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Noninvasive methods for assessing left ventricular performance in man
EDITORIALS
Noninvasive Methods for Assessing Left Ventricular Performance ARNOLD
M. WEISSLER,
in Man MD,
FACC
Detroit, Michigan
techniques offer reflections of the fundamental contractile events of the heart. When properly interpreted, and applied in combination, they can offer insights into the presence, extent and functional consequence of cardiovascular disease which, in many respects, add to those derived from more popular invasive methods. It is the purpose of this presentation to summarize present approaches to the estimation of left ventricular contractile performance by noninvasjve methods and to discuss some problems and perspectives relative to the investigation of such methods. With use of noninvasive techniques, it is currently possible to estimate five general indexes of cardiac performance in man: (1) left ventricular diameter and volume, (2) left ventricular muscle mass, (3) left ventricular wall motion, (4) left ventricular outflow dynamics, and (5) the time sequence of the left ventricular cycle.
When the physiologist wishes to express the performance of cardiac muscle in quantitative terms, he regularly resorts to three basic parameters, the length, force and time of contraction. He recognizes that in portraying the function of cardiac muscle in these terms he must take into cognizance the initial stretch of the muscle, the load that the muscle is contracting against, the mass of the muscle and the synchrony of the individual muscle fibers comprising the body of the muscle. In physiologic terms he is interested in the preload, the afterload, the synchrony and mass of contracting muscle. When he translates these characteristics of muscle performance into the function of the heart as a whole, the physiologist must resort to common hemodynamic measurements. Thus, the preload of the ventricles is reflected in the enddiastolic pressure and volume, the afterload in measures of aortic resistance and pressure or ventricular systolic force, or both, the length of muscle contraction in the stroke volume expressed relative to the end-diastolic volume, the force of contraction in the pressure-volume changes occurring during systolic contraction, and the time of contraction in the duration of the phases of the cardiac cycle and the time derivatives of the contractile events. It is apparent that hemodynamic measures reflect fundamental characteristics of muscle contraction through which one can derive information pertinent to the contractile properties of heart muscle. In approaching the evaluation of the diseased heart, changes in performance of the cardiac chambers, as reflected in pressure, volume, flow and time alterations, permit one to discriminate the presence, extent and functional effect of cardiovascular disease. Often, a combination of hemodynamic measurements must be obtained to define the presence of cardiac malfunction. Such combinations of hemodynamic measurements allow better definition of cardiac performance than single measures alone. These considerations are pertinent to the application of noninvasive methods in evaluating cardiac performance. It is because noninvasive techniques can reflect hemodynamic events within the cardiac chambers that they are, in fact, useful. Like measures of pressure, volume and flow, modern noninvasive
Left Ventricular Diameter and Volume
From the Department of Medicine, Wayne State University School of Medicine, Detroit, Mich. Address for reprints: Arnold M. Weissler. MD, Department of Medicine, Wayne State University School of Medicine, 540 E. Canfield Ave., Detroit, Mich. 4820 1.
Echocardiography: One of the most promising additions to cardiovascular medicine in recent years has been the echocardiogram. Although echocardiography has attained wide popularity as a means of estimating valvular motion and in delineating the presence of various anatomic defects and pericardial effusion, it is also useful in assessing the motion of the internal minor axis of the left ventricle. By directing the ultrasonic beam just below the plane of the mitral valve one can obtain remarkable delineation of the extent of motion of the left side of the interventricular septum and of the endocardial surface of the left ventricular posterior wall. When these recordings are made simultaneous with the electrocardiogram on a continuous photographic recording, the motion of the minor axis of the left ventricle is readily estimated. Measurements of the extent and rate of change in this diameter provide valuable data relative to the performance of the left ventricle. Such measurements have been employed by several investigators in the estimation of left ventricular stroke volume, ejection fraction and mean rate of circumferential fiber shortening.1-7 The estimates of ventricular volume derived by use of these techniques agree closely with those measured by angiographic or other direct techniques, thereby attesting to the accuracy of the echocardiographically determined minor axis dimension. Some investigators prefer to confine estimates of ventricular function by the echocardiogra-
July 1974
The American Journal of CARDIOLOGY
Volume 34
111
EDITORIALS
phic method to measurement of the changes in the minor axis of the left ventricle alone without extrapolation to determinations of volume change. Indeed, at present, the measurement of change in the minor axis is the primary basis for the functional estimates of left ventricular contractile performance by the echocardiographic technique.
