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A critical appraisal of ballistocardiography
Editorials
A Critical Appraisal of Ballistocardiography
E. E. EDDLEMAN, WALTER
Jr.,
MD,
K. HARRISON,
DAVID
H. JACKSON,
HENRY
L. TAYLOR,
Birmingham,
FACC*
PhDt MD*
PhDt
Alabama
The ballistocardiograph is an apparatus for recording body motion, generally in a head-foot direction, secondary to cardiovascular phenomena. The first records were obtained by Gordon1 in 1887, and since then there have been many efforts to determine the mechanisms of these body movements and their relation to cardiovascular events. Useful clinical interpretations of the abnormal wave forms that occur in patients with heart disease have been difficult to achieve. In spite of a vast amount of investigative work over the past 50 or so years, the technique has not reached a satisfactory state of scientific or clinical acceptability. Many investigators, particularly in recent years, have avoided ballistocardiographic studies, and in the medical community there is a fair amount of skepticism concerning the merits and clinical uses of the technique. Several factors have contributed to these negative attitudes.
Problems in Methodology Probably the most important problems have been those of methodology. Many of the published reports have been concerned with techniques of recording the movements of the body, and various methods have been proposed. Consequently, the contours of the ballistocardiogram have varied according to the techniques employed, and conflicts have arisen about the nature of the “true” ballistocardiogram. Confusion on methodology can often be attributed to lack of understanding of (1) the physical problems related to coupling of the body From the Department of Medicine, University of Alabama School of Medicine and the Veterans Administration Hospital, Birmingham, Ala.*; the Department of Biomedical Engineering, the Johns Hopkins University School of Medicine, Baltimore, Md.t; and the School of Public Health, University of Minnesota, M,inneapolis, Minn.t This study was supported by U. S. Public Health Service G,rants HE-11310 and HE-13050. Address for reprints: E. E. Edd,leman, Jr., MD, Veterans Administration Hospital, 700 South 19th St., Birmingham, Ala. 35233. 120
to the platform from which the motion is sensed (whether this be a bed which a patients rests upon or a platform which rests upon a patient) ; (2) the physical characteristics of the transducer used to record the motion; and (3) the relations between displacement, velocity and acceleration of body motion. Physical problems concerning the accurate registration of body movement: These problems, discussed by Talbot and Harrison2 in 1955, are still not fully appreciated. The latters’ studies demonstrated that the body has a tendency to oscillate on the platform due to its elastic coupling, thus producing artifacts in the records, These artifacts can be made negligible by reducing the weight of the moving part of the bed to less than 10 lb, eliminating or making minimal use of bed damping, and keeping the period of the bed below 0.3 Hz. When these principles are observed, such a platform closely follows the movements of the body, and the records can be satisfactorily interpreted with relative ease. Lack of proper recognition of physical difficulties has rendered almost useless a number of published studies. There are now several different ways of constructing suitable platforms. Lightweight beds can be made of honeycomb aluminum and can be suspended in different ways yet still yield essentially the same records. The platform can be hung by long cables from the ceiling so that the period of the bed is near the ideal figure cited. More practically, the bed can be equipped with an air-bearing type of suspension. Records obtained with this technique are essentially identical to those obtained during weightlessness,3s4 thus confirming that these traces represent the ideal form of the ballistocardiogram. Since the ultra-low frequency ballistocardiogram has been shown to reflect accurately the ideal body motion, this method is clearly the most advantageous one for recording ballistocardiograms. Understanding of the mathematical relationships between velocity, acceleration and displaceis essential for the ment : Such understanding correct interpretation of ballistocardiograms. Motion can be recorded by any or all 3 types of sensors, depending upon the purpose of the investigation. Physicians often poorly appreciate that all 3 types of records really contain the same information, since velocity is the first mathematical time derivative of displacement, and acceleration is the second time derivative of displacement. ThereThe
American
Journal
of
CARDIOLOGY
EDITORIALS
fore, the informational content in all 3 records is the same, and the only basis for choosing a particular type of sensing transducer is the purpose of the investigation. Often some abnormalities are more easily visualized in one type of recording than another. It is now generally agreed that the acceleration record is probably the most useful, since abnormalities are more easily recognized in these records than in the velocity or the displacement traces.
