Routinely used imaging techniques for detection of viable myocardium: usefulness, discrepancies and new methods

Introduction

Identification of viable and hibernating myocardium allows for a better selection of patients who need revascularisation. Techniques evaluating these myocardial states have included metabolic and perfusion imaging and assessment of contractile reserve. Consequently, the noninvasive determination of viable myocardium is becoming increasingly important. In theory, those segments of the ischemic heart that are akinetic but remain viable may be expected to improve in contractility once perfusion has been restored. In this sense, a number of diagnostic methods are used in daily clinical practice to establish myocardial viability.

Among the techniques routinely used to detect the presence of viable myocardium, special mention should be made of dobutamine stress echocardiography (DSE), isotopic testing in the form of Thallium-201 redistribution scintigraphy and, more recently introduced, the myocardial contrast echocardiography (MCE) techniques and magnetic resonance.

Imaging techniques

Low-dose dobutamine infusion is able to demonstrate the contractile reserve of myocardium as it shows an improved contractility following infusion of this positive inotropic drug. This technique is based on the capacity of echocardiography to visualise the contractile reserve of myocytes after administrating this positive inotropic drug. In those situations where increased contractility is not induced by dobutamine, this technique defines the corresponding segment as non-viable.

Thallium-201 with redistribution is a classical isotopic test used to detect viable myocardium in patients with ischemic heart disease and depressed ventricular function. Following injection, the uptake of the radiodrug in the different cardiac segments is measured at baseline and after a number of hours (redistribution). Thallium uptake requires an active sodium-potassium pump (i.e., cell, membrane integrity). If an intact cell membrane is considered to imply a live cell, then the detection of TL-201 in a given segment is indicative of tissue viability.(1)

Reason for discrepancies between DSE and Thallium 201

Although DSE and Thallium-201 with redistribution have been validated for the diagnosis of viable myocardium, they frequently yield conflicting results. When both DSE and Thallium-201 are performed in the same patient, discordant results are often obtained In fact, in comparative studies where both techniques have been used prior to revascularisation procedures, the sensitivity, specificity and predictive value for the detection of viable myocardium have been found to differ. Indeed, comparative studies have been reported in which dysfunctional segments failing to respond to dobutamine (i.e., classified by dobutamine stress echocardiography as being non-viable) show thalium-201 uptake in redistribution, and are thus classified as viable by this test, highlighting the different sensitivity and specificity of both techniques. This discrepancy might be explained by the fact that the minimum viable myocytes count required in a given segment for each technique to diagnose myocardial viability is different –i.e., the minimum critical cell mass for the identification of viability differs from one technique to the other. In a recent study, we hypothesized that a minimum critical mass of viable myocytes is required for each technique to identify a segment as being viable or non-viable. The results obtained in this study suggest that the discrepant viability findings of DSE and Thallium 201 redistribution testing are attributable to differences in the minimum critical mass of live myocytes required for each technique to diagnose myocardial viability. In this sense, Thallium 201 requires less live tissue in each segment than DSE to diagnose viability. In addition, maximum diagnostic efficiency (sensitivity and specificity) is obtained for less viable tissue per segment with Tl-201 than for dobutamine echocardiography. These considerations should be taken into account when both diagnostic tests are used for viability in the pre-revascularisation period.(2)

New and promising tools: myocardial contrast echocardiography and magnetic resonance

MCE is an emerging technique to assess myocardial viability. It has been reported that myocardial viability depends on the presence of myocardial contrast enhancement and on microvascular integrity as necrosis of myocardial cells results in collapse of the surrounding microvasculature. As concentration of microbubbles in different myocardial beds represents relative capillary density or the sum of cross-sectional area in these vascular beds, in this setting there will be a lack of myocardial contrast in a concrete myocardial region. Recent studies in post acute myocardial infarct patients have shown a good agreement between myocardial contrast echocardiography and nuclear imaging for the detection of myocardial perfusion.(4)

In the acute myocardial infarction setting, recanalisation of an occluded artery does not always signify myocardial salvage. The detection of microvascular integrity in this situation can be very useful in identifying the presence of viability. Several recent studies concluded that microvascular integrity demonstrated by MCE is an indicator of preserved viability and predicted functional recovery. In the setting of chronic left ventricular dysfunction secondary to chronic coronary artery disease, MCE parameters help differentiate hibernating myocardium from myocardium with irreversible dysfunction, taking a predominant role in predicting functional recovery contrast indexes of myocardial blood velocity over blood volume indexes. Furthermore, MCE can provide information that can be added to the information obtained from conventional DSE. (3)

Magnetic resonance imaging is proving to be a very useful non-invasive imaging technique in the study of the patient suffering from coronary artery disease. Not only is it applicable to the diagnosis of atherosclerotic disease but also to the characterisation of the cellular mechanisms implicated in the development of vascular damage. Furthermore, magnetic resonance is a useful technique in the field of myocardial viability. Magnetic resonance has shown its high accuracy for detection of reversible myocardial dysfunction before coronary revascularisation thanks to contrast-enhanced imaging. (6) This is a very promising technique that avoids the limitation of the acoustic window, which is troublesome in a large number of patients. But it also has several limitations. The most important one is the availability of the technique in the different clinical centers.

Practical approach

Among the daily used techniques to non invasively detect myocardial viability, DSE and Thallium 210 are helpful. Nevertheless they provide different diagnostic efficiencies (sensitivities and specificities). A better diagnostic efficiency is obtained for less viable tissue per segment with Thallium 201 than from DSE. This information should be taken into account when both diagnostic tests are used for viability in the pre-revascularisation period. Of increasing interest are the new echo contrast techniques that are being developed, especially the evaluation of myocardial perfusion using MCE, that provides a rapid, easy and bedside assessment of myocardial viability.(5) Magnetic resonance provides a very useful tool for the identification of myocardial viability, although its main limitation is the availability of the technique.

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.