Current Status of Ballistocartliography

Current Status of Ballistocardiography By W. R. ScAnBonoucn HIS YEAR marks the twentieth anniversary of the modern era of ballistocardiography, which dates back to 1939 when Starr 1 published the first of his classic studies in this field. There has been much progress during these 20 years and a great deal in the last five years. It is not our purpose to chronicle the development of the field or to review the extensive literature pertaining to it but rather to present, in crystalized form, our views as to where ballistocardiography stands at present and in what direction it may be expected to proceed in the future. Such views have been expressed editorially on several occasions in the past -~,s and this communication represents a fresh appraisal which takes into consideration recent developments in the field. During the first decade since 1939 ballistocardiographic research was carried out almost exclusively in this country. Since then there has been an ever increasing volume of work from other countries, notably Holland, Germany, France and Italy. It will probably come as no surprise to learn that the method has been under investigation in Russia during the last few years.

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INSTRU.~IENTALCONSIDERATIONS Ballistocardiography is a relatively young discipline and is still a rather long way from maturity. Stated in the simplest terms the purpose of ballistocardiography is to obtain records of the motions of tile body known to be produced by cardiovascular activity and from such records to glean information on certain aspects of circulatory function. The pattern or waveform of these body motions is highly dependent on, and virtually determined by, the manner in which the motions are measured. Much of the confusion which exists about ballistocardiography stems from the great diversity of methods and technics which have been used. Instrumental considerations have constituted a serious barrier to progress in this field and it was to the penetration of this barrier that much of the recent progress must be credited. Extensive biophysical and engineering studies (vere carried out on tile various existing ballistocardiographic systems at about the same time by Burger, 4,~ von XVittern,6 and Talbot and Harrison. 7 The results were in substantial agreement and pointed to a common solution. This solution was to support the human body on a light platform or frame suspended in such a manner that the coupling between platform and "earth" is so weak that there is little resistance to platform motion. Under these circumstances body and platFrom the Johns Hopkins Medical School and ttospital, Baltimore, Md. The views expressed herein are those of a group which includes in addition to the author, Drs. Benj. M. Baker, Jr., Sam'l. A. Talbot, Frank W. Davis, Jr., ,Martin L. Singewald, Robert E. Mason, and Patricla M. Smith. This work has been supported in part by a Research Grant (H-327) from the National Heart Institute, National Institutes of Itealth, U.S.P.II.S. 263

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form tend to move together as a unit. Ballistocardiographic systems of this sort have been designated "ultra-low frequency" (ULF) s because the very low natural frequency (less than 0.5 e/s) is a reflection of the weakness of coupling between platform and earth. A number of different designs have been described for realizing tlle required conditions (including flotation in water or mercury9 and the use of ball bearings l°,~t) but the method most frequently used and probably the most satisfactory is some sort of pendular suspension. The simplest example of this is a light platform suspended from above by a number of wires each about 10 feet or more in length. 4 Various modifications have been employed to reduce the length of the suspension wires. 12a:~ The bed in routine use in this laboratory stands only 2 feet above the floor and has no overhead wires; this was made possible through the use of special differential pendular legs. 1~ A detailed consideration of the mechanical characteristics of the BCG methods described by Starr I (high frequency), Nickerson 15 (low frequency, critically damped), and Dock ~G (direct body) is beyond the scope of this communication, and for such the reader is referred elsewhere.4s,~7 It has become evident that the use of these methods involves errors which are often quite serious; these can be minimized or obviated through the use of the ULF systems. In addition, the latter provide information of different types extending over a much broader frequency range than do the other methods. This is not to suggest that the classic methods of Starr, Dock and Nickerson have not been useful. On the contrary, they have yielded much important information and have made possible large scale clinical studies not yet feasible with the newer systems. Despite certain deficiencies the Starr bed does reflect the major and slower cardiowlscular forces and its record most closely approximates the ULF acceleration (force) ballistocardiogram. (See figure 1 for comparison of records.) In our opinion it is by far the most rugged and dependable method available and yields the most consistently reproducible records. By decreasing the weight of the platform and tightening tlle couI)ling between it and the patient Starr is has recently been able to improve the performance of his bed considerably. In skilled hands and With proper attention to detail during test procedures the "direct-body" methods yield (force) records generally similar to those from the Starr bed. The ULF ballistocardiographic systems are capable of supplying several different types of mutually complementary information. To paraphrase Newton's Third Law, we may state that when a body (in our case the human body and supporting platform) is suspended in space so that it is relatively free of external forces, then any internal mass motion will induce an equal and opposite mass motion of the rest of the body so that t h e center of gravity of tile body as a whole will be unclmnged. Since the ULF systems generally meet the Newtonian requirements for minimal external forces, records of the external motions of body and platform accurately reflect internal motions which, for our purposes, are those associated with the circulation of blood. The three types or aspects of motion generally recorded are displacement, velocity and acceleration. When multiplied by the bed-body mass these be-

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come the body's mass-displacement, momentum and force and are equal and opposite to the blood's mass-displacement, momentum and force, respectively. To offer a concrete example, if 100 Gm. of blood were displaced 7 cm. in a footward direction in the arterial system, then the body of a 70 Kg. man should be displaced 100 t~ (or 0.1 mm.) in the headward direction, the product, 700 Gm.-cm., being the same for both blood and body. Suitable transducers which sense tile motion, of tlle platform on which the body is supported therefore make it possible to obtain information on the blood's mass-displacement, momentum and force continuously t!troughout the cardiac eye!e. Simultaneous displacement (D), velocity (V) and acceleration (A) records from 3 normal females are shown in figure 2. The intrinsic relationship of these three kinds of records has been described and a nomenclature has been devised for them. s Displacement records of the kind shown in figure 2 are not new, and in point of fact the two earliest investigators, Gordon 19 and Henderson, -~° obtained records of this sort since no velocity or acceleration transducers were then available. Both used •equivalents of ULF systems and it is quite possible that difficult), with respiration was the reason Henderson did not continue hiswork. Inasmuch as the displacements during normal breathing are approximately 10 times the size of those due to cardiovascular

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Fla. 2.--Nomml displacement (D), velocity (V) and acceleration (A) ballistocardiograms from an ultra-low frequency (ULF) system. A, B and C are from three young, healthy females and were obtained during suspended respiration. Lat-A refers to the lateral acceleration record, Le to the lead 2 electrocardiogram, and phono to the heart sounds at the apex. In this and all subsequent photographic reproductions of records, darkest time lines are 0.1 sec. apart (paper speed 5.0 cm./scc.). (Reproduced by permission of the American Journal of Cardiology.) activity, it is necessary to have the subject hold his breath, avoiding if possible Valsalva or Muller procedures. This is sometimes difficult, especially with untrained subjects. Efforts have been made by various means to eliminate tile motions due to breathing. Tile problem has not yet. been solved, ~7.21 although it should not be regarded ,'is insoluble. At present respiration must be suspended in order to obtain satisfactory displacement and velocity records. However, acceleration ballistocardiograms may be recorded without difficulty during normal breathing since tlle accelerometer does not sense the slow (ca. 0.25 e/s) respirat(Jry motions; in this respect such records are comparable to those from the Starr bed. The foregoing has dealt exclusively with records of motion along tile headfoot (longitudina l) axis of the body, a natural choice because of the orientation of the great vessels. There has been good reason for tlle belief that usefid information might be supplied by records obtained along tlle other two axes (side to side or "lateral" and anteroposterior). Various modifications of the Starr and "direct-body" methods have been used to obtain bi- or tridirectional records, "~2-2. and some quite interesting observations have been made, especially relating to abnormal diastolic impacts ~-5 and to changes with age. -~c,'°r Certain reservations regarding records of this sort w e r e justified because of uncertainty about their physical validity and significance and because tile different recording technics did not yield records which were similar. Lateral ballistocardiograms from ULF systems have been obtained by Honig "~s and are being recorded routinely in this laboratory (see figures 2 and 3). However, there is evidence suggesting that records of this kind represent a mixture of translation and rotation (or body roll) and as such cannot be combined with