Problems
Related
to Aim of Investigator
The second problem that has resulted in a negative attitude toward ballistocardiography is related to the purpose or aims of the investigators. A few examples are worth mentioning. Initially, a great deal of attention was directed toward the calculation of stroke volume. Reasonable results were obtained in normal subjects; however, the technique was generally not successful in disease states in which the information concerning stroke volume would be particularly useful. For example, studies revealed a fairly close correlation between stroke volume and that calculated from the ballistocardiogram in normal persons”; however, in shock the relation was inaccurate. Most of these studies were performed on platforms not conforming to the physical principles cited. Several recent studies have revived this problem using the low frequency displacement ballistocardiogram.6.7 From a theoretical standpoint these calculations should be more accurate than the earlier studies. However, there is a paucity of data on direct comparisons between stroke volume calculated from the ideal ballistocardiogram, and from reference methods, under the necessary carefully controlled conditions. Another discouraging approach to the ballistocardiogram was the attempt to diagnose many specific cardiovascular disease states from the abnormal patterns encountered. It became quite obvious to most investigators working in the field that very few disease entities were associated with specific wave contours which would aid in the clinical diagnosis. Thus, the general opinion gradually developed that the ballistocardiogram was an interesting phenomenon but generally useless when applied to the problems of clinical medicine.
Ballistocardiogram Myocardial
as Reflection Function
of
The purpose of this editorial is to point out that in spite of these discouraging problems, the potential use of the technique in clinical medicine could be exceedingly significant, depending upon the questions asked. It now appears quite clearly that the ballistocardiogram is a very sensitive reflection of myocardial function which is not related to specific disease entities or merely to stroke volume alone. VOLUME
29, JANUARY
1972
This concept should have been recognized earlier from the studies of Starr and his co-workers.x-l” The fact that he was able to generate abnormal patterns in the cadavers by altering the way that blood was injected into the aorta clearly supports this concept. Since Starr’s classic work on the ballistocardiogram, there have been several significant studies that support this hypothesis, The first is that of Harrison et al.,“” who measured in a group of 20 patients the first derivative of pressure rise (dp/dt) in the left ventricle simultaneously with ultra-low frequency ballistocardiograms. A close correlation was found between the amplitude of the IJ wave and the maximal first derivative of the left ventricular pressure pulse. Since the maximal dp/dt of the left ventricle is an index of cardiac function, this relation offers significant evidence that the ballistocardiogram does indeed reflect myocardial function. The second investigation is that of Jackson et al.?’ in a group of patients with hypertrophic subaortic stenosis. Significant relations between certain ballistocardiographic variables and the ejection fraction, as well as the rate of volume ejected by the left ventricle, were demonstrated, again supporting the concept that the ballistocardiogram reflects myocardial function. BargeronZ2 has also recently demonstrated that severity of aortic ventricular gradients in children with aortic stenosis is highly correlated with ballistocardiographic observations, Ballistocardiogram in early detection of ischemic heart disease: There have now been 3 very exciting investigations relating to the question of early detection of ischemic heart disease. Starr2”?” initially observed in a long-term follow-up study of so-called normal subjects that patients with initial abnormal contours died earlier than patients whose ballistocardiogram was normal. Similar results were obtained by the group at Hopkins,27 and recently Taylor and co-workerP have observed confirming results. Thus, an abnormal pattern in the ballistocardiogram appears to be a sensitive predictor of probable early mortality. These studies were performed on ballistocardiographic beds that do not meet the standards cited. Artifactual content of the records could only reduce prognostic sensitivity below that obtainable with the optimal apparatus. Even though older recording techniques were used, the results cannot be overlooked since 3 independent studies have arrived at the same conclusion. A plausible interpretation of these data is as follows : It is well recognized that coronary atherosclerosis begins in early life, particularly in the male, and tends to progress over the years. Since even minimal coronary artery disease is usually associated with areas of patchy fibrosis in the myocardium, one may assume that those subjects who had abnormal ballistocardiograms at an early age already had sufficient decrease in myocardial func121
EDITORIALS
tion (undetectable by other means) which altered the pattern of the record. Therefore, it can be reasonably concluded that ballistocardiography may very well be the best and most reliable technique for recognizing incipient heart disease or the presence of early changes in myocardial function due to minimal ischemia. In summary, it would appear that the approach to the study of the ballistocardiogram in general has been in the wrong direction and that its greatest usefulness lies in early detection of heart disease and in evaluating myocardial functional
abnormalities. Techniques are now available for quantifying myocardial function more precisely, such as the use of the ejection fraction, left ventricular dp/dt, and instantaneous aortic root blood flow in patients. Other indexes of function can be determined by quantitative angiocardiography, and a new look at ballistocardiography should and undoubtedly will be undertaken. With the present emphasis on early detection of heart disease, we may well have overlooked the most sensitive method available, and one that is easy to apply.