Scintiphotographic measurement of left ventricular volume: Another method for estimating
ventricular volume involves use of newly developed nuclear techniques8-lo The scintiphotographic method for measuring left ventricular volume employs the injection of a radioactive tracer, usually technetiumlabeled albumin. Images of the heart at end-systole and end-diastole are obtained using a scintillation camera (Anger camera) and an electronic gate triggered by the patient’s electrocardiogram. The gamma radiation from the technetium is recorded over a period of several beats, permitting inscription of the volume characteristics of the cardiac chambers at end-diastole and end-systole. The volumes so calculated agree closely with those measured directly by angiographic methods. This technique offers great promise as a noninvasive means of detecting alterations of left ventricular function as well as defects in the pattern of ventricular contraction.
Left Ventricular Muscle Mass A new and promising application of echocardiography is the estimation of left ventricular wall thickness. From such measurements and knowledge of the specific gravity of ventricular muscle, estimates of left ventricular mass can be derived. When compared with left ventricular thickness and mass calculated from angiographic measures, the echocardiographic data provide remarkably accurate estimates.lr Indeed, if estimates of left ventricular mass by the echocardiogram could be made accurately and correlated with other measures of ventricular performance derived by either invasive or noninvasive techniques, a major advance in evaluating left ventricular performance by noninvasive technique will have been achieved.
Left Ventricular Wall Motion Apex cardiogram: The pattern of left ventricular wall motion can be evaluated by several noninvasive methods. The apex cardiogram is an older technique that was not generally accepted as a means of assessing cardiac performance until recent years. In large measure, the failure of the apex cardiogram to gain acceptance as a measure of left ventricular dynamic events was related to problems of instrumentation. It has recently been emphasized that major distortions in recordings of the configuration of the apex cardiogram result from the use of uncorrected nonlinear, partially differentiated piezoelectric transducers.r2J3 With high fidelity equipment for recording undistorted apical movements, the apex cardiogram offers valuable information on the timing of the events of the cardiac cycle and in detecting altered left ventricular performance in such circumstances as left ventricular hypertrophy and ventricular aneurysm.14 112
July 1974
The American Journal of CARDIOLOGY
Kinetocardiogram: The kinetocardiogram records the precordial motion imparted by the contraction and relaxation of the cardiac chambers.15 The instrument is designed to allow measurement of absolute motion at various points on the anterior chest wall. In contrast to the apex cardiogram, which measures motion of the apex relative to the adjacent thoracic wall, the kinetocardiogram measures the extent of local motion. Distortions of precqrdial motion have been shown to accompany many forms of cardiac disease, and recent attempts to quantitate kinetocardiographic patterns have yielded a detailed expression of the sequence of precordial cardiac motion that other methods have not approached.16 Radarkymography: Another technique for recording cardiac wall motion is that of radarkymography. This method, which employs a video tracking device combined with image intensification fluoroscopy, allows quantitative expression of the motion of the interface of the cardiac chambers and the pulmonary fields. l7 The recordings permit detailed expression of localized alterations in the outer wall of the left ventricle.r8 This technique appears to be a promising addition to the noninvasive approach for detecting alterations in the synergy of contraction and in detecting the presence of ventricular aneurysm. Echocardiogram: Recent studies suggest that the echocardiographic method may prove useful in detailing left ventricular chamber motion and in detecting alterations in the synchrony or extent, or both, of muscle contraction. By scanning the left ventricular chamber with the ultrasonic beam, it is possible to visualize the pattern of contraction of the interventricular septum and the left ventricular outer wall in the area from the mitral valve to the apex. Left Ventricular Ejection Dynamics Ballistocardiogram: Two noninvasive methods for assessing the dynamics of left ventricular ejection are the ballistocardiogram and the impedance plethysmogram. The ballistocardiogram has been recognized as an instrument with potential for evaluating cardiac performance for almost a century. It has met with various degrees of popularity as a clinical technique in recent decades. Much of the reluctance to accept the technique of ballistocardiography in past decades related to problems in instrumentation. The early instruments typically generally had intrinsic frequency responses that were similar to the natural frequency of the cardiac impulse. Thus, each ballistocardiographic impulse reflected not only the motion of the heart but the simultaneous response of the instrument as well, yielding distorted ballistic recordings of the body. In recent years, new attention has been given to the development of better ballistocardiographic instrumentation. The ultra low frequency ballistocardiograph, in which the natural frequency of the instrument is low relative to that of the cardiac pulsations, appears to lend a more quantitative measure of the bodily movements imparted by the heartbeat than has been heretofore available. In a unique version of this type of instrumentation, the body is supported on a thin column of pressurized air as a