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Fro. 3.--Sinmltaneous ULF ]mad-foot displacement (HF-D) and acceleration (HF-A) ballistocardlograms and lateral displacement (Lat-D) and acceleration (Lat-A) ballistocardiograms. Obtained from'a young normal female during suspended respiration. Polarity of the tracings is indicated by the arrows. (Reproduced by permission of the American Journal of Cardiology.) head-foot records (true translation) for vector presentation. 14,29 This problem has been under investigation for some time by Talbot, who has now developed a tridirectional "roll-free" bed. 3° Three plane vector loops have been obtained on a few normal persons. 3~ This instrument is of considerable mechanical and electronic complexity but it may prove to be a valuable research model by pointing up leads that could be explored by simpler instruments. It may be concluded that for ULF systems the major problems in instrumental development have been solved as f a r as tim longitudinal mode is concerned. The rec.ording of ballistocardiograms in the other two modes still requires further investigation. In discussing the current status of ballistocardiographie instruments most o f the space has been devoted to the ULF systems because most of the recent technical developments have been with them. Furthermore, there lms been a gradual transition I~oward the use of instruments of this kind by investigators active in this field. The reasons for lack of a more rapid transition are all too clear. No ULF bed of any kind is available commercially. Of those in use, a number were made from components supplied at no charge by a medical instrument company* for investigational use. As a consequence tile medical investigator who is interested in this kind of work has often been *Mr. Maurice Rappaport, Sanborn Company,Waltham, Mass.

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in tile position of having to build his own, an unprofitable job unless technihal facilities and personnel are available. Transducers, amplifiers and recorders are rather expensive, especially when the nature of the research requires the recording of multiple variables. Progress would unquestionably be faster if some form of ULF ballistocardiograph were available commercially. No two ULF beds now in use are identical, although some are similar. Fortunately, the mechanical requirements (weight, natural frequency and damping) With this type of system are not critical and provided certain minimal conditions are met, s performance characteristics and BCG records from different beds should be reasonably comparable. Nevertheless, a standardized instrument would further increase our confidence in comparing records and results from different research laboratories. In discussing the requirelnents for "distortion-free" BCG systems it was implied, but not explicitly stated, that these were largely based on certain physical properties of the human body. When a single sharp impact is applied longitudinally to a body lying on a rigid surface there results a series of low-frequency (ca. 4 e/s), underdamped oscillations. The body is simply vibrating passively on its own "dorsal spring" with respect to the rigid support. This is the source of the distortion which is minimized when a ULF platform is used; body and support move together as a unit and no passive body vibrations are excited. There is, however, another source of distortion and this should be briefy mentioned. The "cardiovascular generator" (heart, great vessels and blood within them) may be considered a small, long mass suspended internally througla multiple springs (the "internal coupling network") to the much larger mass of the body. Little is known about the mechanical behavior of this part of the system although it has been under study. ~m,'~-" It appears that some distortion may be expected at higher frequencies but little or none at low frequencies. PtIYSIOLOGIC SIGNIFICANCE OF TItE BALLISTOCARDIOGRAXI AND ITS RELATIONStIIP TO CIRCULATORY EVENTS

The critical issue about which this methodologie approach turns is that dealing with the precise physiologic meaning of tim ballistoeardiogram. Much insight has been gained in recent years on this subject but a great deal more information is needed before a definitive answer can be provided. Some, if not all, of the determinants of the normal ballistocardiogram are known, but their multiplicity, relative importance, mutual dependence and inaccessability make the problem a very complicated one. Among the major determinants may be listed: motion of the heart's mass; acceleration of blood from the heart dttring systole; instantaneous velocity of blood flow through the major arteries and veins; size, spatial orientation and:elasticity of the larger vessels. Information on most of these lms been difficult or impossible to obtain from intact human subjects or even from the experimental animal. Nevertheless there is some direct evidence, and even more of an indirect nature, that can be brought to bear on this subject. For a more comprehensive treatment of this evidence and its implications the reader is referred to several recent publications, xa,'7,33,3~ A brief account will be given here of (1) the

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nature and physical meaning of the information supplied by ballistocardiography and (2) its relation to certain specific aspects of circulatory physiology. The human circulatory system in essence consists of a pump and a closed circuit of elastic tubing filled with blood. The mass-motions which take place within this system are depicted by the externally recorded ballistocardiogram. More precisely the latter represents tile algebraic sum of all mass-motions which occur within the system; mass-motions, equal in magnitude but opposite in direction, cancel and their sum is zero. If the tubing which carries the blood were indistensible, tile sum of mass-motions would necessarily be zero regardless of the velocity of blood flow or of the size and shape of tile circuit; no external motion of the body mass (or ballistocardiogram) would be induced under these circumstances. It is tlle distensibility of tile tubing (vessels) which makes possible changes in distribution of blood mass (and its center of gravity) within the system as a whole. The blood's center of gravity shifts in space as, with the onset of ejection, the wave of distension moves from the root of the aorta out over the arterial tree. The analysis of changes in the systemic circulation is somewhat complicated by the spatial orientation of the aorta. The blood's center of gravity initially moves headward as the proximal aorta is distended, but then reverses direction and moves footward as the aorta distal to the arch is progressively distended. During early diastole it reverses direction again and moves headward, rapidly at first and more slowly later, throughout the remainder of diastole until it is at, or near, the original point of departure. These shifts are highly consistent and repetitive from beat to beat. We have already seen from Newton's law that for ULF systems mass-motions of the blood and the body are equal in magnitude, but opposite in direction. Thus, if the ULF displacement record (labeled D in figure 4) is inverted (fig. 5), this then represents displacements of blood's mass, headward motion being inscribed upward. This is in general agreement with what is known, on anatomic and physiologic grounds, about shifts of blood within the systemic circulation during the cardiac cycle. The pulmonary circulation must also be" taken into account in this general description. While it undoubtedly contributes to the over-all mass-motion pattern, the part it plays is not believed to be very large. The morphology of the pulmonary circulation is such that beyond the bifurcation the vessels fan out radially in all directions and it is probable that mass-motions in them are mutually canceled. Wlmt remains then are the mass-motions associated with the displacement of the stroke volume from the mid right ventricle to .the bifurcation, and the return (through the pulmonary veins) of a similar quantity of blood from that point to the left ventricle; both distances are short. The main contribution would be to add to the initial headward displacement of blood ("I-"J segment in figure 5) in early systole. The point has now been reached where some general statements may be made regarding the origin of the normal displacement ballistocardiogram in qualitative terms. In figures 4 and 5 a small, presystolic wave marked "G is shown. This appears to be associated with the footward flow of blood from atria to ventricles during atrial contraction. The "H-"I segment (fig. 5) is a transient displacement which may be due to an initial footward movement of