References 1. Gordon JW: On certain molar movements of the human body produced by the circulation of the blood. J Anat Physiol 11:533-536, 1877 of cur2. Talbot SA, Harrison WK Jr: Dynamic comparison rent ballistocardiographic methods. l-3. Circulation 12: 577-587,845-857, 1022-1033, 1955 Triaxial BCG-ECG flight and 3. Hixson WC, Beischer DE: laboratory data, chap 8. In, Biotelemetty of the Triaxial Ballistocardiogram and El,ectrocardiogram in a Weightless Environment. Monograph 10, US Naval School of Aviation Medicine, US Naval Aviation Medical Center, Pensacola, Florida, 1964, p l-15 in 4. Beischer DE, Hixson WC: Triaxial ballistocardiogram a weightless environment. In, Proceedings of the 1st World Congress in Ballistocardiographic Cardiovascular Dynamics, Amsterdam (Knoop AA, ed). Base1 and New York, S Karger, 1966, p 85-89 5. Nickerson JL, Warren JV, Brannon ES: The cardiac output in man; studies with the low-frequency, critically damped ballistocardiograph, and the method of ‘right atrial catheterization. J Clin Invest 26:1-10, 1947 WK: A ballistocardiographic stroke volume 6. Harrison formula recognizing blood distribution effects. Bibl Cardiol 20:29-34, 1968 H. Schaede A. Thurn P. et al: Comoarative 7. Klensch study of cardiac output with the ballistic and th’e direct Fick methods. Pfluegers Arch Physiol 269232-239, 1959 8. Starr I: The relation of the ballistocardiogram to cardiac function. Amer J Cardiol 2:737-747, 1958 9. Starr I, Horwitz 0, Mayock RL, et al: Standardization of the ballistocardiogram by simulation of th,e heart’s function at necropsy; with a clinical method for the estimation of cardiac strength and normal standards for it. Circulation 1:1073-1096, 1950 10. Starr I, Schnabel TG Jr, Mayock RL: Studies made by simulating systole at necropsy. 2. Experiments on the relation of card,iac and peripheral factors to the genesis of the pulse wave and the ballistocardiogram. Circulation 8:4461, 1953 11. Starr I, Schnabel TG Jr: Studies made by simulating systole at necropsy. 3. On the genesis of-the systolic waves of the ballistocardioeram. J Chin Invest 33:10-22. 1954 12. Starr I, Schnabel TG, Askovitz SI, et al: Studies made by simulating systole at necropsy. 4. On the relation between pulse pressure and card,iac stroke volume, leading to a clinical method of ‘estimating cardiac output from blood pressure and age. Circulation 9648-663, 1954 13. Starr I: Studies made by simulating systole at necropsy. 6. Estimation of cardiac stroke volume from the baltistocardiogram. J Appl Physiol 8:31#5-329, 1955 14. Starr I, Askovitz SI, Feder W, et al: Studies made by simulating systole at necropsy. 7. Clinical methods for estimating the work of the left ventricle. With a note on
122
15.
16.
17.
18.
19. 20.
21.
22.
23.
24.
25.
26.
27.
