ECHOCARDIOGRAPHY Validity of echocardiographic determination of left ventricular systolic wall thickening

Previous direct measurements of left ventricular systolic wall thickening (SWT) in animal studies have yielded values approximately one-half those found echocardiographically in humans, suggesting a possible overestimation of SWT by echocardiographic techniques. To test the validity of echocardiographic SWT measurements, the relationship between echocardiographically determined end-diastolic and end-systolic left ventricular short-axis myocardial cross-sectional areas (ED Myo CSA and ES Myo CSA, respectively) was assessed in 18 normal subjects. Since Myo CSA is a function of wall thickness and wall circumference, overestimation of SWT by echocardiography would be expected to produce an overestimation of ES Myo CSA relative to ED Myo CSA. SWT, as determined by both M mode (52%) and two-dimensional echocardiography (48%), was consistent with previously reported echocardiographic values, but exceeded that reported in animal studies. By least squares linear regression analysis. ES Myo CSA was 1.078 x ED Myo CSA -0.385 cm2 (r .947, SEE = 1. 183 cm2) when assessed by one observer and was 1.042 x ED Myo CSA 0. 142 cm2 (r .906, SEE = 1.831 cm2) when assessed independently by another. The close relationship observed between echocardiographically determined ES Myo CSA and ED Myo CSA was consistent with constant left ventricular myocardial mass throughout the cardiac cycle and thus did not suggest an overestimation of SWT by echocardiographic techniques. Circulation 70, No. 2, 226-232, 1984. ECHOCARDIOGRAPHIC STUDIES of systolic wall thickening (SWT) of the free wall of the normal human left ventricle have generally yielded values ranging from 40% to 80%. 1' In contrast, studies of SWT in animals in which various more direct techniques have been used have produced mean values ranging from 10% to 30%.1-8 The major explanation given for this discrepancy has been that echocardiographic measurements of SWT include the contribution of compressed trabeculae carnae to apparent end-systolic wall thickness, while the more direct measurement techniques used in animal studies do not.',' In as much as systolic trabecular compression displaces blood volume (albeit incompletely) and thus contributes to left ventricular stroke volume, it also contributes to effective SWT.2 SWT determinations are derived from wall thickness measurements. The accuracy of wall thickness measurements relies on the accuracy of epicardial and endocardial edge detection. The aim of our investigation was to determine whether From the Cardiovascular Unit and Department of Medicine, St. Vincent's Hospital, and the University ofNew South Wales, Sydney, Australia. Address for correspondence: Dr. Michael P. Feneley, St. Vincent's Hospital, Darlinghurst, N.S.W. 2010, Australia. Received Nov. 27, 1983; revision accepted March 22, 1984. 226 echocardiographic techniques overestimate the systolic movement of the endocardial surface of the left ventricle relative to the epicardial surface, resulting in overestimation of SWT. Assuming that myocardial density remains constant throughout the cardiac cycle, myocardial volume would be expected to remain constant in order to preserve constant myocardial mass. In fact, a negligible reduction of approximately 0.3% in left ventricular myocardial volume has been estimated to occur during systole due to a reduction in intramyocardial blood volume.9 Myocardial volume may be considered virtually constant, therefore, for the purposes of wall thickness determinations. Left ventricular myocardial volume may be calculated, with the use of Simpson's rule, as the sum of serial myocardial cross-sectional areas (Myo CSAs) along the length of the left ventricle (E Myo CSAs) times the length of the interval separating consecutive cross-sections.4" ` In view of the relatively small change in left ventricular long-axis length during systole (5% to 10%), 1 Simpson's rule would predict only a small increase in Myo CSA from enddiastole to end-systole. For example, assuming an ellipsoid model for calculating left ventricular myocardial volume and 10% left ventricular long-axis systolic shortening, end-systolic Myo CSA (ES Myo CSA) CIRCULATION by gest on A ril 7, 2017 http://ciajournals.org/ D ow nladed from DIACGNOSTIC METHODSECHOCARL)IOGRAPHY would be approximately equal to I . I X end-diastolic Myo CSA (ED Myo CSA). Assuming 5% long-axis shortening, ES Myo CSA would be approximately equal to 1.05 x ED Myo CSA (Appendix 1). Myo CSA is a function of wall thickness and wall circumference. An overestimation of wall thickness would thus result in an overestimation of Myo CSA. In particular, an overestimation of SWT (viz an overestimation of end-systolic wall thickness relative to enddiastolic wall thickness) would result in an overestimation of ES Myo CSA relative to ED Myo CSA. In our investigation, the relationship between twodimensional echocardiographic measurements of ED Myo CSA and ES Myo CSA was determined in human subjects as an indicator of the reliability of echocardiographic measurements of SWT.