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the heart and of blood in tile inferior vena cava. The early systolic "I-"J segment represents the headward displacement of blood as the proximal aorta and pulmonary artery are distended~ The large "J~'M, footward motion of blood, reflects the predominant uptake of blood by the vascular tree below tile heart; it extends fi.om mid-systole into early diastole. The "M-'N segment,

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which corresponds in time with the rapid filling phase of the ventricles, may be due to rapid headward flow through the inferior vena cava, to run-off of blood from the vascular tree below the heart, or to both. Displacements beyond "N are variable and represent a summation of mass-motions in the arteries and veins during this rather quiescent portion of the cardiac cycle. Until recently most investigators in this country were concerned primarily with the force ballistocardiogram because this was the kind of information most methods supplied. In the foregoing attention was purposely focused on the U L F displacement record because it is the simplest and the discussion of its origin is the most readily comprehensible. Since the velocity (momentum) and acceleration (force) ballistocardiograms are mathematical derivatives of the displacement record, they are therefore determined by the same events which produce the lattei-. Ea'ch differentiation has the effect of magnifying the faster motions and attenuating the slower ones. Figure 4 serves to indicate the temporal relationship of the normal displacement, velocity, and acceleration ballistocardiograms. The characteristics of these records have been described elsewhere 14,35 and need not be discussed here, but a few points should be mentioned. The wave tips or flexures of displacement and acceleration records coincide in time but are opposite in direction; the acceleration record depicts the cttrvatttres of the displacement record. The corollary to this is that there is no direct relationship between the amplitz,des of the two records. For example, the accelerations are small during the period when the largest displacement (the "J-"M deflection) occurs. It will be noted in figure 4 that the maior force events of the acceleration record, the I and J waves, occur in the first half of systole. This is in general accord with observations 3~ indicating that ejection from the normal heart begins with great rapidity and reaches maximum velocity early in systole; as a consequence more than two-thirds of the stroke volnme is discharged during the first half of the cardiac ejection phase. A number of attempts have been made since Starr's original work to quantitatively derive a summated pattern of mass-motions in the circulatory system which might be compared with externally recorded ballistocardiograms. In 1945 Hamilton and his co-workers ~7 computed a summated force curve representing mass-motions in the aorta and pulmonary artery but at that time Fie. 4.--Normal human displacement (D), velocity (V) and acceleration (A) ballistocardiograms from an ultra-low [rcquency (ULF) system and their temporal relation to events in tile cardiac cycle. The BCG, heart sounds (phono) and EKG tracings were based on tile measurements from 25 males, 20 to 29 years of age. Tile block diagrams at th"-. bottom represent tile timing of cardiac events on tile two sides of tile heart and are based on data supplied by Braunwaldzs and by Wiggcrs.ZO IC ---- isometric contraction (crosshatched), E = ejection phase (solid block), RF ---- rapid filling and D _-- diastasis. Calibrations for the three ballistocardiograms are shown on the right, mG represents milli-G; 1.0 mG (0.001G) is approxlnmtely equal to 1.0 cm./sec.-"; ,it represents micron; 20 IX is equal to 0.02 nlillimcters. The BCG tracingg represent external.motions of the body; headward motions are upwards. Tile tips of displacement waves should coincide in time with the tips of corresponding acceleration waves; however, tile displacement waves are slightly advanced in time owing to the presence of a small phase lead. (Reproduced by permission from tile American Journal of Cardiology.)

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ULF systems were not in use and there was no undistorted force ballistocardiogram with which to compare their results. The pattern of this coni.puted force curve is rather similar to the acceleration (force) records now being obtained with ULF ballistocardiographs although its magnitude is considerably greater. The most recent and probably the most successful attempt to compute circulatory mass-motions was made by Noordergraaf. lr,3~ Using anatomic measurements, an average value for arterial distensibility and a normal central blood pressure pulse contour, he calculated the uptake of blood by successive segments of all tile larger arteries and plotted them with respect to time. The uptake curve for each" segment was multiplied by its distance from tile heart and these products were then summated algebraically. The resulting summated uptake curve and its first ~and second derivatives were surprisingly similar to normal ULF displacement, velocity and acceleration ballistocardiograms not only in waveform but in magnitude as well.34 The relation of the features of normal human ULF ballistocardiograms

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to events in the cardiac cycle is also shown diagrammatically in figure 4. The timing of events for tile two sides of the circulation depicted at the bottom of the illustration was based on measurements by Braunwald and co-workersss and by Wiggers. ~ In the acceleration record tile presystolie waves are those which precede tlle H wave. The systolic waves are H, I and J. The end of systole is frequently marked by a small flexure, Lo, and this point coincicles rather closely in time with the onset of the second heart sound. The diastolic waves are L, M and N. Certain differences in the timing of right and left sided events, evident in Braunwald's data, led some observers 39,4° to suggest that the onset and end of ejection from the two sides of the heart can be identified separately in the acceleration ballistocardiogram. However, tlle evidence presented thus far is rather unconvincing. A clear-cut and significant relationship has been demonstrated3"~m between the duration of left ventricular systole (S~-S~ interval) and the duration of tile systolic BCG complex (H-L interval). RELATION OF TIlE BALLISTOCAHDIOGRA.XI TO CERTAIN SPECIFIC ASPECTS OF

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Stroke Volume and Cardiac Output It is of historical interest that in his paper published in 1905 Yandel Henderson 2° stated that his work with what was later to be named the ballistocardiograph was aimed at estimating in man changes in ventrieular volume (cardiac output) which he had been directly measuring in dogs with cardiometers. His BCG records were of the displacement type and are comparable with those from ULF systems now in use. Starr's initial interest was also in the measurement of cardiac output. In the construction of his bed Starr 1 was able to circumvent the distortion due to breathing which had made it necessary for Henderson to obtain his records only during suspended respiration. However, Starr's records were" quite different from Henderson's and were more closely related to cardiac force than to stroke volume. Nevertheless there was an obvious relationship between the amplitude of the Starr ballistoeardiogram and stroke volume. In conditions ,associated with high cardiac output (as in hyperthyroidism, A-V fistulae and after adrenalin and exercise) "the ballistocardiogram was abnormally large and in those associated with low cardiac output (as in hypothyroidism) it was unusually small. Furthermore, the changes in amplitude of the ballistocardiogram during the respiratory cycle (increasing in inspiration, decreasing in expiration) were in good agreement with observations42, 44 on changes in total cardiac output during respiration. Start derived an equation for stroke volume based on the area under the I and j waves of ballistocardiograms from his bed. When compared with other methods of measuring cardiac output there was fairly good agreement and this was improved when his equation was modified somewhat. However, this method is imreliable when waveform is abnormal and this is usually the case in paticnts with cardiovascular disease. In recent years the emphasis has been on the relationship of the ballistocardiogram to cardiac force rather than to cardiac output.

