abstract
An exptl. and theoretical study of the human ballistocardiograph was undertaken with a view to designing an apparatus which will measure accurately and easily the motion of the body which occurs as a result of the heart beat. Both approaches to the problem have demonstrated that: a) It is absolutely necessary to have the bed damped by a device which introduces no friction. Since both over and under damping yield errors which are difficult to evaluate, the damping should be critical for the total load. b) The restoring force on the bed is most easily specified by the undamped frequency of oscillation of the loaded bed. True ballistic records are obtained only when fre-quencies somewhat less than 1/sec. are used. When higher frequencies are used, the errors in amplitude and wave form increase with the frequency. c) Measurements at very low frequencies are difficult to make. However at frequencies between 1 and 1.5/sec, the difficulties are reduced somewhat and the errors are not serious. d) It was found that at frequencies above 2/sec. the subject and the bed do not move in unison, and the pattern recorded depends in part on the elastic properties of the tissues of the subject. This leads to serious errors at high frequencies. Complete specifications are given for the construction of the bed used in this work.