Fundamental concepts in myocardial viability assessment revisited: When knowing how much is "alive" is not enough
In this article we have re-examined some fundamental concepts in the assessment of myocardial viability. Although counterintuitive, we have explored the proposal that "knowing how much is alive is not enough" to provide a comprehensive assessment of myocardial viability. In part, we believe this is the consequence of two physiological precepts. First, normal hearts have significant regional variability in the transmural extent of viable myocardium. This intrinsic heterogeneity leads to uncertainty regarding the presence and extent of non-viable myocardium when using a technique that is able to detect or assess only viable myocardium. Second, for a given region, the absolute amount of viable myocardium may be dynamic and thus decrease or increase over time. A patient example was demonstrated in which a "thinned" anterior wall nearly doubled in diastolic wall thickness following coronary revascularisation. For this patient, the ratio of viable to total myocardium (viable plus non-viable) in the dysfunctional region was more accurate that the absolute amount of viable myocardium alone in predicting functional improvement. It was postulated that non-invasive methods should identify and assess non-viable myocardium as well as viable myocardium since the combined data appear to provide a more accurate and comprehensive evaluation of myocardial viability.