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Nickerson also found a correlation between stroke volume and the amplitude of tlle records from his low-frequency bed, and reasonably good agreement with the Fick method was obtained. I~,4z As a result of some recent work he believes that the accuracy of the method in estimating stroke volume has been increased. 4~ His new method of calculation does not rely on constants obtained by comparison with other methods. There has been a recent revival of interest in the estimation of stroke volume as a consequence of the availability of ULF ballistocardiograms of the displacement type. On the basis of what is currently known of the origin and physical meaning of these records it seems quite likely that they bear tile closest relationship to stroke vohune of any kind of ballistoeardiogram. Based on work with lmmans and with hydraulic models several methods for co,nputing stroke volume from the ULF displacement ballistocardiogram have been proposed recently. 1°,~7-51 However, the results are not entirely convincing and judgment must be withheld until more evidence is available. This may well be a productive area for future investigation. Cardiot:asc,lar Force

A distinction should be made at the outset between the forces reflected by the ballistocardiogram and the forces of myocardial contraction and cardiac ejection. The former represent the summated forces arising from the whole cardiovascular system while the latter are restricted to the heart itself. Even though these forces are not exactly the same, they appear to be closely related. Most of the direct experimental evidence bearing on this relationship has come from a series of cadaver injection studies by Starr and his co-workers, the first of which was reported in 1950.52 In fact, the conceptual basis for evahmting the 10hysiologic significance of the waveform of the ballistocardiogram derives, in large measure, from this work. While some reservations may be entertfiined about certain aspects of the injection technic, the significance of the results cannot be doubted. In essence the technic was designed to simulate systole in human bodiOs shortly after death. The cadaver '¢ballistocardiogram" was recorded during the injection of fluid ( b l o o d o r water) through rigid tubes into the roots of the aorta and pulmonary artery. The volume and rate of injection Could be controlled and measured and the time of onset of injection into the two vessels could be varied. Thus it was possible 'to compare the time-course of ejection with the force ballistocardiogram which resulted from it. The results of these experiments can only be touched on here and the reader is referred to a recent paper by Starr 33 summarizing them. It was possible with this technic to produce normal ballistocardiograms and to reproduce most of the various abnormal patterns observed in patients with cardiovascular disease. The normal ballistocardiogram was produced only when injection was begun abruptly and initial velocity was made to increase very rapidly. To produce this kind of injection a large initial force had to be applied to the pump pistons and tile greater the applied force the larger was the resulting force ballistocardiogram. If the velbcity of injection were slow, a small and/or abnormal ballistocardiogram was produced. One clinically common form of abnormality, the "late downstroke" type, was produced if

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early injection was slow and made to end abruptly. Other forms of abnormality were associated with uneven iniection or asynchronous iniection into tile two vessels. Starr compared tile applied force of injection with the amplitude of the force ballistocardiogram and found a high degree of correlation (r ~-- 0.92). Tile over-all results suggest that in living humans the size and waveform of tile ballistocardiogram is closely related to the time-course of cardiac eiection. Tile large, normal ballistocardiograms of young, healthy individuals would seem to indicate that the normal heart characteristically accelerates blood very rapidly into the great vessels. On the other hand, the small and abnormal ballistocardiograms of patients with cardiac disease suggest that tile diseased heart is incapable of ejecting in this manner and accelerates blood more slowly. It should be pointed out that m e a n ejection velocity and stroke volume may be the same for normal and diseased hearts. In all of the cadaver injection studies Starr used his higla-frequency bed, but this does not appreciably alter the results or the conclusions because the ballistocardiograms from this bed are predominantly force records and because he chose tile least distorted portion of tile record (I and J waves) for his calculations. One final point regarding Starr's cadaver work is worth mentioning. Among the bodies he nsed for injection studies, tile presence or absence of aortic atherosclerosis had little or no effect on the force record produced by simulating systole. PIIYSIOLOGIC STUDIES IN EXPEIIIXIENTAL ANIMALS

Tile difficulty of evahmting in the intact human the relative importance of the various determinants of the normal and abnormal ballistocardiogram resulted in efforts to carry out ballistocardiographie studies on dogs in whom a greater number and variety of circulatory measurements could be made. The work in this field proved more difficult and progress has been slower than was anticipated. One of the reasons for this has been the difficulty of obtaining or constructing the specialized, equipment needed for this work. Attempts have been made to record ballistocardiograms from dogs with modified high-frequency bed systems and with "direct-body" ballistocardiographs applied to various parts of the bodyY ,~3-~' More recently, ULF "beds" have been specially constructed for use in dog studies. ~r-~9 As a consequence of the technical difficulties only a few laboratories are currently engaged in such work and therefore research experience has been neither varied nor extensive. Tile habitus and physical properties o f the dog's body make it much less suitable for BCG studies than the human and this creates a special problem in the design of ballistocardiographic instruments for use with them. One of the chief diflqculties encountered in using ULF systems is that the weak coupling between bed and earth must not be compromised. An), tubing or cables which must be attached to the becl should be lightweight and led onto the bed in such a manner that they xx~illnot add "spring" or otherwise produce distortion of tile records. Still another source of difficulty arose from the necessity of using general anesthesia. I't has not been possible to obtain worthwhile ballistocardiograms from nnanesthctized dogs because of panting respiration and voluntary movements of the body, head

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and limbs. In this laboratory it was found that the use of anesthesia (morphine and pentobarbital witl'l or without dial-urethane) alone was associated with striking changes in the dog (ULF acceleration) ballistocardiogram. ~s In a high proportion of dogs there was progressive deterioration of the BCG with time and this abnormality occurred earlier and was more marked when positive pressure artificial respiration was used. It became obvious that unless a solution to this vexing problem could be found the anesthetized dog would be rather unsuitable for work aimed at providing a physiologic basis for the ballistocardiogram. Fortunately, a rather simple means was found for preventing or correcting the spontaneous deterioration of the dog's ballistocardiogram. The application and maintenance of abdominal compression proved a rather effective solution and in most cases made it possible to obtain normal and reproducible ballistocardiograms for several hours. Evidence has been presented :'s that the deterioration of the anesthetized dog's ballistocardiogram is a reflection of real circulatory changes known to occur during anesthesia. Available data suggest that, by impairing vasomotor control, anesthesia produces depletion of intrathoracic blood volume, pooling of blood in tlle splanehnic and peripheral venous beds, decrease in venous return, cardiac output and blood pressure. Tlle deleterious effect of artificial respiration on tile dog's BCG is readily explicable on tlle grounds that this procedure tends to augment the circulatory derangements produced by anesthesia. The improvement in the ballistocardiogram following tlle application of abdominal compression may be due to the fact that the blood pooled in the splanchnic bed is squeezed out into the active circulation and is not allowed to reaccumulate there. The explanations offered for the observed changes seem reasonable enough and are supported by a considerable amount of indirect evidence, but direct proof is lacking. If the interpretations are correct, then the matter should be of practical as well as academic interest and will have a direct bearing on the functional state of the circulation in the experimental animal and the surgical patient during anesthesia. These observations on dogs and those by Malt 6° on humans suggest that the ballistocardiograph may be a sensitive, useful instrument for evaluating the effects of anesthetic agents in man and animal. Aside from the physiologic mechanisms by which anesthesia produces abnormality of the dog BCG, there emerged • the important practical point that such changes may be prevented or corrected by abdominal compression or some similar procedure. However, abdominal compression is neither always nor completely effective and further refinements may be necessary. A pressure suit applied to the lower body up to the lower thoracic cage might be an improvement. In spite of the various difficulties encountered in BCG studies on dogs, some modest progress can be recorded. Until it becomes possible to record instantaneous velocity of blood flow and chaiages in circumference from multiple points on the larger vessels, it will probably not be possible to provide a completely sound physiologic basis for ballistocardiography. In the meantime, information is being obtained from experiments of an indirect kind. Thus far, the results have been so limited that they can only be mentioned in passing.