28.
the diminution of heart work as age advances. Circulation 12:1005-1021, 1955 Starr I: Studies made by simulating systole at necropsy. 8. Significance of the pulse pressure. Circulation 14: 1117-1128, 1956 Starr I: Studies made by simulating systole at necropsy. 10. State of peripheral circulation in cadaver preparations. J Appl Physiol 11:174-180, 1957 Starr I: Studies made by simulating systole at necropsy. 11. On the higher dynamic functions of the h,eart, and their reflections in the pulse wave. Circulation 17:589600,1958 Starr I: Studies made by simulating systole at necropsy. 12. Estimation of initial cardiac forces for the ballistocardiogram. Circulation 20:74-87, 1959 Starr I, Schnabel TG Jr, Mayock RL: Studies made by simulating systol,e at necropsy. Circulation 8:1-14, 1953 Harrison WK, Friessinger GC, Johnson SL, et al: Relation of the ballistocardiogram to left ventricular pressure measurements in man. Amer J Cardiol 23:673-678, 1969 Jackson DH, Eddleman EE Jr, Bancroft WH Jr, et al: Ballistocardiographic and angiographic correlative study in idiopathic hypertrophic subaortic stenosis. Bib1 Cardiol 27:14-20, 1971 Bargeron LM Jr: The force ballistocardiogram as an index of severity in congenital aortic stenosis. Circulation 37:238-243,1968 of heart disease in Starr I: On the later development apparently healthy persons with abnormal ballistocardiograms. Eight- to ten-year after-histories of 90 persons over 40 years of age. Amer J Med Sci 214:233-242, 1947 Starr I: Prognostic value of ballistocardiograms. Studies on evaluation of the doctor’s experience. JAMA 187:511517, 1964 Starr I, Askovitz SI, Mandelbaum EM: Items of prognostic value in the clinical study; the relationship of symptoms, heart size, blood-pressure, electrocardiogram, and ballistocardiogram to after-histories and to each other. JAMA 192:83-87, 1965 Starr I: Prognostic value of ballistocardiograms; as judged by after-histories of 211 healthy persons and 221 patients followed from 5 to 25 years. In Ref. 4, p 7-20 Baker BM, Scarborough WR, Davis FW Jr, et al: Ballistocardiography and ischemic heart disease: predictive considerations and statistical evaluation. Proc Roy Sot Med 60:1290-1296, 1967 Taylor HL, Keys A, Blackburn H: The contribution of the low frequency ballistocardiogram to the prediction of death from all causes and the development of coronary heart disease in 18 years among Minnesota business and professio,nal men: a multivariate analysis. In preparation.
The
American
Journal
of
CARDIOLOGY
A Critical Appraisal of Ballistocardiography
E. E. EDDLEMAN, WALTER
Jr.,
MD,
K. HARRISON,
DAVID
H. JACKSON,
HENRY
L. TAYLOR,
Birmingham,
FACC*
PhDt MD*
PhDt
Alabama
The ballistocardiograph is an apparatus for recording body motion, generally in a head-foot direction, secondary to cardiovascular phenomena. The first records were obtained by Gordon1 in 1887, and since then there have been many efforts to determine the mechanisms of these body movements and their relation to cardiovascular events. Useful clinical interpretations of the abnormal wave forms that occur in patients with heart disease have been difficult to achieve. In spite of a vast amount of investigative work over the past 50 or so years, the technique has not reached a satisfactory state of scientific or clinical acceptability. Many investigators, particularly in recent years, have avoided ballistocardiographic studies, and in the medical community there is a fair amount of skepticism concerning the merits and clinical uses of the technique. Several factors have contributed to these negative attitudes.
