Myocardial viability

Echocardiographic assessment of viable myocardium

Left ventricular function is one of the most important determinates of long-term prognosis in patients with coronary artery disease. In recent years, it has become apparent that left ventricular dysfunction in patients with coronary artery disease is not always an irreversible process stemming from myocardial necrosis and fibrosis. Myocardial tissue can undergo both a state of potential reversible dysfunction because of prolonged sustained ischemia (hibernating myocardium) or episodes of acute ischemia (stunned myocardium). Revascularization of this tissue may improve regional and global left ventricular function and therefore prognosis. Numerous studies have now firmly established dobutamine echocardiography as a safe, reliable, and accurate imaging modality in the assessment of reversible left ventricular dysfunction. Furthermore, dobutamine echocardiography has been shown to have good sensitivity, specificity, and, more importantly, positive predictive accuracy in identifying both acute and chronic reversible left ventricular dysfunction for risk satisfaction and prognosis.

Reversible left ventricular dysfunction

The extent and degree of myocardial viability are important parameters in the risk stratification of patients with significant left ventricular dysfunction secondary to coronary artery disease. Although several imaging modalities can identify viable myocardium, dobutamine stress echocardiography has gained considerable importance as an accurate, safe, and reliable method. In patients with significant left ventricular dysfunction secondary to coronary artery disease, identification of the presence and extent of contractile reserve and, therefore, viable myocardium during low-dose dobutamine infusion can predict the recovery of left ventricular function after revascularization, survival rate, and future cardiac events.

The use of myocardial contrast echocardiography in clinical evaluation after myocardial infarction

Because disruption of the microvasculature is a hallmark of myocyte necrosis, MCE may be able to distinguish between viable and infarcted tissue. In order to interpret images appropriately following myocardial infarction, however, one should be versed in the pathophysiology of post-ischemic reflow, and understand that reperfusion to infarcted tissue is a heterogeneous combination of hyperemia, low-reflow, no-reflow, and impaired microvascular flow reserve. Furthermore, the relative mix of these perfusion patterns changes both temporally and spatially, which has implications for the timing of MCE following reperfusion. The identification of no- and low-reflow by MCE predicts regions unlikely to demonstrate segmental functional recovery, and is associated with adverse clinical events. To date, studies documenting the utility of MCE in the AMI setting have been performed using intracoronary injections in the cardiac catheterization laboratory. With the advent of intravenous contrast agents and innovations in ultrasound imaging systems, it may be possible to make these determinations without the need for coronary instrumentation, thus expanding the role of MCE in acute infarction and reperfusion to settings such as the emergency room and intensive care unit.

Prognostic implications of myocardial contractile reserve in patients with ischemic cardiomyopathy

The extent and degree of myocardial viability is an important parameter in the risk stratification of patients with significant left ventricular dysfunction due to coronary artery disease (CAD). Although several imaging modalities can identify viable myocardium, dobutamine stress echocardiography has gained considerable importance as an accurate, safe, and reliable method. In patients with significant left ventricular dysfunction secondary to CAD, identifying the presence and extent of contractile reserve and, therefore, viable myocardium, during low dose dobutamine infusion can predict recovery of left ventricular function postrevascularization, survival, and future cardiac events.

Are technetium-99m-labeled myocardial perfusion agents adequate for detection of myocardial viability?

The noninvasive assessment of myocardial viability in patients with coronary artery disease and depressed left ventricular function has proven clinically useful for identifying those patients with ischemic cardiomyopathy who benefit most from coronary revascularization. Thallium-201 (201Tl) imaging at rest has been the radionuclide imaging technique most often utilized for distinguishing viable myocardium from scar. However, new technetium-99m (99mTc) perfusion agents such as 99mTc-sestamibi and 99mTc-tetrofosmin have emerged as alternatives to 201Tl for imaging of regional myocardial perfusion. Whether these new agents, which have better physical properties for imaging with a gamma camera than 201Tl, are valid for use in assessing myocardial viability is still uncertain. Recent clinical studies have demonstrated that these agents, when imaged using quantitative SPECT, can identify patients with myocardial hibernation who exhibit improved regional systolic function following revascularization. Experimental laboratory studies have shown that the uptake of 99mTc-sestamibi and 99mTc-tetrofosmin in ischemic myocardium is only slightly lower than the uptake of 201Tl. These 99mTc-labeled agents remain bound intracellularly in mitochondria of viable myocytes under conditions of myocardial stunning and short-term hibernation, producing severe myocardial asynergy. With respect to determination of viability, the inferior wall region is at times problematic since attenuation of 99mTc-sestamibi and 99mTc-tetrofosmin is greatest in this area. Demonstration of preserved systolic thickening on ECG-gated SPECT images is indicative of viability in the instance of decreased regional 99mTc counts due to attenuation and not scar. Administration of nitrates prior to tracer injection improves the sensitivity for identifying viable myocardial segments using rest imaging with 99mTc-sestamibi or 99mTc-tetrofosmin. Thus, it appears that the new 99mTc perfusion imaging agents can be successfully employed for the determination of myocardial viability in the setting of severe regional dysfunction and chronic coronary artery disease. The greater the myocardial uptake of these agents in the resting state, the greater the probability of improved systolic function after coronary revascularization.

The viable myocardium: epidemiology, detection, and clinical implications

The success of fibrinolytic and other therapies has reduced the mortality of myocardial infarction. However, many survivors develop congestive heart failure. Medical treatment of this disorder has limited efficacy, and cardiac transplantation has limited availability. Contrary to previous teaching about ischaemic injury, roughly 40% of segments involved in myocardial infarction may subsequently recover, either spontaneously or after revascularisation. The persistence of such viable myocardium means that previous approaches to treatment of myocardial infarction must be reappraised. This review examines the pathogenesis of this response, the techniques that may be used to identify the salvageable tissue, and the clinical implications. Myocardial revascularisation may improve symptom status, exercise capacity, and prognosis in selected patients with viable myocardium.