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6s:0 FIc. 6.--Comparison of normal acceleration (force) ballistoeardiograms from a human and a dog--Single head-foot (HF) complexes with EKG, pbono, and lateral BCG complexes (Lat.). Both records were obtained with ULF ballistoeardiographs. The dark vertical line, marked Q, has been drawn in to make easier the comparison of time relations between the Q wave of the EKG and file ballistle waves. A, from a 10 year old girl weighing 87 pounds. B, from a dog weighing 32 pounds. In both records the major waves are similar in amplitude and direction and they occupy thh same relative positions in the cardiac cycle, although file absolute duration of systole is considerably less in file dog. The wave marked L o in record A marks the end of systole and its counterpart is present in record B also. (Ileprodueed by permission of the American Heart Journal.)

The normal ULF force ballistoeardiogram from the dog is similar to that from the human, as figure 6 shows. A prominent K wave is frequent in dogs but uncommon in humans. The duration of the ballistic complex (H-L) is shorter in dogs and this is related to the fact that systole is shorter in the dog than in man at any given heart rate. Still another difference is that whereas in man the amplitude of the systolic complexes increases during inspiration, in dogs it usually decreases. Sinus arrhythmia appears to b e more marked in dogs than in man and this may provide a partial explanation for tl~s phenomenon. The asynchronous behavior of the two sides of the heart in the human during the cardiac cycle,3s has also been observed in the dog by Moseovitz and Wilder. °~ The general temporal relationship between the ballistoeardiographic waves and events of the cardiac cycle in dogs appears to be similar to that shown for man in figure 4.c~

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Other experimental studies on the dog which have been carried but or are in progress include: (1) simulation of certain types of congenital or acquired cardiovascular lesions observed in humans, (2) assessment of the relative importance of heart motion and blood flow in tile great vessels in producing the ballistocardiogram, (3) segregation of the contributions of the pulmonary and systemic circulations, (4) circulatory changes associated with respiration, and (5) relationship between cardiac contractile forces (as measured with the Walton strain gage arch) and BCG amplitude before and after the administration of various drugs. Research in two of these areas should be touched on. Darby, Walton and their colleagues55,G3,6~ at tile Medical College of South Carolina have obtained some interesting results on dogs relating cardiac contractile force to tile ballistocardiogram. Walton strain gage arches were sutured to the walls of tile ventricles and the dogs were allowed to recover. Later, under light anesthesia, measurements of cardiac contractile force were made from tile implanted gages along with simultaneous ballistocardiograms and blood pressures, before and after the injection of sympathomimetics and other drugs. They observed that changes in contractile force were associated with parallel changes in tile amplitude and the I-J interval of the ballistocardiogram. Increased contractile force was associated with increased I-J amplitude, decreased I-J interval, or both, depending on the blood pressure response. These observations were made with a special direct-body device which recorded acceleration but similar results have been obtained recently with force ballistocardiograms from an ULF system,c5 Studies have been carried out in a number of laboratories in an effort to evaluate the relative importance of motions of the heart's mass and of blood flow in the origin of the ballistocardiogram. Whether the size and shape of the ballistocardiogram are largely determined by the "clanking" of the cardiac pump or by mass-motions in the vessels is a point of no little importance. It is generally conceded that cardiac motion does make a contribution to the over-all mass-motion pattern but tile amount it contributes is unknown. Although motions of the center of gravity of tile heart and the blood within it may be small, its mass is normally some four to five times as large as that of the blood ejected and for this reason cardiac mass-motions cannot be dismissed. No entirely satisfactory technical method has yet been devised to segregate heart motion from blood flow. Tile methods used have been rather crude and involved inflow obstruction (vena caval occlusion), outflow obstruction (pulmonary artery and aortic occlusion) or both. Thomas, Frederick, Eddleman and others G6 recorded displacement ballistocardiograms from tile skulls of dogs partially imbedded in sand. They found that after complete infow obstruction there was usually some decrease iri the amplitude of tile systolic waves (though in two of nine dogs th'ere was an increase) but the timing of tile waves remained tile same. Similar results were apparently obtained more recently with force, records from a n ULF dog bed. ~9 The significance of the results obtained by these obser~;ers after complete outflow obstruction alone and after inflation of balloons in the heart with air seems highly conjectural. While conceding that blood flow plays a significant role in tile origin of tile ballistocardiogram, it was concluded that forces

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Fro. 7.---Ballistocardiograms from a dog before (A) and 30 sec. after (B) complete occlusion ~)f both vena cavas with pneu,natic cuffs. HF-A and Lat-A refer to head-foot and lateral acceleration records, respectively. PA is puhnonary artery pressure (somewhat dampcd). RESP refers to respirator air pressure during artificial respiration. After caval occlusion the HF-A ballistocardiogram is very small and the waves are difficult to identify with any certainty. The position of the electrocardiographic Q wave is indicated near the BCG tracings to facilitate timing of the waves. associated with cardiac motion m a k e an important contribution. Studies on inflow obstruction b y Honig and Tenney ~7 and b y us ~-~ p r o d u c e d rather different results, for the systolic waves w e r e . virtually eliminated and their identity was often uncertain. Figure 7 shows the effect on the dog ballistocardiogram of complete occlusion of both vena cavae b y inflating pneumatic cuffs a b o u t them. Neither this method nor Honig's p r o d u c e d any abnormal

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coupling of tile beating heart to the chest wall or to the rest of tile body and this may explain, in part, the differing results. In any case, experiments of this sort do not constitute a fair test inasmuch as both cardiac motion and blood flow are altered. Therefore, the question of the relative importance of cardiac motion remains unsettled. \Vhen viewed as a whole, it seems fair to say that studies on experimental animals have not, as yet, contributed much important information on tile origin of the ballistocardiogram, but it is expected that they will in the future. CLINICAL APPLICATION OF BALLISTOCARDIOGRAPttIC~{ETttODS

The bulk of the literature on ballistocardiography deals with the clinical application of the method to the study of patients and this has been reviewed and discussed in other publicationsY G'~ We will be concerned here with some rather broad generalizations and with connnents on recent developments. Most of the interest in clinical application at present revolves around the use of the newer ULF ballistocardiographic systems. For reasons already described, however, clinical studies with them have been limited and information is only gradually accumulating. Two large groups of young normal subjects have been studied and detailed time and amplitude measurements have been carried ont on the displacement, velocity and a'cceleration recordsY .35 These should serve as a backgrotmd against which studies on patients with cardiovascular disease may be projected. Comparisons between normal and abnormal Starr and ULF force records have been made. 9 In general, when one of tJmse is "abnormal," the other is likely to be "abnormal" also but as yet no precise criteria of normality for the ULF records have been defined. The relative simplicity of Starr records makes them rather easy to classify while the complexity of U L F force records presents much more diificulty. The best and most comprehensive clinical study yet conducted with an ULF bed is that reported in a Dutch monograph* by Elsbach, 3~ who used the Burger bed design. He first "studied normal subjects, made detailed time and amplitude ratio measurements from the records and demonstrated clearcut age trends. Then, concentrating on patients under the age of 40 (to avoid the problem of abnormality in older individuals) he studied a variety of congenital and acquired, cardiovascular diseases. He found characteristic "patterns of abnormality in some of these conditions and showed tlmt these alterations correlated rather well with the results of other physiologic measurements made on these patients. The displacement, velocity and acceleration records presented complementary information; in some conditions the most marked changes were found in one type of record while in other conditions the most striking findings appeared in one of the other two. It is an odd, but not inexplicable, occurrence that in hvo patients the displacement records may be similar while the corresponding acceleration records *This monograph has been translated inio English, and copies may be obtained from the author of this paper.