Problems in Methodology Probably the most important problems have been those of methodology. Many of the published reports have been concerned with techniques of recording the movements of the body, and various methods have been proposed. Consequently, the contours of the ballistocardiogram have varied according to the techniques employed, and conflicts have arisen about the nature of the “true” ballistocardiogram. Confusion on methodology can often be attributed to lack of understanding of (1) the physical problems related to coupling of the body From the Department of Medicine, University of Alabama School of Medicine and the Veterans Administration Hospital, Birmingham, Ala.*; the Department of Biomedical Engineering, the Johns Hopkins University School of Medicine, Baltimore, Md.t; and the School of Public Health, University of Minnesota, M,inneapolis, Minn.t This study was supported by U. S. Public Health Service G,rants HE-11310 and HE-13050. Address for reprints: E. E. Edd,leman, Jr., MD, Veterans Administration Hospital, 700 South 19th St., Birmingham, Ala. 35233. 120
to the platform from which the motion is sensed (whether this be a bed which a patients rests upon or a platform which rests upon a patient) ; (2) the physical characteristics of the transducer used to record the motion; and (3) the relations between displacement, velocity and acceleration of body motion. Physical problems concerning the accurate registration of body movement: These problems, discussed by Talbot and Harrison2 in 1955, are still not fully appreciated. The latters’ studies demonstrated that the body has a tendency to oscillate on the platform due to its elastic coupling, thus producing artifacts in the records, These artifacts can be made negligible by reducing the weight of the moving part of the bed to less than 10 lb, eliminating or making minimal use of bed damping, and keeping the period of the bed below 0.3 Hz. When these principles are observed, such a platform closely follows the movements of the body, and the records can be satisfactorily interpreted with relative ease. Lack of proper recognition of physical difficulties has rendered almost useless a number of published studies. There are now several different ways of constructing suitable platforms. Lightweight beds can be made of honeycomb aluminum and can be suspended in different ways yet still yield essentially the same records. The platform can be hung by long cables from the ceiling so that the period of the bed is near the ideal figure cited. More practically, the bed can be equipped with an air-bearing type of suspension. Records obtained with this technique are essentially identical to those obtained during weightlessness,3s4 thus confirming that these traces represent the ideal form of the ballistocardiogram. Since the ultra-low frequency ballistocardiogram has been shown to reflect accurately the ideal body motion, this method is clearly the most advantageous one for recording ballistocardiograms. Understanding of the mathematical relationships between velocity, acceleration and displaceis essential for the ment : Such understanding correct interpretation of ballistocardiograms. Motion can be recorded by any or all 3 types of sensors, depending upon the purpose of the investigation. Physicians often poorly appreciate that all 3 types of records really contain the same information, since velocity is the first mathematical time derivative of displacement, and acceleration is the second time derivative of displacement. ThereThe
American
Journal
of
CARDIOLOGY
EDITORIALS
fore, the informational content in all 3 records is the same, and the only basis for choosing a particular type of sensing transducer is the purpose of the investigation. Often some abnormalities are more easily visualized in one type of recording than another. It is now generally agreed that the acceleration record is probably the most useful, since abnormalities are more easily recognized in these records than in the velocity or the displacement traces.
Problems
Related
to Aim of Investigator
The second problem that has resulted in a negative attitude toward ballistocardiography is related to the purpose or aims of the investigators. A few examples are worth mentioning. Initially, a great deal of attention was directed toward the calculation of stroke volume. Reasonable results were obtained in normal subjects; however, the technique was generally not successful in disease states in which the information concerning stroke volume would be particularly useful. For example, studies revealed a fairly close correlation between stroke volume and that calculated from the ballistocardiogram in normal persons”; however, in shock the relation was inaccurate. Most of these studies were performed on platforms not conforming to the physical principles cited. Several recent studies have revived this problem using the low frequency displacement ballistocardiogram.6.7 From a theoretical standpoint these calculations should be more accurate than the earlier studies. However, there is a paucity of data on direct comparisons between stroke volume calculated from the ideal ballistocardiogram, and from reference methods, under the necessary carefully controlled conditions. Another discouraging approach to the ballistocardiogram was the attempt to diagnose many specific cardiovascular disease states from the abnormal patterns encountered. It became quite obvious to most investigators working in the field that very few disease entities were associated with specific wave contours which would aid in the clinical diagnosis. Thus, the general opinion gradually developed that the ballistocardiogram was an interesting phenomenon but generally useless when applied to the problems of clinical medicine.
