Viability assessment: clinical applications

Myocardial viability assessment is indicated in patients with severe coronary artery disease (CAD) and severe left ventricular (LV) dysfunction (Table 1) [1–10]. In many other patients, viability assessment is not indicated (Table 2). There are different endpoints for viability assessment (Table 3). These endpoints need to be assessed quantitatively, and be tailored to the individual patient, for example, in an asymptomatic patient improvement in survival is desirable, while in an elderly patient with heart failure improvement in the quality of life may be more important. In some patients, all endpoints may be achievable because they are interdependent ie., improvement in wall motion, ejection fraction (EF), quality of life and survival. Improvement in survival may occur without improvement in wall motion or EF, possibly because coronary revascularization in patients with viable myocardium may prevent further deterioration of LV dysfunction or death (Table 4). It is also conceivable that the timing after coronary revascularization is crucial for the detection of improvement of LV function. Histological changes of de-differentiation in severe cases of hibernation may be slow to recover; improvement may occur at 6 to 12 months rather than 3 to 6 weeks, which is the conventional time period for follow-up studies [4–6]. In other patients, improvement may not occur because of subendocardial scarring. It is presumed that the inner most layer of the myocardium is responsible for the resting wall motion. Therefore, though there is sufficient viable myocardium in the outer layer to result in improvement in quality of life and survival after coronary revascularization, there is no improvement in regional function. More recently using magnetic resonance imaging with a tagging technique, a significant role of the subepicardium to regional LV function has been demonstrated. After coronary revascularization, the subepicardial viable myocardium has been shown to result in improvement in regional function and if such improvement involves a sufficient zone of the myocardium, there is also increase in EF [11]. Thus, while improvement in wall motion (or EF) are appropriate endpoints for viability assessment, it is possible that improvement in quality of life and survival could be achieved with modest or no change in regional function. It should also be noted that improvement in wall motion is often assessed subjectively and precise registration on segmental basis is difficult when different methods are used to assess perfusion and function (see below). There are other potential reasons for discordance between viability assessment and endpoints (Table 5) [1–3]. It is possible that LV volume affects significantly the ability of revascularization to improve regional and global LV function and survival even if there is reversible myocardial ischemia [12].