A new index for quantitative ballistocardiography: the velocity of body displacement.

Calculation of the velocity of body displacement and of the momentum of the body from 100 normal ballistic tracings shows that the values of the velocity of body displacement present a small scatter (mean value 68.3 + 13.9 mm. per second) and are independent of body weight and surface area. For uniformity of values and simplicity of calculation, which can be effected also on many abnormal tracings, it is suggested that the velocity of body displacement be used as an index of quantitative ballistocardiography. AT PRESENT the examination of ballistic tracings includes a qualitative and a quantitative analysis. The qualitative examination is concerned with the shape of the tracings, independent of the amplitude of the various deflections, and does not require that the tracings be recorded with special calibration. Quantitative analysis, on the other hand, can be applied only to tracings recorded with ballistocardiographs that employ a swinging bed and that may be precisely calibrated. Up to now the indexes used for the quantitative analysis of the ballistocardiogram have been: (a) the calculation of the stroke volume (SV), and (b) the calculation of the maximal cardiac force (A1CF), according to Starr.' Several formulas have been applied to the calculation of stroke volume, among which the one most used and most practical is the formula of Tanner2: SV = 100A/21 + JVQC where I and J are the areas under the I and J waves of the ballistocardiogram; C is the duration of the cardiac cycle in seconds. The values of the stroke volume obtained with this formula differ somewhat from the real values obtained with the gas analysis method which makes use of the principle of Fick.3' 4 Moreover , it must be admitted that the calculation of the stroke volume from the ballistocardio-gram is open to criticism from both a theoretic and practical point of view. (a) It has not been From the Istituto di Patologia Medica of the University of Rome. 276 convincingly demonstrated that the areas under I and J are related to the amount of blood ejected from the ventricle. (b) The formula for the calculation of stroke volume should include the value of the cross-sectional area of the aorta, which is difficult to assess and can be estimated only approximately from the tables of Bazett: for this reason the formula of Tanner does not include this value but only a fixed coefficient. (c) Accurate location …