Ballistocardiogram Myocardial
as Reflection Function
of
The purpose of this editorial is to point out that in spite of these discouraging problems, the potential use of the technique in clinical medicine could be exceedingly significant, depending upon the questions asked. It now appears quite clearly that the ballistocardiogram is a very sensitive reflection of myocardial function which is not related to specific disease entities or merely to stroke volume alone. VOLUME
29, JANUARY
1972
This concept should have been recognized earlier from the studies of Starr and his co-workers.x-l” The fact that he was able to generate abnormal patterns in the cadavers by altering the way that blood was injected into the aorta clearly supports this concept. Since Starr’s classic work on the ballistocardiogram, there have been several significant studies that support this hypothesis, The first is that of Harrison et al.,“” who measured in a group of 20 patients the first derivative of pressure rise (dp/dt) in the left ventricle simultaneously with ultra-low frequency ballistocardiograms. A close correlation was found between the amplitude of the IJ wave and the maximal first derivative of the left ventricular pressure pulse. Since the maximal dp/dt of the left ventricle is an index of cardiac function, this relation offers significant evidence that the ballistocardiogram does indeed reflect myocardial function. The second investigation is that of Jackson et al.?’ in a group of patients with hypertrophic subaortic stenosis. Significant relations between certain ballistocardiographic variables and the ejection fraction, as well as the rate of volume ejected by the left ventricle, were demonstrated, again supporting the concept that the ballistocardiogram reflects myocardial function. BargeronZ2 has also recently demonstrated that severity of aortic ventricular gradients in children with aortic stenosis is highly correlated with ballistocardiographic observations, Ballistocardiogram in early detection of ischemic heart disease: There have now been 3 very exciting investigations relating to the question of early detection of ischemic heart disease. Starr2”?” initially observed in a long-term follow-up study of so-called normal subjects that patients with initial abnormal contours died earlier than patients whose ballistocardiogram was normal. Similar results were obtained by the group at Hopkins,27 and recently Taylor and co-workerP have observed confirming results. Thus, an abnormal pattern in the ballistocardiogram appears to be a sensitive predictor of probable early mortality. These studies were performed on ballistocardiographic beds that do not meet the standards cited. Artifactual content of the records could only reduce prognostic sensitivity below that obtainable with the optimal apparatus. Even though older recording techniques were used, the results cannot be overlooked since 3 independent studies have arrived at the same conclusion. A plausible interpretation of these data is as follows : It is well recognized that coronary atherosclerosis begins in early life, particularly in the male, and tends to progress over the years. Since even minimal coronary artery disease is usually associated with areas of patchy fibrosis in the myocardium, one may assume that those subjects who had abnormal ballistocardiograms at an early age already had sufficient decrease in myocardial func121
EDITORIALS
tion (undetectable by other means) which altered the pattern of the record. Therefore, it can be reasonably concluded that ballistocardiography may very well be the best and most reliable technique for recognizing incipient heart disease or the presence of early changes in myocardial function due to minimal ischemia. In summary, it would appear that the approach to the study of the ballistocardiogram in general has been in the wrong direction and that its greatest usefulness lies in early detection of heart disease and in evaluating myocardial functional
abnormalities. Techniques are now available for quantifying myocardial function more precisely, such as the use of the ejection fraction, left ventricular dp/dt, and instantaneous aortic root blood flow in patients. Other indexes of function can be determined by quantitative angiocardiography, and a new look at ballistocardiography should and undoubtedly will be undertaken. With the present emphasis on early detection of heart disease, we may well have overlooked the most sensitive method available, and one that is easy to apply.
