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Abstract
Ideally, the accurate definition of viability is as simple as that mentioned in the Oxford English Dictionary meaning ”capable of living” (Box et al., 2003).
The assessment of myocardial viability is one of the most fascinating and challenging areas in modern cardiology. The true standard for viability is the improvement of global or regional wall motion or both following myocardial revascularization (Anne et al., 2006).
Ischemic heart disease remains a leading cause of morbidity and mortality. The survival after acute ischemic events has improved substantially in recent years, but new syndromes have emerged.
Cardiac remodeling following a large myocardial infarction, with dilatation of the left ventricle, has deleterious hemodynamic consequences and is a major cause of chronic heart failure The increased incidence of potentially lethal arrhythmias is related to the presence of scar tissue but also to altered properties of the surviving myocardium, as characterized in experimental studies ( Heusch and Sipido, 2004).
In contrast to the extensive characterization of cellular remodeling following myocardial infarction, much less information is available on the changes in hibernating myocardium. the importance of this syndrome for clinical management is increasingly recognized, more specifically the need for its diagnostic recognition and therapy by timely revascularization
( Murray and Pfeffer, 2002).
This is supported by the recent meta-analysis in patients with chronic coronary artery disease and left ventricular dysfunction: in patients with evidence of myocardial viability, revascularization decreased mortality (relative to medical therapy) from 16% to 3.2% during a 25±10 month follow-up interval, whereas in patients without viability, mortality was intermediate, and neither revascularization nor medical therapy was superior in terms of mortality 7.7% versus 6.2% (Allman et al., 2002).
Excess death in the population with hibernating myocardium is, to a large extent, sudden presumed arrhythmic death. This is also underscored by the results of the Multicenter Automatic Defibrillator Implantation Trial II demonstrating the increased survival with implantation of an automatic defibrillator in patients with ischemic cardiomyopathy.
Many imaging techniques have been proposed over the last 2 decades. These techniques rely on different characteristics of dysfunctional but viable myocardium. The most tested and clinically used techniques include nuclear imaging by PET (evaluating glucose use with 18F-FDG), nuclear imaging by SPECT (evaluating perfusion, cell membrane integrity, and intactness of mitochondria with Tl-201 or Tc-99m labeled agents), echocardiography with dobutamine (to assess contractile reserve), echocardiography with intravenous contrast agents (to assess perfusion), MRI with dobutamine (to assess contractile reserve), MRI with intravenous contrast agents (to assess scar tissue), and CT with intravenous contrast agents (to assess scar tissue) (Arend et al., 2007).
Over the years, echocardiography has been used extensively for the assessment of myocardial viability. The simple assessment of LV end-diastolic wall thickness can already be used to obtain a first impression of the presence or absence of viable tissue. It has been demonstrated that severely thinned walls most likely represent scar tissue. In a study of a large registry, Schinkel et al., demonstrated that segments with LV end-diastolic wall thicknesses of less than 6 mm virtually never exhibited contractile reserve. On the other hand, the majority of segments with relatively well preserved end-diastolic wall thicknesses ( 6 mm) had contractile reserve. Low-dose dobutamine echocardiography has been used to further refine viability assessment.( Arend et al., 2007).
Dobutamine stress-echocardiography does have limitations, Inducing Myocardial Ischemia, Down regulated β- receptors and β blocker therapy, Severity of baseline dyssynergy, Critical stenosis or total coronary occlusion, Image quality and interpretation (Meluzin et al., 2001).
Imaging technology has developed very rapidly over the past decade. In particular, the application of imaging to basic research in medical sciences have moved much of our understanding of cellular and molecular mechanics from the realm of inferred understanding to that of direct observation (Elliot and Veigh, 2006).
In SPECT imaging, a radionuclide tracer is injected into the blood. It accumulates at different locations in the body depending on the relative perfusion and affinity for the compound containing the radionuclide. The SPECT camera then estimates the relative amount of activity at each position in the body by detecting the emission of gamma rays from a series of angular views. The resolution of the resulting image is dependent on the aperture through which the radiation is detected (Elliot and Veigh, 2006).
Nuclear-Imaging-Versus-Dobutamine-Stress-Echocardiography:
A controversial issue in clinical cardiology concerns the relative merits of nuclear imaging and dobutamine echocardiography for the assessment of viability. Various direct comparisons of nuclear imaging and low-dose dobutamine stress echocardiography have reported substantial disagreement between the techniques. For example, Panza et al., reported an agreement between Tl-201 imaging and low-dose dobutamine echocardiography of only 68%. The disagreement was mainly related to segments with viability on nuclear imaging but without contractile reserve on echocardiography (Arind, 2007).
The largest head-to-head comparison included 114 patients who had ischemic cardiomyopathy and who underwent resting perfusion imaging and low-dose dobutamine echocardiography . The agreement between the techniques was 72%; 92% of segments without perfusion did not have contractile reserve, but 47% of segments with perfusion also lacked contractile reserve. Accordingly, the available studies showed a higher sensitivity of nuclear imaging than of dobutamine echocardiography for detecting myocardial viability (Arind, 2007).
Extensive CAD is also marked by an early onset of ischemia, at a low heart rate and rate-pressure product, or at a low dose of dobutamine. However, the sensitivity of stress echocardiography for single-vessel CAD has been quite limited in several studies and is probably less than that of TI-201 myocardial perfusion scintigraphy.
This reflects the need for ischemia to involve a significant extent of myocardium for the stress echocardiogram to be positive, which may not be fulfilled if the involved vessel is small or distal or if the stenosis is only mildly flow limiting.
Conclusion
●In a meta-analysis, the superior sensitivity of TI-201 was balanced by a greater specificity with echocardiography, so the accuracy of the techniques was comparable.
●The slightly lower sensitivity of stress echocardiography than TI-201 perfusion scintigraphy may reflect superiority of perfusion imaging (which does not require the development of ischemia in a metabolic or functional sense) for the identification of patients with single-vessel disease.
●Similarly, patients receiving therapy with antianginal agents may be better studied using TI-201 perfusion scintigraphy (because ischemia does not need to be induced for the test result to be positive).
●TI-201 perfusion scintigraphy also appears to be more sensitive for the recognition of multivessel disease and may be superior for the detection of ischemia in the setting of resting wall motion abnormalities, although this benefit cannot be appreciated by consideration of sensitivity and specificity.
●TI-201 scintigraphy is superior in patients with poor echocardiographic windows (e.g., resulting from pulmonary disease).
●There is a particular benefit of echocardiography over perfusion scintigraphy with respect to specificity.
●In patients with LV hypertrophy and left bundle branch block. Breast attenuation artifacts and other issues have led to lower reported levels of accuracy of perfusion imaging in women, and the results of a meta-analysis suggest this may be a group better studied with dobutamine stress echocardiography .
●Dobutamine stress echocardiography is versatile, rapidly performed, and less expensive than perfusion imaging.
●In situations in which other cardiac problems are present as well as ischemia (e.g., valvular and pericardial diseases), the selection of dobutamine stress echocardiography avoids duplicate testing.
●the major limitations of DSE are endocardial border definition and subjective interpretation of stress-induced wall motion abnormalities. Enhancement of border definition (and thus interpretation) is currently under investigation with gray-scale B-mode color encoding, intravenous contrast agents, tissue Doppler interrogation, tissue characterization techniques and backscatter analysis.