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MFR PAPER 1046 



Ballistocardiography as a Technique 

 for Comparative Physiology 



N. TY SMITH and ERIC A. WAHRENBROCK 



ABSTRACT 



The ultra low-frequency haUistocarttioi^rcini was recorded on a young Califor- 

 nia gray whale. The tracing /.v remarkably similar to those obtained from man 

 and mouse, both in amplitude and in form. The IJ amplitudes fr>r mouse, man. 

 and whale were 2.6, 4,3, ami 4.6 cntlsec'-. We conclude that greater differences 

 are caused by poor recording technique or by disease than by species differences. 

 The major interspecies differences were seen in the timing of cardiac events, 

 such as preejeclion or ejection time. These differences could be caused by 

 differences in heart size. 



The ballistocardiograph (Beg) is a 

 device for evaluating the mechanical 

 function of the heart. It has been 

 recorded in an incredible array of 

 animals, ranging from egg embryos to 

 cattle. One of the more interesting 

 facts to arise from these recordings 

 is that the tracings are remarkably 

 similar among species, particularly 

 mammals. This similarity holds both 

 in form and in amplitude. It was 

 therefore an excellent opportunity to 



N. Ty Smith is an Associate 

 Professor of Anesthesia at the 

 University of California at San 

 Diego, Veterans Administration 

 Hospital, San Diego, CA 92161, 

 and Eric A. Wahrenbrock is an 

 Assistant Professor of Anesthesia 

 at the University of California, 

 San Diego, 



extend these observations to Gigi, 

 an animal with an entirely different 

 mass and configuration from other 

 mammals previously used. 



The Beg records the movements 

 of the body caused by movements of 

 blood in the body. First recorded in 

 1887. the Beg has undergone a series 

 of ups and downs in its attempts to 

 become a useful tool for measuring 

 cardiovascular function noninvasively. 

 Not until the 1950"s when physicists 

 and engineers entered the held, did the 

 Beg finally re-emerge as an accurate, 

 relatively simple technique. 



Essentially, the Beg works on the 

 principle that an attempted shift in the 

 center of mass of a floating body is 

 compensated for by a movement of the 

 body in the opposite direction, so that 

 the center of mass remains constant in 



relation to a fixed point. Thus, if blood 

 moves in one direction after ejection 

 by the left ventricle, the body will 

 move in the opposite direction. These 

 movements are quite small, but the 

 reader has certainly noticed a slight 

 bodily movement as he lies quietly 

 on a bed or a slight movement of 

 the pointer on a weighing scale, each 

 movement synchronous with the 

 heart beat. This minute body move- 

 ment can be recorded as displace- 

 ment, velocity, or acceleration. Figure 

 1 shows examples of normal tracings 

 in man. The important fact to note 

 is that the major components of the 

 Beg occur during ejection of blood, 

 particularly during the early portion. 



METHODS 



When the physical scientists entered 

 the field, they laid down certain 

 standards for recording the Beg, 

 standards which were to convert bal- 

 listocardiography from a haphazard 

 technique to a precise one. The first 

 requirement is that a very light bed 

 is necessary, in contrast to the heavy 

 ones formerly used. A ratio of 10:1 

 for subjectibed is minimal. Second, 

 coupling, or binding, of subject to bed 

 must be as tight as possible. Third. 

 coupling to ground must be minimal, 

 so that ambient vibrations can be 

 attenuated. The Beg is an extremely 

 sensitive instrument. Peak displace- 

 ment is about \00iJ. peak acceleration, 

 a few millig"s. g being the accelera- 

 tion of gravity. With older instruments, 

 vibrations from a truck outside the 

 building were able to destroy a bal- 