Volume 34
EDITORIALS
means of accurately recording the ballistic impulses with each heartbeat. In recent studies, the ballistocardiogram was shown to be a sensitive and effective means of detecting altered left ventricular performance in patients having coronary artery bypass surgery.ly With the ballistocardiogram, major improvement in left ventricular dynamics in some patients could be shown by comparing the records obtained before and after operation in the same patient. It. is notable that after less effective surgical procedures for coronary artery disease, the ballistocardiogram revealed no improvement in left ventricular performance. Impedance plethysmogram: The position of the impedance plethysmogram as a noninvasive technique remains somewhat controversial. Essentially, this technique records the electrical impedance changes across the thorax with each cardiac beat. Greatest interest in recent years has been in the first derivative of impedance changes across the thorax.“O A major problem in evaluating the impedance plethysmogram as well as the ballistocardiogram as meaningful expressions of the heart’s performance is the fact that these devices probably reflect cardiovascular functional characteristics that are not measured by conventionally employed hemodynamic methods in man. The acceleration of blood ejected from the heart, which probably accounts in large part for the ballistocardiographic and the impedance impulses, is not readily estimated in man by current laboratory measures. Validation of ballistocardiographic and impedance plethysmographic tracings as a measure of cardiac performance has in the past relied on their correlation with hemodynamic measures that reflect neither the velocity nor the acceleration of blood ejected by the left ventricle. That such correlative studies have yielded varying results is hence not unexpected. Only when acceptable measures of the acceleration or velocity of blood flow are obtained in man will effective tests of these noninvasive measures be provided. Time Sequence
of Left Ventricular Time Intervals)
Cycle (Systolic
One of the most effective noninvasive measures of left ventricular performance is the determination of systolic time intervals. With simultaneous recordings of the electrocardiogram, the phonocardiogram and the carotid or subclavian arterial pulse one can accurately estimate the duration of the preejection and ejection phases of the cardiac cycle.21-“3 The preejection period can be subdivided into various components such as the interval from onset of electrical activity to the first heart sound and the interval from onset of electrical activity to the beginning of left ventricular chamber motion as recorded by the apex cardiogram. In making such measurements, one notes a distinct abnormality in the time sequence of the cardiac cycle in left ventricular dysfunction in which the preejection phase lengthens and the ejection time becomes abbreviated. Studies in recent years have indicated that these changes in the systolic intervals
correlate closely with other hemodynamic consequences of left ventricular dysfunction such as the diminished left ventricular stroke volume and cardiac output, the decrease in left ventricular ejection fraction and depression in indexes of left ventricular contractile performance that are based on measurements of the first derivative of left ventricular pressure.21m24 The use of systolic time interval measurements provides a valuable and sensitive approach in evaluating left ventricular performance, particularly in chronic disease states such as coronary artery disease, hypertension and primary myocardial disease. Used singly or in combination with other measures of left ventricular performance, these measurements provide insight into the presence and ext,ent. of left ventricular dysfunction particularly when bedside clinical signs are not conclusive. In addition, the measurement of the systolic intervals has proved to be of value as a noninvasive means for investigating the transient or sustained effects of various cardiostimulatory agents.25 Problems
and Perspectives
Noninvasive methods vs. clinical state: Interest in noninvasive methods has brought with it problems that any new technology must face. In developing these methods it is most often the goal of the investigator to produce measures of cardiac performance that are more sensitive than conventional bedside techniques. In attempting to test the new methods as a measure of cardiac performance, the investigator often resorts to displaying data in groups based on the severity of the patients’ illness as determined by symptomatic responses. Such an approach is burdened with several major investigative limitations, not the least of which is the fact that the interpretation of “functional class” is subjective. In addit,ion, the symptomatic response in left ventricular disease is often contingent upon the degree of associated pulmonary congestion or the decrease in the cardiac output response, variables that in themselves do not directly reflect the degree of intrinsic cardiac functional impairment. Finally, the testing of newer methods in this manner alone cannot clearly distinguish whether the noninvasive measure is an improvement over the clinical approach in detecting left ventricular dysfunction. Noninvasive techniques vs. hemodynamic measurements: One must hence resort to comparisons with more objective, direct and sensitive measures of cardiac performance when critical evaluation of the noninvasive technique is desired. However, the use of such an approach presents additional technologic and conceptual problems. In attempting to test the validity of the noninvasive measure, comparisons are made with more conventional parameters of ventricular performance based on measurements of blood pressure, volume and flow. As indicated previously, the noninvasive methods may approach dimensions of cardiac chamber performance that more conventional hemodynamic measures do not. Failure to show congruent changes with conventional hemodynamic measures may be interpreted as evidence
