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 4 – Myocardial perfusion scintigraphy with 99m Tc-Sestamibi (upper line) and 18 F-FDG PET (lower line) for assessment of myocardial viability, showing improvement in perfusion/metabolism in anterior (apical, medial and basal), apical septal, anteroseptal (medial and basal) and inferoseptal (medial and basal) segments; “mismatch” pattern. only the addition of some minutes to the angiography protocol; its high spatial resolution, being of great importance in evaluation of small infarctions; near-isotropic resolution and reliable 3D data reconstruction for the small slide thickness; possibility of inclusion of patients with pacemakers and other metallic devices. As disadvantages, we can mention the necessity of higher radioactive emission for acquisition of additional images following coronary images, and poorer localization ability and transmurality as compared with CMR. 43 Assessment of myocardial viability with MR imaging MR is a highly efficient method for myocardial viability study 26 and has played an important role in the clinical practice. It has also been considered a gold standard method in the assessment of left ventricular function. MR allows assessment of left ventricular dysfunction associated with chronic ischemic disease by evaluation of contractile reserve using dobutamine at low dose and, most importantly, evaluation of fibrosis by late gadolinium enhancement. In a metanalysis, Romero et al. 44 concluded that MR with low-dose dobutamine has high sensitivity and specificity (81% and 91%, respectively), whereas late gadolinium enhancement MR has 95% sensitivity and 51% specificity, and high accuracy in determining some parameters, including ejection fraction, left ventricular volume, regional wall motion, and myocardial thickness. 45,46 Left ventricular wall thickness at end diastole is important to exclude viability. The most notable characteristic of MR is its high spatial resolution, and, for this reason, the method stands out for its high imaging quality and capacity to diagnose ischemic areas that would not be detectable by other methods. MR may also be particularly useful in the assessment of myocardial blood flow at rest in hibernating areas of narrowed coronary artery and improvement of local myocardial contractility after coronary revascularization. 13,47 The use of gadolinium as a contrast medium in MR allows the detection of the effects of perfusion, microvascular obstruction and differentiation between transmural and subendocardial necrosis. 48 Gadolinium has a low risk of nephrotoxicity, except for patients with end-stage renal disease, in which the risk of systemic toxicity is real. Although chelated-gadolinium compounds are distributed in the extracellular space, and do not penetrate in intact cells, they may accumulate in myocytes with ruptured cell membrane (e.g. acute myocardial infarction) and fibrotic areas 10 (Figure 5). The likelihood of functional recovery after revascularization is proportional to the transmurality of acute myocardial infarction. A very important marker of improvement of myocardial function is the amount of delayed enhancement by MR imaging since there is a progressive improvement in myocardial function with the increase of transmurality of scar tissue. Kim et al. 46 evaluated the ability of contrast- enhanced MR imaging to predict functional recovery after revascularization. Approximately 80% of segments with less than 25% of transmural fibrosis had functional recovery after revascularization, whereas only 10% of the segments with transmurality higher than 50% recovered after revascularization. Selvanayagam et al. 47 showed that delayed- enhancement cardiovascular MR imaging is a strong predictor of myocardial viability after surgical revascularization. Left ventricular wall thickness may reveal valuable information about viability. Schinkel et al. 26 showed that segments with an end-diastolic wall thickness of less than 5 mm was associated with higher likelihood of recovery after revascularization. Taken together, these findings suggest that segments with an end-diastolic wall thickness of less than 5.5 mm never show recovery of function after revascularization, which may be related to the presence of nontransmural infarction. These segments contain subendocardial scar tissue, with residual viability in the epicardium. Therefore, significant wall thinning indicates scar tissue, with low likelihood of recovery after revascularization; nevertheless, evidence suggests that recovery of function may occur, but only when contrast- enhanced MR excludes scar tissue. 10 Geber et al. 49 demonstrated that cardiac MR was important in identifying patients with ischemic cardiomyopathy and severe left ventricular dysfunction who would benefit from myocardial revascularization. CMR can be performed in ischemic cardiomyopathy with left ventricular dysfunction to characterize myocardial viability. 50 Limitations of this technique, however, include its high cost, difficulty of performing scans in patients with implanted devices, and limited availability. 10 282