Assessment of myocardial viability: guide to prognosis and clinical management

Over the past two decades it has become clear that depressed left ventricular function in patients with chronic coronary artery disease is not necessarily an irreversible process, but that improvement of left ventricular function following revascularization may be anticipated in patients with dysfunctional but viable myocardium. Numerous studies employing a variety of techniques have demonstrated that the majority of the dysfunctional but viable segments improved in function following adequate revascularization. Currently, four techniques are predominantly used for the identification of viable myocardium, each based on a different ‘hallmark’ of viable tissue (Table 1). These different characteristics include (1) preserved glucose utilization, (2) cell membrane integrity, (3) intact mitochondria, and (4) preserved contractile reserve. The first three characteristics are usually evaluated by scintigraphic techniques: fluorine18-fluorodeoxyglucose traces glucose utilization, thallium-201 uptake reflects cell membrane integrity, and technetium-99m sestamibi (or tetrofosmin) uptake corresponds (at least in part) to intact mitochondria. The fourth characteristic, i.e. preserved contractile reserve, can be probed by echocardiography using dobutamine stress. Alternatively, dobutamine stress using magnetic resonance imaging has been put forward as an accurate measure for assessing myocardial ischaemia and viability. All these imaging techniques have reported high accuracies to predict improvement of regional left ventricular function following revascularization (Fig. 1). Advantages of the nuclear imaging modalities and magnetic resonance imaging over dobutamine stress echocardiography is the option of quantitative, operatorindependent analysis of the former, as compared to the visual, operator-dependent analysis of the latter. Moreover, transthoracic dobutamine stress echocardiography is not feasible in a substantial number of patients (varying from 15% to 40%) due to an inadequate acoustic window. In the current paper by Baer et al., the authors have proposed two possible alternative approaches to overcome the latter of the two shortcomings of transthoracic echocardiography: dobutamine transoesophageal echocardiography and dobutamine magnetic resonance imaging. They convincingly showed that dobutamine transoesophageal echocardiography and dobutamine magnetic resonance imaging were feasible in 97% and 98% of the patients, respectively. Moreover, both approaches yielded comparable accuracies for the prediction of improvement of regional left ventricular function following revascularization, as compared to the other techniques (Fig. 1). Assessment of contractile reserve by quantitative analysis was not addressed, as both the dobutamine magnetic resonance imaging and the dobutamine transoesophageal echocardiography data were analysed visually. In particular, dobutamine magnetic resonance imaging is very well suited for quantitative analysis. In a previous work from the authors, however, it was shown that dobutamine magnetic resonance imaging, when analysed quantitatively, yielded only slightly higher sensitivities and specificities for the prediction of improvement of function as compared to the results in the current, visual analysis. However, it would be of interest to perform a head-to-head comparison between quantitative and visual analysis using either dobutamine magnetic resonance imaging or dobutamine transoesophageal echocardiography. Another important observation in the study by Baer et al. is the translation of improvement of contractile function of segments in improvement of global left ventricular function when sufficient viable segments were present. Several studies have recently demonstrated comparable results. Based on the current results and other data it seems likely that recovery of left ventricular ejection fraction may be anticipated when 25% to 40% of the myocardium is viable. Besides the end-points used in the study by Baer et al., i.e. recovery of regional and global left ventricular function, additional end-points may be at least as important, including improvement of heart failure symptoms, exercise capacity and long-term prognosis. Di Carli et al., employing fluorine-18fluorodeoxyglucose and positron emission tomography, demonstrated a direct relationship between the pre-operative extent of viable tissue and the postoperative improvement in exercise capacity. Moreover, retrospective analyses of patients with scintigraphically viable tissue demonstrated a low incidence of (peri-)operative events and an excellent long-term survival following surgical revascularization. In contrast, patients with viable tissue who