July 1974
TheAmerIcan
Journal of CARDlOLOGY
Volume 34
113
EDITORIALS
against the usefulness or applicability of the newer method whereas different performance characteristics may have been compared. A corollary problem may be produced when the measurements provided by a new noninvasive technique agree closely with data derived from the cardiac catheterization laboratory. Such agreement in observations may be interpreted as evidence that the noninvasive method measures the hemodynamic parameter with which it correlates, when the two measures may merely reflect independent alterations in performance that change in parallel. Such an interpretation can result in the extension of the use of the noninvasive method for quantitating hemodynamic measures under a variety of circumstances in which the correlation was never tested. In attempting objective validation of newer methods, the investigator must be aware that the complex performance of the cardiac chamber is reflected in a wide spectrum of dynamic events not all of which change simultaneously or to equivalent degrees. Further, methods currently used in the cardiac catheterization laboratory for quantitating the performance of the cardiac chambers do not yield definitive measures of the effectiveness of the contractile function of the heart. There is little agreement as to which hemodynamic measure or measures offer the most sensitive index of altered cardiac chamber or muscle performance; that is, there are no measures of ventricular performance that can be applied singly or in com-
bination as a regularly accepted standard for comparison. These considerations must be kept in mind by those who attempt to introduce or test new methods for determining the state of cardiac performance in man. Although the stark empiricism that our predecessors were forced to use has little place in laboratory investigation today, we must be cognizant that overzealous interpretation of the correlation or regression approach can lead to unnecessary rejection of potentially valuable methods or to uncritical acceptance of others. While we must apply objective criteria in judging new methods, we must guard against interpreting these new approaches strictly in the image of those measures which are conventional or with which we have greatest familiarity. It is my belief that we are entering an exciting era in the development of clinical cardiology, one in which noninvasive technology will serve to augment and extend current laboratory methods. The application of these techniques should provide new means for quantitating altered left ventricular performance in man. The methods should not be construed as competitive techniques designed to displace currently established hemodynamic modes. Rather, they will supplement current diagnostic practice and will provide dynamic measures of cardiac performance that can be obtained and repeated more readily than catheter methods permit. When properly understood and applied, such techniques will greatly enhance current cardiovascular practice.
References 1. Pombo JR, Troy BL, Russell RO: Left ventricularvolume and ejection fraction by echocardiography. Circulation 43:480-490, 1971 2. Fortuin NJ, Hood WP, Sherman ME, et al: Determination of left ventricular volumes by ultrasound. Circulation 44575-584, 1971 3. Popp RL, Harrison DC: Ultrasonic cardiac echocardiography for determining stroke volume and valvular regurgitation. Circulation 411493-502, 1970 4. Fortuin NJ, Hood WP Jr, Cralge E: Evaluation of left ventricular function by echocardiography. Circulation 46:26-35, 1972 5. Feigenbaum H, Chang S: Analysis of left ventricular wall motion by reflected ultrasound. Circulation 46:14-25, 1972 6. Murray JA, Johnaton W, Reid JM: Echocardiographic determination of left ventricular dimensions, volumes and performance. Am J Cardiol30:252-257, 1972 7. Coooer RH. O’Rourke RA. Kariiner J. et al: Comoarison of uitrasound and cineangiographic measurements of mean rate of circumferential fiber shortening in man. Circulation 46: 914923, 1972 8. Sullivan RW, Bergeron DA, Vetter WR, et al: Peripheral venous scintillation angiography in determination of left ventricular volume in man. Am J Cardiol 28:563-567, 1971 9. Strauss HW, Zaret BL, Harley PJ, et al: A scintiphotographic method for measuring lefl ventricular ejection fraction in man without cardiac catheterization. Am J Card/o1 28575-580, 1971 10. Zaret BL, Strauss W, Harley PJ, et al: A noninvasive scintiphotographic method for detecting regional ventricular dysfunction in man. N Engl J Med 284:1165-1170, 1971 11. Troy BL, Pombo J, Rackiey CE Measurement of left ventricular wall thickness and mass by echocardiography. Circulation 45: 602-611. 1972 12. Johnson JM, Siegel W, Biomquist 0: Characteristics of transducers used for recording the apexcardiogram. J Appl Physiol 31:796-800, 1971 13. Kesteioot H, Wiiiems J, Van Voiienhoven E: On the physical
114
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Volume 34
14. 15.
