Assessment of myocardial viability by radionuclide and echocardiographic techniques: is it simply a sensitivity and specificity issue?

Myocardial viability: what we knew and what is new

Some patients with chronic ischemic left ventricular dysfunction have shown significant improvements of contractility with favorable long-term prognosis after revascularization. Several imaging techniques are available for the assessment of viable myocardium, based on the detection of preserved perfusion, preserved glucose metabolism, intact cell membrane and mitochondria, and presence of contractile reserve. Nuclear cardiology techniques, dobutamine echocardiography and positron emission tomography are used to assess myocardial viability. In recent years, new advances have improved methods of detecting myocardial viability. This paper summarizes the pathophysiology, methods, and impact of detection of myocardial viability, concentrating on recent advances in such methods. We reviewed the literature using search engines MIDLINE, SCOUPS, and EMBASE from 1988 to February 2012. We used key words: myocardial viability, hibernation, stunning, and ischemic cardiomyopathy. Recent studies showed that the presence of viable myocardium was associated with a greater likelihood of survival in patients with coronary artery disease and LV dysfunction, but the assessment of myocardial viability did not identify patients with survival benefit from revascularization, as compared with medical therapy alone. This topic is still debatable and needs more evidence.

The cardiac magnetic resonance (CMR) approach to assessing myocardial viability

Cardiac magnetic resonance (CMR) is a noninvasive imaging method that can determine myocardial anatomy, function, perfusion, and viability in a relative short examination. In terms of viability assessment, CMR can determine viability in a non-contrast enhanced scan using dobutamine stress following protocols comparable to those developed for dobutamine echocardiography. CMR can also determine viability with late gadolinium enhancement (LGE) methods. The gadolinium-based contrast agents used for LGE differentiate viable myocardium from scar on the basis of differences in cell membrane integrity for acute myocardial infarction. In chronic myocardial infarction, the scarred tissue enhances much more than normal myocardium due to increases in extracellular volume. LGE is well validated in pre-clinical and clinical studies that now span from almost a cellular level in animals to human validations in a large international multicenter clinical trial. Beyond infarct size or infarct detection, LGE is a strong predictor of mortality and adverse cardiac events. CMR can also image microvascular obstruction and intracardiac thrombus. When combined with a measure of area at risk like T2-weighted images, CMR can determine infarct size, area at risk, and thus estimate myocardial salvage 1-7 days after acute myocardial infarction. Thus, CMR is a well validated technique that can assess viability by gadolinium-free dobutamine stress testing or late gadolinium enhancement.

The cardiac magnetic resonance (CMR) approach to assessing myocardial viability

Cardiac magnetic resonance (CMR) is a noninvasive imaging method that can determine myocardial anatomy, function, perfusion, and viability in a relative short examination. In terms of viability assessment, CMR can determine viability in a non-contrast enhanced scan using dobutamine stress following protocols comparable to those developed for dobutamine echocardiography. CMR can also determine viability with late gadolinium enhancement (LGE) methods. The gadolinium-based contrast agents used for LGE differentiate viable myocardium from scar on the basis of differences in cell membrane integrity for acute myocardial infarction. In chronic myocardial infarction, the scarred tissue enhances much more than normal myocardium due to increases in extracellular volume. LGE is well validated in pre-clinical and clinical studies that now span from almost a cellular level in animals to human validations in a large international multicenter clinical trial. Beyond infarct size or infarct detection, LGE is a strong predictor of mortality and adverse cardiac events. CMR can also image microvascular obstruction and intracardiac thrombus. When combined with a measure of area at risk like T2-weighted images, CMR can determine infarct size, area at risk, and thus estimate myocardial salvage 1-7 days after acute myocardial infarction. Thus, CMR is a well validated technique that can assess viability by gadolinium-free dobutamine stress testing or late gadolinium enhancement.

Role of myocardial perfusion scintigraphy post invasive coronary angiography in patients with Myocardial Infarction

BACKGROUND

The presence of severe hypokinesia or akinesia and near complete stenotic lesions on coronary angiography, in a patient with acute myocardial infarction raises a question of viability in the involved territory and its response to revascularization. The decision of revascularization can be effectively taken after myocardial perfusion scintigraphy (MPS).

AIM

To evaluate the role of MPS in patients with acute or recent myocardial infarction after invasive coronary angiography.

MATERIALS AND METHODS

Thirty-five patients (27 Males, 8 Females; Mean age 54 years) with acute myocardial infarction, who underwent invasive angiography, were included prospectively. Invasive angiography was attempted during the episode of acute chest pain in 20 patients. Fifteen patients underwent angiography without MPS because of non-availability of MPS at the time of initial presentation in the referring hospital. Revascularization was deferred because of complete / near complete block of artery with hypokinesia / akinesia of the distal LV segments in 32 / 35 patients and 50 to 70% block in 3 / 35. These patients were subjected to MPS.

RESULTS

Twenty patients underwent stress MPS and 15 underwent nitrate-augmented rest re-distribution study (RR study). Imaging was performed using the hybrid SPECT / CT system. The average defect size of the perfusion defect was 34% (5 - 57% range). Sixteen patients (46%) had fixed perfusion defects. Reversible ischemia was present in 19 (54%). Ten patients had a < 10% reversible perfusion defect. Nine patients had reversible ischemia, > 10% of the LV myocardium, and underwent the invasive revascularization procedure.

CONCLUSION

MPS is invaluable in patients who have total / near total occlusion of the coronary artery and distal segment hypokinesia or akinesia on invasive angiography. One in four patients, deemed to have non-viable myocardium, underwent an invasive revascularization after undergoing MPS.

Myocardial perfusion imaging with contrast echocardiography

Advances in myocardial perfusion imaging have firmly established the use of noninvasive techniques capable of providing useful information over a broad range of diagnostic and therapeutic cardiovascular problems. Evaluating regional myocardial perfusion abnormalities is a cornerstone for the diagnosis of coronary artery disease, risk assessment in those with known disease, and determination of myocardial viability. The clinical use of myocardial perfusion imaging and the current limitations of existing techniques continue to promote the development of new technologies capable of assessing microvascular and capillary perfusion abnormalities on a global myocardial level. Myocardial contrast echocardiography is an emerging technique capable of rapidly assessing myocardial perfusion at the capillary level in many different clinical settings. This article focuses on myocardial contrast-enhanced ultrasound perfusion techniques, emphasizing the unique information this modality provides compared with other noninvasive perfusion imaging techniques.