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FIG. 8.wULF ballistoeardiograms from a young female with mitral stenosis and sinus rhythm. All three head-foot records (D, V and A) are abnormal. The giant displacement (D) wave marked "G + "I represents a large abnormal displacement of blood (and heart) first footward, then headward; it is probably a counterpart of tile giant "a" wave observed in the left atrial pressure records from patients with mitral stenosis and sinus rhytbna. In B the "J-"M detlection, normally the largest of the displac.cmcnt record, is almost nonexistent. The acceleration record (A) shows abnormal footward forces in presystole and early systole; this pattern is similar to that observed in Starr recorcJs. Compare with figures 2 and 3.

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FIG. 9.--ULF ballistoeardiograms (D, V and A) from a normal subject (.4) and a patient

(B) who sustained a myocardial infarction in the past. The men were comparable in age, weight and habitns. Calibration is the same for both. All three headfoot records in B are abnormal and are quite different from the normal records in A, both in amplitude and waveform, are quite different and vice versa. 14 Studies of the type carried out by Elsbach deserve, and will undoubtedly receive, more and more attention from investigators in the future. "Figures $ and 9 illustrate the sort of alterations which mhy be found; these should be compared with the normal ballistocardiograms in figures 2 to 4. The records in figure 8 are from a 21 year old female with relatively pure mitral stenosis and sinus rhythm. Tile acceleration records show abnormal footward forces in presystole and early systole which distort this portion of the record. This resembles the pattern observed in Starr bed records from patients with mitral stenosis, 7°,TI although it is clearer here that the abnormal forces begin before the onset of systole. The displacement record shows a massive "G wave which is fused with the "I. This represents an unusually large and abnormal footward displacement of cardiovascular mass (heart and blood) beginning about the time of atrial systole; it is probably a counterpart of the giant "a'" wave observed in the left atrial pressure tracings from patients with mitral stenosis and sinus rhythm. It is most likely due, at least in part, to the footward displacement of the enlarged heart produced by the vigorous contraction of the ]eft atrium in its effort to force blood through the stenotic orifice; other events in early systole may also contribute. In figure 8B the "J-"M deflection, normally the largest in the displacement record, is almost nonexistent,

C U R R E N T STATUS OF BALLISTOCARDIOGRAPtlX"

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Figure 9 shows the ULF records from two young men of approximately the same age, weight and habitus. Those in A are from a normal man while those in B are from a patient who had sustained a myocardial infarction in the past. The marked differences in all three records, both in waveform and in amplitude (calibration is the same in both), are clearly apparent. In the past extensive studies with the Starr and "direct-body" methods have been carried out on patients with various types of cardiovascular disease. In a few conditions characteristic patterns were found but in most cases abnormalities of a nonspecific kind were observed. Tile lack of specificity as regards the anatomic lesion may have been due, in part, to the fact that the records from these instruments were distorted and lesion-specific patterns were obscured. There is evidence that the comparable (force) record from the ULF ballistocardiograph, because it is relatively free of distortion, may show paterns which are more lesion-specific. However, a great deal of this should not be expected because the ballistocardiogram gives information primarily about the [,nctiomtl state of the circulation. If the same functional derangement is produced by different cardiovascular diseases, the changes in the ballistocardiogram are usually the same. In this respect, it is in the same category with stroke volume or cardiac output, neither of which provide any direct information about anatomic abnormality or etiology. Nevertheless, the differing aspects of circulatory function depicted by ULF disp;acement, velocity and acceleration ballistocardiograms may make it possible to define more precisely the nature of various functional derangements and by so doing be more useful clinically. Still more information may accrue if motions along the other two body axes arc recorded. The exact importance and clinical utility of the information obtained with ULF ballistocardiographic systems cannot be clearly defined until more extensive studies are carried out. The study of patients with valvular and congenital heart disease should prove a fruitful area of investigation. The clinical findings with the Starr, Dock and Nickerson methods are well documented although the latter has not'been applied nearly so extensively as the former two. Most of this work has been in the nature of empirical clinical correlation wherein the BCG pattern is related to the type of heart disease and to the circulatory alterations known or though t to be associated with it. In general, the ballistocardiogram is abnormal in most patients with overt heart disease and in a higher proportion of those in whom the process is well advanced. Rather characteristic patterns have been observed consistently in a few specific types of cardiovascular disease. These include: mitral stenosis, coarctation of the aorta, early hypertension, aortie insufficiency and constrictive pericarditis. Abnormalities confined to diastole are not infrequent in congestive heart failure and in gallop rhythm. In disorders associated with high or low cardiac output tbe amplitude of the systolic, waves may be abnormally great or small but there is usually no particularly specific pattern. In most other conditions nonspecifie abnormality of the systolic waves is found. A general correlation has been observed between 'changes in the ballistocardiogram and in clinical course. Improvement in the patient's clinical condition, whether spontaneous or as a result of surgical or drug therapy, is usually associated with improvement in the BCG. This kind of relation has

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led to the conviction that changes in the ballistocardiogram may reflect real changes in circulatory function even though gross clinical signs of such are not evident. - While there is evidence which supports this conviction, opinion should be reserved until it can be more convincingly proven that a definite relationship of this sort exists. The changes in the ballistocardiogram with age have been the subject of much work and speculation. Tile trends are clearly apparent. In healthy subjects under tile age of 40, the BCG is almost invariably normal in waveform but from the age of 20 on there is a progressive, linear decrease ill amplitude. In clinically healthy individuals above 40, there is a progressive and rapid rise in the frequency of almormal ballistocardiograms, reaching approximately 90 per cent in the eighth decade. The reason for such abnormalities are still unknown. Although this is tile age span in which coronary disease manifests itself, there is no direct proof that the abnormal ballistocardiograms are a consequence of subclinical coronary disease in "apparently" healthy persons. Other factors such ~ls changes in aortic elasticity must be excluded. Aortic elasticity is known to decrease with age and in a small gronp of s:d~jects with and without coronary disease it was observed that the BCG was not normal in anyone whose pulse wave velocity (subclavian-femoral) was greater than 8 meters/secY Mean sernna cholesterol increases with age in normal persons (tlaough it falls in later decades) but no relationship has been demonstrated in these individuals between serum cholesterol and normality or abnorma.lity of the ballistocardiogramY ',rs It must be conceded that the cause of the abnormal ballistocardiogram in older normal persons remains uncertain. However, the results of long-term follow-up studies have begun to suggest that the development of manifest coronary disease is more frequent in normal individuals with abnormal (or unusually small) ballistocardiograms than in those with normal ones. 74,7~ More information of this sort will be of vital importance to the solution of this problem. The importance of ballistocardiography in the diagnosis and management of coronary heart disease is still ari unsettled, and rather controversial, matter. ;6 There is no question that the ballistocardiogram is abnormal in a high • o 8,} proportion (about 80 per cent of all ages) of patients with this condttlon. The limitation of the method in the diagnosis of coronary disease stems from the fact, already referred to, that in the coronary age span, abnormal ballistocardiograms are not infrequent among normal persons. In both groups the abnormality is of the nonspecific variety anti both show similar progressive increases in frequency of abnormality with age. The nature of these relationships is such that the most significant findings are abnormal records from young persons antl normal records from older ones. A variety of stress procedures has been used in conjunction with the ballistocardiograph and these have improved somewhat the discrimination between stibjects with and without coronary disease. 76 In this laboratory the "cigarette test ''rr has proved the most useful of these procedures but it is not without its drav~backs. When the control ballistocardiogram is abnormal, as it usually is in liatients with coronary disease, it is sometimes difficult to determine whether it has become significant1); more abnormal afterlsmoking. Exercise is a more physiologically normal