16. 17. 18.
19.
20. 21. 22.
23.
24.
25.
principles and methodology of mechanocardiography. Acta Cardiol 24:147-160, 1969 Craige E: Clinical value of apexcardiography. Am J Cardiol 28: 118-121, 1971 Eddieman EE Jr, Harrison TR: The kinetocardiogram in patients with ischemic heart disease. Prog Cardiovasc Dis 6:189-211, 1963 Eddieman EE Jr, Bancroft WH Jr: Quantification of the kinetocardiogram. Comput Biomed Res 1:237-250, 1967 Schuette WH, Simon AL: A new device for recording cardiac motion. Med Res Engin 7~25-27, 1968 Cohen LS, Simon AL, Whitehouse WC, et al: Heart motion vkteo tracking (radarkymography) in diagnosis of congenital and acquired heart disease. Am J Cardiol 22:678-684, 1968 Starr L, MacVaugh H iii: Objective tests of cardiac contractility before and after three operative procedures designed to improve the coronary circulation. Trans Assoc Am Physicians 85: 259-266, 1972 Lababidi J, Ehmke DA, Durnim RE, et al: The first derivative thoracic impedance cardiogram. Circulation 4 1:65 l-658, 1970 Weiesier AM, Harris WS, Schoenfeid CD: Systolic time intervals in heart failure in man. Circulation 37:149-159, 1968 Weissier AM, Harris WS, Schoenfeid CD: Bedside techniques for the evaluation of ventricular function in man. Am J Cardiol 23:577-583, 1969 Garrard CL Jr, Weiaeier AM, Dodge HT: The relationship of alteratfons in systolic time intervals to ejection fraction in patients with cardiac disease. Circulation 42:455-462, 1970 Ahmed SS, Levinson GE, Schwartz CJ, et al: Systolic time intervals as measures of the contractile state of the left ventricutar myocardium in man. Circulation 46:559-571, 1972 Weiasier AM, Lewis RP, Leighton RF: The systolic time intervals as a measure of left ventricular performance in man. In. Progress in Cardiology (Vu PN. Goodwin JF. ed). Philadelphia, Lea & Febiger, 1972, p 155-183
Noninvasive Methods for Assessing Left Ventricular Performance ARNOLD
M. WEISSLER,
in Man MD,
FACC
Detroit, Michigan
techniques offer reflections of the fundamental contractile events of the heart. When properly interpreted, and applied in combination, they can offer insights into the presence, extent and functional consequence of cardiovascular disease which, in many respects, add to those derived from more popular invasive methods. It is the purpose of this presentation to summarize present approaches to the estimation of left ventricular contractile performance by noninvasjve methods and to discuss some problems and perspectives relative to the investigation of such methods. With use of noninvasive techniques, it is currently possible to estimate five general indexes of cardiac performance in man: (1) left ventricular diameter and volume, (2) left ventricular muscle mass, (3) left ventricular wall motion, (4) left ventricular outflow dynamics, and (5) the time sequence of the left ventricular cycle.
When the physiologist wishes to express the performance of cardiac muscle in quantitative terms, he regularly resorts to three basic parameters, the length, force and time of contraction. He recognizes that in portraying the function of cardiac muscle in these terms he must take into cognizance the initial stretch of the muscle, the load that the muscle is contracting against, the mass of the muscle and the synchrony of the individual muscle fibers comprising the body of the muscle. In physiologic terms he is interested in the preload, the afterload, the synchrony and mass of contracting muscle. When he translates these characteristics of muscle performance into the function of the heart as a whole, the physiologist must resort to common hemodynamic measurements. Thus, the preload of the ventricles is reflected in the enddiastolic pressure and volume, the afterload in measures of aortic resistance and pressure or ventricular systolic force, or both, the length of muscle contraction in the stroke volume expressed relative to the end-diastolic volume, the force of contraction in the pressure-volume changes occurring during systolic contraction, and the time of contraction in the duration of the phases of the cardiac cycle and the time derivatives of the contractile events. It is apparent that hemodynamic measures reflect fundamental characteristics of muscle contraction through which one can derive information pertinent to the contractile properties of heart muscle. In approaching the evaluation of the diseased heart, changes in performance of the cardiac chambers, as reflected in pressure, volume, flow and time alterations, permit one to discriminate the presence, extent and functional effect of cardiovascular disease. Often, a combination of hemodynamic measurements must be obtained to define the presence of cardiac malfunction. Such combinations of hemodynamic measurements allow better definition of cardiac performance than single measures alone. These considerations are pertinent to the application of noninvasive methods in evaluating cardiac performance. It is because noninvasive techniques can reflect hemodynamic events within the cardiac chambers that they are, in fact, useful. Like measures of pressure, volume and flow, modern noninvasive
Left Ventricular Diameter and Volume
From the Department of Medicine, Wayne State University School of Medicine, Detroit, Mich. Address for reprints: Arnold M. Weissler. MD, Department of Medicine, Wayne State University School of Medicine, 540 E. Canfield Ave., Detroit, Mich. 4820 1.
