ABC | Volume 110, Nº3, March 2018

Review Article Ket et al Practical Implications of Myocardial Viability Studies Arq Bras Cardiol. 2018; 110(3):278-288 Figure 2 – Images of rest (upper line) and nitrate-enhanced rest (lower line) myocardial perfusion scintigraphy, showing improvement of perfusion in anterior (apical, medial and basal) and anterolateral (medial and basal) segments. In fasting conditions, myocardium uses preferentially free fatty acids as energy source, whereas in post-prandial phase, its metabolism is shifted to glucose (with increased levels of circulating insulin). 5 As the metabolism of free fatty acids depends on oxygen, during myocardial ischemia, glucose is the preferred substrate (glycolytic pathway), which is the hallmark of myocardial viability. 35,32-35 PET with 18 F-FDG has mean sensitivity of 92% and specificity of 63% in assessing the likelihood of functional improvement of the muscle in the after revascularization. Many studies have used comparative data of perfusion and 18 F-FDG uptake, defining myocardial viability as hypoperfused areas with preserved glucose metabolism. 26,28,32-34 (Figure 4). Overall improvement of left ventricle may also be evaluated by 18 F-FDG. Left ventricular ejection fraction (LVEF) improves from 37% to 47% (mean values) in patients with myocardial viability detected by 18 F-FDG PET after revascularization. In patients without viable myocardium, LVEF remained almost unchanged (39% x 40%). 31,34-39 Assessment of myocardial viability with computed tomography (CT) CT is the most recent and widely used method for coronary angiography. Three techniques are currently used for cardiac CT – coronary angiography, CT with iodinated and non‑contrast CT – and all of them can provide information on myocardial viability. 40-42 CT coronary angiography has high negative predictive value (> 95%) in excluding epicardial CAD, with increasing role in the assessment of chest pain. It may also provide valuable information in the evaluation of patients with left ventricular systolic dysfunction, with suspected congenital heart disease or coronary anomaly. 42 Delayed enhancement CT uses a similar principle to gadolinium-basedmagnetic resonance (MR) imaging for imaging studies of myocardial scarring. In CT, the use of iodinated contrast causes an increase in Hounsfield units in contrasted tissues, due to attenuation of X-rays, allowing the visualization of cardiac muscle in the early arterial phase, and discrimination of macro and microvascular obstruction. When evaluated 5‑10 minutes after injection of iodinated contrast and increased enhancement, the obstruction is suggestive of infarction, due to extracellular contrast accumulation. 41,42 Finally, non-contrast CT can reveal calcified aneurysms in the left ventricle, for showing similar images to those obtained during attenuation correction scans or calcium scoring. 41,42 Some advantages of cardiac CT include the possibility of being performed in combination with coronary CT, requiring Figure 3 – Myocardial perfusion scintigraphy with 201 Tálio for assessment of myocardial viability; stress imaging (upper line) and 24-hour redistribution imaging after injection of the radiotracer 201 Tálio (lower line), showing improvement of perfusion in anterior (apical, medial and basal) and anterolateral (medial and basal) segments. 281