Comparison of Results of the Normal Ballistocardiogram and a Direct Fick Method in Measuring the Cardiac Output in Man 1

Starr and his associates (1) have demonstrated several characteristics of the ballistocardiographic method of cardiac output determination which rec- ommend it for clinical use.The technique is simple, the apparatus is highly sensitive to small variations in stroke volume, the results in the same individual are consistent, and the limits of normality are well defined.The usefulness of the method for investigative work, however, de- pends in addition upon its degree of accuracy.Preliminary information as to the accuracy of the method has been provided by Starr et al. by comparing results calculated from the ballistocar- diogram with those obtained by other methods of cardiac output determination.Almost simultane- ous cardiac output determinations by both the ethyl iodide method and the ballistocardiograph were performed on 30 subjects (2).Of these, the 25 best cases showed values of cardiac output averaging 3.9 per cent smaller by the ballisto- cardiograph than by the ethyl iodide method.A larger discrepancy was found between a group of 106 normal subjects studied by the bal- listocardiograph (1) and two other groups of normal subjects studied by the ethyl iodide (3) and acetylene methods (4).Values for cardiac output by the ballistocardiograph averaged 14.4 per cent smaller than by the ethyl iodide method and 13.6 per cent smaller than by the acetylene method.It must be noted in addition that stand- ard basal conditions were not obtained in the group studied with the ballistocardiograph.If more rigid basal conditions had prevailed as in the two series studied by the foreign gas methods, it is fair to assume that the figures for cardiac output by the ballistocardiograph would have been somewhat lower, thus increasing the discrepancy.The comparisons presented to date do not satis-