Echocardiography: One of the most promising additions to cardiovascular medicine in recent years has been the echocardiogram. Although echocardiography has attained wide popularity as a means of estimating valvular motion and in delineating the presence of various anatomic defects and pericardial effusion, it is also useful in assessing the motion of the internal minor axis of the left ventricle. By directing the ultrasonic beam just below the plane of the mitral valve one can obtain remarkable delineation of the extent of motion of the left side of the interventricular septum and of the endocardial surface of the left ventricular posterior wall. When these recordings are made simultaneous with the electrocardiogram on a continuous photographic recording, the motion of the minor axis of the left ventricle is readily estimated. Measurements of the extent and rate of change in this diameter provide valuable data relative to the performance of the left ventricle. Such measurements have been employed by several investigators in the estimation of left ventricular stroke volume, ejection fraction and mean rate of circumferential fiber shortening.1-7 The estimates of ventricular volume derived by use of these techniques agree closely with those measured by angiographic or other direct techniques, thereby attesting to the accuracy of the echocardiographically determined minor axis dimension. Some investigators prefer to confine estimates of ventricular function by the echocardiogra-
July 1974
The American Journal of CARDIOLOGY
Volume 34
111
EDITORIALS
phic method to measurement of the changes in the minor axis of the left ventricle alone without extrapolation to determinations of volume change. Indeed, at present, the measurement of change in the minor axis is the primary basis for the functional estimates of left ventricular contractile performance by the echocardiographic technique.
Scintiphotographic measurement of left ventricular volume: Another method for estimating
ventricular volume involves use of newly developed nuclear techniques8-lo The scintiphotographic method for measuring left ventricular volume employs the injection of a radioactive tracer, usually technetiumlabeled albumin. Images of the heart at end-systole and end-diastole are obtained using a scintillation camera (Anger camera) and an electronic gate triggered by the patient’s electrocardiogram. The gamma radiation from the technetium is recorded over a period of several beats, permitting inscription of the volume characteristics of the cardiac chambers at end-diastole and end-systole. The volumes so calculated agree closely with those measured directly by angiographic methods. This technique offers great promise as a noninvasive means of detecting alterations of left ventricular function as well as defects in the pattern of ventricular contraction.
Left Ventricular Muscle Mass A new and promising application of echocardiography is the estimation of left ventricular wall thickness. From such measurements and knowledge of the specific gravity of ventricular muscle, estimates of left ventricular mass can be derived. When compared with left ventricular thickness and mass calculated from angiographic measures, the echocardiographic data provide remarkably accurate estimates.lr Indeed, if estimates of left ventricular mass by the echocardiogram could be made accurately and correlated with other measures of ventricular performance derived by either invasive or noninvasive techniques, a major advance in evaluating left ventricular performance by noninvasive technique will have been achieved.
