Echo Wall Thickness and Myocardial Viability
abstract & commentary
Synopsis: End-diastolic wall thickness is an important measure of viability, and a value of less than 6 mm almost excludes the possibility for recovery of function.
Source: Cwajg JM, et al. J Am Coll Cardiol 2000; 35:1152-1161.
Controversy still exists concerning the optimal method of determining myocardial viability. Rest-redistribution thallium scintigraphy and dobutamine stress echocardiography are popular techniques, but their accuracy is less than ideal and they are involved, expensive tests for which there are no standardized protocols. Thus, Cwajg and colleagues tested the hypothesis that thinned myocardium on echocardiography represents infarction and noncontractile myocardium of normal thickness is viable. They performed rest and dobutamine echocardiograms and rest-redistribution thallium scans on 45 patients with coronary artery disease (CAD) and left ventricular dysfunction. Without regard to these studies, revascularization was performed and a rest echocardiogram was performed two months later. Wall thickness measurements were done in whichever views showed the myocardial segment best. The resting echocardiogram exhibited 555 of the 585 possible segments and wall thickness could be measured in 85% of the dysfunctional segments. The poorly visualized segments were all in the apex of the left ventricle. Thirty-eight percent of the dysfunctional segments recovered resting function after revascularization (angioplasty in 31, bypass surgery in 14 patients). The segments that recovered function were thicker (9.4 mm vs 6.7 mm; P < 0.001). A receiver operating curve showed that an end-diastolic thickness of more than 8 mm provided the best sensitivity (75%) and specificity (80%), but at a cut-off of 6 mm, sensitivity was 94%, specificity was 48%, and the negative predictive value was 93%, which may be of more use clinically since maximum sensitivity for viability is probably desirable. Similarly, a thallium uptake of more than 60% had a sensitivity of 91% and a specificity of 50%. A combination of wall thickness and any contractile reserve on dobutamine stress had a sensitivity of 88% but a specificity of 77%. Cwajg et al conclude that end-diastolic wall thickness is an important measure of viability, and a value of less than 6 mm almost excludes the possibility for recovery of function.
Comment by Michael H. Crawford, MD
Pathologic studies have shown that irreversibly damaged and necrotic myocardium is associated with early thinning. Also, biopsies of viable segments have shown less than 10-20% muscle loss. Thus, the hypothesis tested had other support that made it reasonable. Surprisingly, wall thinning was just as good as thallium scintigraphy and is much more simple and less costly. The touted biphasic response to dobutamine stress echocardiography was more sensitive but less specific for the detection of resting wall motion recovery after revascularization. The combination of resting wall motion and the response to dobutamine had the best results, but if wall thickness in the acontractile segment is less than 6 mm, recovery is virtually implausible, and the stress part of the test can be omitted.
The major drawback to this approach is that only 85% of segments can be visualized well enough on echo to make reliable measurements of thickness. However, new improvements in echocardiography, such as harmonic imaging, may improve these results. The decision not to revascularize seems fairly straightforward, but what about the patient with a wall thickness in the noncontractile segment of more than 6 mm? Some would argue that all these patients should undergo revascularization if feasible since all the testing modalities, including position emission imaging, have inaccuracies and any improvement in left ventricular function will improve survival. Therefore, wall thickness greater than or less than 6 mm may be all you need to make the revascularization decisions in patients with reduced left ventricular function due to segmental wall motion abnormalities.
Myocardial viability can be accurately predicted by:
a. rest-redistribution thallium imaging.
b. dobutamine stress echocardiography.
c. resting echo wall thickness.
d. All of the above