Evaluation of myocardial function in cardiomyopathic states

Abstract

T HE CARDIOMYOPATHIES include diverse pathologic processes that involve heart muscle. They have been defined as myocardial disorders of unknown cause and, for this reason, any process for which the etiology can be defined may be excluded from the diagnostic category of cardiomyopathy. However, many investigators favor the concept of a functional classification that includes three subgroups: congestive (dilated), hypertrophic, and restrictiveobliterative cardiomyopathies.‘-4 The functional classification, which will be used in this article, does not depend on the absence of a specific etiologic factor, but rather relies on structural and functional characteristics. A large dilated left ventricle with poor systolic function is thus classified as a congestive cardiomyopathy regardless of whether the process is known to be alcoholic, infectious, toxic, or is of an obscure undiagnosed nature. The second category is hypertrophic cardiomyopathy. In this condition, inappropriate (excessive) myocardial hypertrophy is associated with normal or supernormal systolic shortening and abnormal diastolic stiffness of the left ventricle. Increased chamber stiffness is caused not only by increased myocardial mass, but is also likely related to myocardial fiber disarray and increased intrinsic stiffness of the left ventricular wall. Prolonged and asynchronous myocardial relaxation may also contribute to altered chamber stiffness. Thus, the cardinal feature of this disorder is massive myocardial hypertrophy (usually asymmetric septal hypertrophy) with normal or small chamber volume, preserved systolic function, and increased diastolic chamber stiffness. The major functional differences between congestive (dilated) cardiomyopathies and hypertrophic cardiomyopathies are presented in Fig 1. In congestive cardiomyopathy, chamber volume is increased and the systolic ejection fraction (SEF) is decreased. In spite of reduced shortening (SEF = 25%), the ventricle may eject a nearly normal stroke volume (35 mL/m’); this is accomplished by virtue of a large end diastolic volume. By contrast, the hypertrophic ventricle may generate a similar stroke volume, but in this case shortening is exaggerated (SEF = 75%). In both disorders, the end diastolic pressure is elevated; in hypertrophic cardiomyopathy the elevated filling pressure is a consequence of increased chamber stiffness (diastolic pressurevolume curve is shifted to the left). In congestive cardiomyopathy, the diastolic pressure is increased primarily as a consequence of increased chamber volume. The characteristics of these two forms of cardiomyopathy are examined further in Fig 2. This outline emphasizes the massive and inappropriate increase in left ventricular muscle mass and the decreased volume/mass ratio in hypertrophic cardiomyopathy.5 Despite the presence of eccentric hypertrophy in congestive (dilated) cardiomyopathy, the increase in left ventricular (LV) mass is inadequate to maintain a normal volume/mass ratio; the volume/mass ratio is increased. As a consequence of these changes in volume and mass, the average values for systolic wall stress are decreased in hypertrophic cardiomyopathy and increased in congestive cardiomyopathy. Because systolic shortening is inversely related to systolic wall stress, shortening is increased in hypertrophic cardiomyopathy. As will be seen, diastolic chamber stiffness and intrinsic myocardial stiffness are increased in hypertrophic cardiomyopathy; in contrast the diastolic pressure-volume relations in congestive cardiomyopathy are displaced to the right (decreased chamber stiffness) in spite of increased myocardial stiffness. Thus the major functional defect in congestive cardiomyopathy is reduced systolic shortening; in mild forms the ejection fraction may be depressed with little or no chamber dilatation. In