Left Ventricular Wall Motion Apex cardiogram: The pattern of left ventricular wall motion can be evaluated by several noninvasive methods. The apex cardiogram is an older technique that was not generally accepted as a means of assessing cardiac performance until recent years. In large measure, the failure of the apex cardiogram to gain acceptance as a measure of left ventricular dynamic events was related to problems of instrumentation. It has recently been emphasized that major distortions in recordings of the configuration of the apex cardiogram result from the use of uncorrected nonlinear, partially differentiated piezoelectric transducers.r2J3 With high fidelity equipment for recording undistorted apical movements, the apex cardiogram offers valuable information on the timing of the events of the cardiac cycle and in detecting altered left ventricular performance in such circumstances as left ventricular hypertrophy and ventricular aneurysm.14 112
July 1974
The American Journal of CARDIOLOGY
Kinetocardiogram: The kinetocardiogram records the precordial motion imparted by the contraction and relaxation of the cardiac chambers.15 The instrument is designed to allow measurement of absolute motion at various points on the anterior chest wall. In contrast to the apex cardiogram, which measures motion of the apex relative to the adjacent thoracic wall, the kinetocardiogram measures the extent of local motion. Distortions of precqrdial motion have been shown to accompany many forms of cardiac disease, and recent attempts to quantitate kinetocardiographic patterns have yielded a detailed expression of the sequence of precordial cardiac motion that other methods have not approached.16 Radarkymography: Another technique for recording cardiac wall motion is that of radarkymography. This method, which employs a video tracking device combined with image intensification fluoroscopy, allows quantitative expression of the motion of the interface of the cardiac chambers and the pulmonary fields. l7 The recordings permit detailed expression of localized alterations in the outer wall of the left ventricle.r8 This technique appears to be a promising addition to the noninvasive approach for detecting alterations in the synergy of contraction and in detecting the presence of ventricular aneurysm. Echocardiogram: Recent studies suggest that the echocardiographic method may prove useful in detailing left ventricular chamber motion and in detecting alterations in the synchrony or extent, or both, of muscle contraction. By scanning the left ventricular chamber with the ultrasonic beam, it is possible to visualize the pattern of contraction of the interventricular septum and the left ventricular outer wall in the area from the mitral valve to the apex. Left Ventricular Ejection Dynamics Ballistocardiogram: Two noninvasive methods for assessing the dynamics of left ventricular ejection are the ballistocardiogram and the impedance plethysmogram. The ballistocardiogram has been recognized as an instrument with potential for evaluating cardiac performance for almost a century. It has met with various degrees of popularity as a clinical technique in recent decades. Much of the reluctance to accept the technique of ballistocardiography in past decades related to problems in instrumentation. The early instruments typically generally had intrinsic frequency responses that were similar to the natural frequency of the cardiac impulse. Thus, each ballistocardiographic impulse reflected not only the motion of the heart but the simultaneous response of the instrument as well, yielding distorted ballistic recordings of the body. In recent years, new attention has been given to the development of better ballistocardiographic instrumentation. The ultra low frequency ballistocardiograph, in which the natural frequency of the instrument is low relative to that of the cardiac pulsations, appears to lend a more quantitative measure of the bodily movements imparted by the heartbeat than has been heretofore available. In a unique version of this type of instrumentation, the body is supported on a thin column of pressurized air as a
Volume 34
EDITORIALS
means of accurately recording the ballistic impulses with each heartbeat. In recent studies, the ballistocardiogram was shown to be a sensitive and effective means of detecting altered left ventricular performance in patients having coronary artery bypass surgery.ly With the ballistocardiogram, major improvement in left ventricular dynamics in some patients could be shown by comparing the records obtained before and after operation in the same patient. It. is notable that after less effective surgical procedures for coronary artery disease, the ballistocardiogram revealed no improvement in left ventricular performance. Impedance plethysmogram: The position of the impedance plethysmogram as a noninvasive technique remains somewhat controversial. Essentially, this technique records the electrical impedance changes across the thorax with each cardiac beat. Greatest interest in recent years has been in the first derivative of impedance changes across the thorax.“O A major problem in evaluating the impedance plethysmogram as well as the ballistocardiogram as meaningful expressions of the heart’s performance is the fact that these devices probably reflect cardiovascular functional characteristics that are not measured by conventionally employed hemodynamic methods in man. The acceleration of blood ejected from the heart, which probably accounts in large part for the ballistocardiographic and the impedance impulses, is not readily estimated in man by current laboratory measures. Validation of ballistocardiographic and impedance plethysmographic tracings as a measure of cardiac performance has in the past relied on their correlation with hemodynamic measures that reflect neither the velocity nor the acceleration of blood ejected by the left ventricle. That such correlative studies have yielded varying results is hence not unexpected. Only when acceptable measures of the acceleration or velocity of blood flow are obtained in man will effective tests of these noninvasive measures be provided. Time Sequence
of Left Ventricular Time Intervals)
Cycle (Systolic