References 1. Gordon JW: On certain molar movements of the human body produced by the circulation of the blood. J Anat Physiol 11:533-536, 1877 of cur2. Talbot SA, Harrison WK Jr: Dynamic comparison rent ballistocardiographic methods. l-3. Circulation 12: 577-587,845-857, 1022-1033, 1955 Triaxial BCG-ECG flight and 3. Hixson WC, Beischer DE: laboratory data, chap 8. In, Biotelemetty of the Triaxial Ballistocardiogram and El,ectrocardiogram in a Weightless Environment. Monograph 10, US Naval School of Aviation Medicine, US Naval Aviation Medical Center, Pensacola, Florida, 1964, p l-15 in 4. Beischer DE, Hixson WC: Triaxial ballistocardiogram a weightless environment. In, Proceedings of the 1st World Congress in Ballistocardiographic Cardiovascular Dynamics, Amsterdam (Knoop AA, ed). Base1 and New York, S Karger, 1966, p 85-89 5. Nickerson JL, Warren JV, Brannon ES: The cardiac output in man; studies with the low-frequency, critically damped ballistocardiograph, and the method of ‘right atrial catheterization. J Clin Invest 26:1-10, 1947 WK: A ballistocardiographic stroke volume 6. Harrison formula recognizing blood distribution effects. Bibl Cardiol 20:29-34, 1968 H. Schaede A. Thurn P. et al: Comoarative 7. Klensch study of cardiac output with the ballistic and th’e direct Fick methods. Pfluegers Arch Physiol 269232-239, 1959 8. Starr I: The relation of the ballistocardiogram to cardiac function. Amer J Cardiol 2:737-747, 1958 9. Starr I, Horwitz 0, Mayock RL, et al: Standardization of the ballistocardiogram by simulation of th,e heart’s function at necropsy; with a clinical method for the estimation of cardiac strength and normal standards for it. Circulation 1:1073-1096, 1950 10. Starr I, Schnabel TG Jr, Mayock RL: Studies made by simulating systole at necropsy. 2. Experiments on the relation of card,iac and peripheral factors to the genesis of the pulse wave and the ballistocardiogram. Circulation 8:4461, 1953 11. Starr I, Schnabel TG Jr: Studies made by simulating systole at necropsy. 3. On the genesis of-the systolic waves of the ballistocardioeram. J Chin Invest 33:10-22. 1954 12. Starr I, Schnabel TG, Askovitz SI, et al: Studies made by simulating systole at necropsy. 4. On the relation between pulse pressure and card,iac stroke volume, leading to a clinical method of ‘estimating cardiac output from blood pressure and age. Circulation 9648-663, 1954 13. Starr I: Studies made by simulating systole at necropsy. 6. Estimation of cardiac stroke volume from the baltistocardiogram. J Appl Physiol 8:31#5-329, 1955 14. Starr I, Askovitz SI, Feder W, et al: Studies made by simulating systole at necropsy. 7. Clinical methods for estimating the work of the left ventricle. With a note on
122
15.
16.
17.
18.
19. 20.
21.
22.
23.
24.
25.
26.
27.
28.
the diminution of heart work as age advances. Circulation 12:1005-1021, 1955 Starr I: Studies made by simulating systole at necropsy. 8. Significance of the pulse pressure. Circulation 14: 1117-1128, 1956 Starr I: Studies made by simulating systole at necropsy. 10. State of peripheral circulation in cadaver preparations. J Appl Physiol 11:174-180, 1957 Starr I: Studies made by simulating systole at necropsy. 11. On the higher dynamic functions of the h,eart, and their reflections in the pulse wave. Circulation 17:589600,1958 Starr I: Studies made by simulating systole at necropsy. 12. Estimation of initial cardiac forces for the ballistocardiogram. Circulation 20:74-87, 1959 Starr I, Schnabel TG Jr, Mayock RL: Studies made by simulating systol,e at necropsy. Circulation 8:1-14, 1953 Harrison WK, Friessinger GC, Johnson SL, et al: Relation of the ballistocardiogram to left ventricular pressure measurements in man. Amer J Cardiol 23:673-678, 1969 Jackson DH, Eddleman EE Jr, Bancroft WH Jr, et al: Ballistocardiographic and angiographic correlative study in idiopathic hypertrophic subaortic stenosis. Bib1 Cardiol 27:14-20, 1971 Bargeron LM Jr: The force ballistocardiogram as an index of severity in congenital aortic stenosis. Circulation 37:238-243,1968 of heart disease in Starr I: On the later development apparently healthy persons with abnormal ballistocardiograms. Eight- to ten-year after-histories of 90 persons over 40 years of age. Amer J Med Sci 214:233-242, 1947 Starr I: Prognostic value of ballistocardiograms. Studies on evaluation of the doctor’s experience. JAMA 187:511517, 1964 Starr I, Askovitz SI, Mandelbaum EM: Items of prognostic value in the clinical study; the relationship of symptoms, heart size, blood-pressure, electrocardiogram, and ballistocardiogram to after-histories and to each other. JAMA 192:83-87, 1965 Starr I: Prognostic value of ballistocardiograms; as judged by after-histories of 211 healthy persons and 221 patients followed from 5 to 25 years. In Ref. 4, p 7-20 Baker BM, Scarborough WR, Davis FW Jr, et al: Ballistocardiography and ischemic heart disease: predictive considerations and statistical evaluation. Proc Roy Sot Med 60:1290-1296, 1967 Taylor HL, Keys A, Blackburn H: The contribution of the low frequency ballistocardiogram to the prediction of death from all causes and the development of coronary heart disease in 18 years among Minnesota business and professio,nal men: a multivariate analysis. In preparation.
The
American
Journal
of
CARDIOLOGY