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form of stress but it is technically difficult to obtain artifact-free ballistocardiograms during the period immediately after exercise. The intravenous injection of ergonovine in conjunction with ballistocardiography may prove useful diagnostically but experience thus far has been too limited to be sure; the same may be said of sublingual nicotine. It should be appreciated that the major source of difficulty in differentiating objectively between normal subjects and patients with coronary disease arises from our uncertainty as to whether the "apparently" normal subjects are, in fact, free of coronary disease. Indeed, tlle evidence indicates that there is a substantial amount of coronary atherosclerosis in asymptomatie individuals over tile age of 40. Sudden death or myocardial infarction occur far too often in asymptomatic, clinically normal persons to leave any doubt that groups of normal individuals will contain some persons with advanced, "sub-clinical' coronary disease. This obviously makes the problem difficult and necessitates long-term, follow-up studies. A solution to the problem could be provided if it were shown that the individuals in the normal group whose control ballistocardiograms were abnormal or whose stress tests were positive developed coronary disease earlier or more frequently than did the remainder of the individuals. Several studies of this kind are in progress. In untreated patients with coronary disease, once the ballistocardiogram becomes abnormal, it usually remains so, barring major changes in clinical state (e.g., during the recovery from myocardial infarction). This does not deny that minor or mo.derate changes may occur but marked spontaneous improvement is unusual. The results of recent studies suggest that abnormality of the BCG is not as irreversible as formerly thought. Significant, and occasionally dramatic, improvement of the ballistocardiogram has been demonstrated in patients with coronary disease while on short- or long-term low fat diets 7s-s° and during treatment with estrogen preparations, sl While this does not guarantee that improvement in circulatory function has occurred in these patients, it does carry a rather strong implication, particularly since these forms of treatment are known to "alter serum ]ipids in a manner believed to be favorable. These observations should be sufficiently impressive to stimulate others to investigate this matter. It is in studies such as these that ballistocardiographi c information is most reliable for the patient serves as his own control. While extracardiac factors such as aortie inelasticity may contribute to the abnormality of the control ballistocardiogram, they are unlikely to change over short time periods and therefore there is less doubt in ascrihing changes to cardiac factors. Str~t~,tAnY Since Starr reawakened interest in it in 1939, a great deal of progress has been made in ballistocardiography. The methods (Starr," Dock, and Nickerson) used to record the ballistocardiogram lmve been subjected to intensive biophysical studies which showed that their performance depended, to a large extent, on the physical properties of the human body and, as a consequence, the information supplied by them differed and was, in all cases, distorted to a greater or lesser degree. Much of the confusion and contr0-

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versy about ballistoeardiography arose from this source and from tile fact that tlle diversity of methods made it difficult or impossible to compare the results obtained with them. Subsequent research led to the development of an improved instrument, called the "ultra low-frequency" (ULF) ballistocardiograph, which largely circumvents tlle errors inherent in tlle other methods and provides information of a different and complementary kind over a broader frequency range than was formerly possible. However, this instrument has been in use for such a short time and to such a limited extent that the accumulated results of clinical and physiologic studies are still rather meager; those obtained thus far have stimulated much interest. Significant progress has also been made in extending tlle BCG recording method from the longitudinal (head-foot) axis to the other two and a tri-directional ULF instrument has been developed. The physical basis of ballistocardiography is now on much firmer ground and the determinants of the ballistocardiogram are more clearly defined. Physiologic studies on humans and animals, though limited as yet, have begun to contribute insight into the origin and significance of the ballistocardiogram and its relationship to other aspects of circulatory function. Much remains to be done, however, in quantitating the relative contributions of the several determinants of the ballistocardiogram and, when waveform is abnormal, in ascribing this to one or another source. There is little reason to doubt that the method is capable of supplying important information about certain mechanical aspects of circulator), function of a kind not obtained by any other method and more directly related to the mechanical pumping function of the heart. A relationship.has been clearly established between the force of cardiac contraction and ejection and cardiovascular force as measured by the ballistocardiogram. Clinical studies with conventional methods have been carried out on a rather broad scale and some important contributions have been made. Most patients with overt heart disease, regardless of type, have ballistocardiograms which are abnormal in waveform: One of the handicaps has been that, in the absence of a thoroughly sound physiologic basis, it has been difficult to be sure what particular factor (or factors) is responsible for the various types of waveform abnormalityl 1,1 some clinical conditions rather characteristic patterns are observed but in most cases the abnormal pattern is Of a n6nspecific kind. There is evidence that the records from the newer ULF systems may be more "lesion-specific" but it seems much more likely that they will provide evidence regarding functional state rather than anatomic lesion. Research now in progress should lead to an improved understanding of what a particular type of waveform abnormality means in terms of specific alterations of circulatory function. C h a n g e s in the ballistocardiogram with age have presented, at once, a problem and a challenge. In normal persons under 40, the ballistocardiogram is almost invariably normal but above this age there is a progressive, rapidly increasing incidence of abnormal records for reasons which are not yet clear. Abnormal records from subjects less than 40 years of age are considered • quite significant and constitute rather definite evidence of derangement of

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circulatory function in some form or other. The significance of the abnormal ballistocardiogram in subjects above the age of 40 is rather uncertain. It has been suggested that sub-clinical coronary disease is responsible for the abnormal ballistocardiograms observed in older normal subjects and there is some indirect evidence which supports this view. Nevertheless, there is no direct proof that this is the case and until it is proven, it seems dangerous to make this assumption. Physiologic work and the results of long-range followup studies should settle this matter. The frequency of abnormal ballistocardiograms in older clinically normal individuals is the chief factor which limits the diagnostic value of ballistocardiography in coronary artery disease, even though abnormal records are observed in a high proportion of patients with this condition. If it turns out that subclinical coronary disease is responsible for the ballistic abnormality in the older normal person, then it will be possible to confirm objectively the diagnosis of coronary disease in most patients with this disease. Ballistocardiography lms been useful in the management of these patients and especially in evaluating the effect of various forms of treatment. Recent results of therapy with low-fat diets and estrogens have been enlightening. Although it is not recommended at present that ballistocardiography be used as a diagnostic method in the imlividual patient above the age of 40, there is no limitation on its application to the study of younger individuals and the newer ultra-low frequency instruments are expected to contribute important new clinical information. It should be emphasized that, regardless of the complexity of the BCG system, as far as the patient is concerned it is a simple test involving no physical discomfort or risk and requires nothing from him other than that he lie quietly on a bed. This discipline is now in a transition period and much more important information about circulatory function should be forthcoming.