One of the most effective noninvasive measures of left ventricular performance is the determination of systolic time intervals. With simultaneous recordings of the electrocardiogram, the phonocardiogram and the carotid or subclavian arterial pulse one can accurately estimate the duration of the preejection and ejection phases of the cardiac cycle.21-“3 The preejection period can be subdivided into various components such as the interval from onset of electrical activity to the first heart sound and the interval from onset of electrical activity to the beginning of left ventricular chamber motion as recorded by the apex cardiogram. In making such measurements, one notes a distinct abnormality in the time sequence of the cardiac cycle in left ventricular dysfunction in which the preejection phase lengthens and the ejection time becomes abbreviated. Studies in recent years have indicated that these changes in the systolic intervals
correlate closely with other hemodynamic consequences of left ventricular dysfunction such as the diminished left ventricular stroke volume and cardiac output, the decrease in left ventricular ejection fraction and depression in indexes of left ventricular contractile performance that are based on measurements of the first derivative of left ventricular pressure.21m24 The use of systolic time interval measurements provides a valuable and sensitive approach in evaluating left ventricular performance, particularly in chronic disease states such as coronary artery disease, hypertension and primary myocardial disease. Used singly or in combination with other measures of left ventricular performance, these measurements provide insight into the presence and ext,ent. of left ventricular dysfunction particularly when bedside clinical signs are not conclusive. In addition, the measurement of the systolic intervals has proved to be of value as a noninvasive means for investigating the transient or sustained effects of various cardiostimulatory agents.25 Problems
and Perspectives
Noninvasive methods vs. clinical state: Interest in noninvasive methods has brought with it problems that any new technology must face. In developing these methods it is most often the goal of the investigator to produce measures of cardiac performance that are more sensitive than conventional bedside techniques. In attempting to test the new methods as a measure of cardiac performance, the investigator often resorts to displaying data in groups based on the severity of the patients’ illness as determined by symptomatic responses. Such an approach is burdened with several major investigative limitations, not the least of which is the fact that the interpretation of “functional class” is subjective. In addit,ion, the symptomatic response in left ventricular disease is often contingent upon the degree of associated pulmonary congestion or the decrease in the cardiac output response, variables that in themselves do not directly reflect the degree of intrinsic cardiac functional impairment. Finally, the testing of newer methods in this manner alone cannot clearly distinguish whether the noninvasive measure is an improvement over the clinical approach in detecting left ventricular dysfunction. Noninvasive techniques vs. hemodynamic measurements: One must hence resort to comparisons with more objective, direct and sensitive measures of cardiac performance when critical evaluation of the noninvasive technique is desired. However, the use of such an approach presents additional technologic and conceptual problems. In attempting to test the validity of the noninvasive measure, comparisons are made with more conventional parameters of ventricular performance based on measurements of blood pressure, volume and flow. As indicated previously, the noninvasive methods may approach dimensions of cardiac chamber performance that more conventional hemodynamic measures do not. Failure to show congruent changes with conventional hemodynamic measures may be interpreted as evidence
July 1974
TheAmerIcan
Journal of CARDlOLOGY
Volume 34
113
EDITORIALS
against the usefulness or applicability of the newer method whereas different performance characteristics may have been compared. A corollary problem may be produced when the measurements provided by a new noninvasive technique agree closely with data derived from the cardiac catheterization laboratory. Such agreement in observations may be interpreted as evidence that the noninvasive method measures the hemodynamic parameter with which it correlates, when the two measures may merely reflect independent alterations in performance that change in parallel. Such an interpretation can result in the extension of the use of the noninvasive method for quantitating hemodynamic measures under a variety of circumstances in which the correlation was never tested. In attempting objective validation of newer methods, the investigator must be aware that the complex performance of the cardiac chamber is reflected in a wide spectrum of dynamic events not all of which change simultaneously or to equivalent degrees. Further, methods currently used in the cardiac catheterization laboratory for quantitating the performance of the cardiac chambers do not yield definitive measures of the effectiveness of the contractile function of the heart. There is little agreement as to which hemodynamic measure or measures offer the most sensitive index of altered cardiac chamber or muscle performance; that is, there are no measures of ventricular performance that can be applied singly or in com-
bination as a regularly accepted standard for comparison. These considerations must be kept in mind by those who attempt to introduce or test new methods for determining the state of cardiac performance in man. Although the stark empiricism that our predecessors were forced to use has little place in laboratory investigation today, we must be cognizant that overzealous interpretation of the correlation or regression approach can lead to unnecessary rejection of potentially valuable methods or to uncritical acceptance of others. While we must apply objective criteria in judging new methods, we must guard against interpreting these new approaches strictly in the image of those measures which are conventional or with which we have greatest familiarity. It is my belief that we are entering an exciting era in the development of clinical cardiology, one in which noninvasive technology will serve to augment and extend current laboratory methods. The application of these techniques should provide new means for quantitating altered left ventricular performance in man. The methods should not be construed as competitive techniques designed to displace currently established hemodynamic modes. Rather, they will supplement current diagnostic practice and will provide dynamic measures of cardiac performance that can be obtained and repeated more readily than catheter methods permit. When properly understood and applied, such techniques will greatly enhance current cardiovascular practice.
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