very case of heart failure requires identification of its cause to inform prognosis and management. The history and non-invasive investigations such as transtho-racic echocardiography usually suffice for this purpose, but invasive procedures such as coronary angiography or endomyocardial biopsy may be required. If the cause is ischemia, specific investigation of its extent is indicated, and/or myocardial mapping to determine viability, particularly after clinical deterioration or coronary accident.
Viability status is a major predictor of short- and longterm outcome in left ventricular dysfunction, carrying significant therapeutic implications. Thus, a Belgian study in 137 consecutive patients with a mean ejection fraction of 35% found that ischemic myocardium (demonstrated by exercise-redistribution-reinjection thallium scintigraphy) and viable myocardium (demonstrated by dobutamine echocardiography) were independent predictors of mortality; 3-year survival was significantly higher after revascularization in the presence of viable myocardium. Similarly, the Echo Dobutamine International Cooperative (EDIC) trial in 314 patients with severe global left ventricular dysfunction after acute uncomplicated myocardial infarction showed that the presence of myocardial viability identified as inotropic reserve after low-dose dobutamine was associated with a higher probability of survival. The higher the number of segments showing improvement of function, the greater the impact of myocardial viability on survival. The presence of inducible ischemia was the best predictor of death (Figure 1).
Hence the importance of mapping studies to identify viability and ischemia in patients with left ventricular dysfunction: failure to submit such patients to revascularization exposes them to the risk of losing the positive prognostic impact of viability and to the negative impact of ischemia. No study has specifically addressed the prognostic effect of timing viability assessment in heart failure patients. However, it has been suggested that recovery of full contractility relies not only on the degree of tissue perfusion, but also on the duration of tissue hypoperfusion. Early assessment is therefore advisable, especially if clinically indicated. Several noninvasive imaging modalities have been established as accurate predictors of myocardial viability and ischemia (Table). They include thallium imaging, positron emission tomography (PET), and dobutamine or dipyridamole echocardiography. PET scanning is the most sensitive in detecting viable myocardium, but is not widely applicable. Thallium imaging is the most commonly employed in practice. Dobutamine echocardiography has high positive predictive accuracy. Dipyridamole echocardiography is similar to low-dose dobutamine in the accurate diagnosis of viability, yielding results that correlate with prognosis, but it is probably less sensitive in detecting ischemia. The choice of technique depends on center and regional expertise. A recent meta-analysis of 24 viability studies reporting survival data in a total of 3088 patients with a mean ejection fraction of 32% followed for a mean 25 months showed that revascularization lowered annual mortality by 79.6% vs medical treatment (16% vs 3.2%, P<0.00001) in patients with viability. In patients without viability, mortality was intermediate, trending to higher rates with revascularization vs medical therapy (7.7% vs 6.2%, P=NS). Patients with viability showed a direct relationship between severity of left ventricular dysfunction and magnitude of benefit with fit between the three testing techniques (thallium per- revascularization. There was no measurable perform- fusion imaging, [18Fifluorodeoxyglucose metabolic ance difference for predicting revascularization bene- imaging, or dobutamine echocardiography). [gallery ids=""]