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REFERENCES Starr, I., Rawson, Ai J., Schroeder, H. A., 5. --~ and --: Physical basis of balllstocardiography. II. Tile quantities that and Joseph, N. K.: Studies on the estimation of cardiac output in man; can be measured with different types of abnomaalities in cardiac function of ballistocardiographs and their mufrom the heart's recoil and the blood's tual relations. Am. Heart J. 51:127, ilppact. Am.J.Physiol. 127:1, 1939. 1956. Baker, B. M., Jr., Scarborough, W. R., 6. yon Wittern, ~V. W.: Ballistocardiography with elimination of tile inDavis, F. W., Jr., Mason, R. E., fluence ofthe vibration properties of Singewald, M. L., and Deuchar, D. C.: Ballistocardiography. (Editorial the body. Am. Heart J. 46:705, 1953.. l)y l)r. A. McG. Harvey) Am.J.Med. 7. Talbot, S. A., and ttarrison, W. K., Jr.: Dynamic comparison, of current bal27:295, 1954. Scarborough, W. R., and Baker, B. M., llstocardiographic metlmds. Part I: Jr.: (Editorial) Ballistoeardiography~ Artifacts in" the dynamically simple Appraisal of current status. Circulaballistocardiographic methods. Part II: Effect of a platform in BCG dytion 16:971, 1957. Burger, tt. C., Noordergraaf, A., and • namics. Part III: Derivation of cardioVerhagen, A. M. ~V.: Physical basis vascular forces from body motion. of the low-frequency ballistocardioCirculation 12:577, 845, and 1022, graph. Am. Heart J. 46:71, 1953. 1955.

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21. 22.

23.

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25.

26.

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dogs to changes in circulation by relations of direct-body ballistocardioadministration of substances with gram with other cardiovascular meascardlovascnlar action. Acta physlol. urements. Fed.Proe. 15:415, 1956. plmrmacol. Neerlandica. 6:53, 1957. 65. - - : Personal communication. Frederick, W. H., Thomas, H. D., 66. Thomas, H. D., Frederick, W. H., Knowles, J. L., Tucker, W. T., and Knowles, J. L., Reeves, T. J., Pappas, Eddleman, E. E., Jr.: The ballistoR., and Eddleman, E. E., Jr.: Tile cardiogram of the normal dog. Am. effects of occlusion of the venae Heart J. 50:416, 1955. cavae, aorta, and pulmonary artery Darby, T. D., Goldberg, L. I., Gazes, on tile dog ballistoeardiogram: Am. P. C., and Arbeit, S. R.: Methods of tteart J. 50:424, 1955. obtaining direct-body displacement- 67. Searborough, \V. R., Davis, F. W., Jr., velocity- acceleration ballistocardioBaker, B. M., Jr., Mason, R. E., and Singewald, M. L.: A review of balgrams of the dog. Proc.Soc.Exper. Biol.& Med. 86:673, 1954. listocardiography. Am. Heart J. 44: Cossio, P., Berreta, J. A., and Mosso: 910, 1952. H. E.: The ballistocardiogram and 68. Deck, W., Mandelbau,n, H., and Manthe blockage of circulation through delbaum, R. A.: Ballistocardiography. ligature of both venae cavae. Am. The Application of the Direct Ballistoeardiograph to Clinical Medicine. Heart J. 49:72, 1955. Honig, G. R., and Tenney, S. M.: The St. Louis, C. V. Mosby, 1953. relationship between the ballisto- 69. Symposium on Ballistocardiography. Am. cardiogram, cardiac movement, and J. Cardiol. Vol. II, No. 4, Oct., 1958, blood flow. Am. Heart J. 52:167, and subsequent issues. 1956. 70. Davis, F. W., Jr., Scarborougb, W. R., Scarborough, W. R.: Some circulatory Mason, R. E., Singewald, M. L., and effects of morphine-barbiturate anesBaker, B. M., Jr.: The ballistocardiothesia, artificial respiration, and abgram in mitral stenosis. Circulation dominal compression based on bal7:503, 1953. listocardiographlc observations on 71. tlenderson, C. B." Tbe abnornral baldogs. With a review of pertinent listocardiogram in mitral stenosls. The literature. Am. Heart J. 54:651, 1957. relationship of the abnormal waves Frederick, W. H., and Eddleman, E. E., to right vcntrlcnlar ejection and to Jr.: Genesis of the force ballistocardiothe mean pulmonary artery pressure. Circulation 12:858, 1955. gram of the dog. J.Appl.Physiol. 13: 109, 1958. • 72. Taylor, H. L., and Anderson, J. T.: Malt, R. A.: Depressant effect of ether Serum cholesterol concentration and on the heart: A study with the ultraballistocardiograpbie characteristics of middle-aged men. 2nd World Conlow-frequency force ballistocardiogress of Cardiology. Washington, graph. Am. Heart J. 55:572, 1958. D.C., Sept. 1954. Abstracts, p. 232. Moscovitz, H. L., and Wilder, R. J.: Pressure events of the cardiac cycle 73. Scarborough, W. R., Smith, E., and' in the dog: Normal right and left Baker, B. M., Jr.: Studies on subjects with and without coronary heart heart. Circulation Research .4:574, disease. Lipid, lipoprotein and protein 1956. determinations and their relation to Scarborough, W. R.: Unpublished obballistocardiographie findings. Am. servations. Heart J. In press. Darby, T. D., Goldberg, L. I., Gazes, P . C., and Arbclt, S. R.: Effects of 74. Starr, I.: On the later development of heart disease in apparently healthy cardiovascular drugs on the accelerapersons with abnormal ballistocardiotion ballistocardiogram of tbe dog. grams. Eight-to-.ten year after-histories J.Pharmacol.& Exper.Therap. 113:14, 1955. of 90 persons over 40 years of age. - - , and Walton, R. P.: Syncbror~ous reAm.J.M.Sc. 214:233, 1947. cordings in the dog to determine 75. - - , and Wood, F. C.: After-histories of

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100 males, originally healthy; followed Until death or for 20 years after their first ballistocardiograms. Abstracts of Communications p. 419--llIrd World Congress of Cardiology--Brussels, Sept., 1958. Davis, F. 'W.: Tile role of the ballistocardiograph in the diagnosis and management of patients with coronary heart disease. AmJ.Cardiol. 3: 103, 1959. Davis, F. W., Jr., Scarborough, W. R., Mason, R. E., Singewald, M. L., and Baker, B. M., Jr.: The ballistocardiographic cigarette test: further observations. Am. Iteart J. 51:165, 1956. Urbach, F., Hildrcth, E. A., and Wackerman, M. T.: The therapeutic uses of low fat, low cholesterol diets. J.Clin.Nt,trition. 1:52, 195"9-,--53. Kuo, P. T., Whereat, A. F., and Horwitz, O.: The effect of lipemia upon coronary and peripheral arterial circulation in patients with essential

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byperlipemia. Am.J.Med. 26:28, 1959. 80. Searborough, W. R., Singewald, M." L., and Scarborough, S. A.: Low fat, low cholesterol diet in coronary artery disease. A follow-up study of ballistocardiograms after long-term diet therapy. Unpublished observations. 81. Davi~, F. W., Jr., Scarborough, W. R., Mason, R. E., Singcwald, M. L., and Baker, B. M., Jr.: Experimental hormonal therapy of atherosclerosis: Preliminary observations on tile effects of two new compounds. Am.J. M.Sc. 235:50, 1958. 82. Scarborough, W. R., Mason, R. E., Davis, F. W., Jr., Singewald, M. L., Baker, B. M., Jr., and Lore, S. A.: A ballistocardiograpbic and electrocardiographic study of 328 patients with coronary artery dis:asc; comparison witb results from a similar study of apparently normal persons. Am. Ileart J. 44:645, 1952.