ABC | Volume 114, Nº2, February 2020

Update Update of the Brazilian Guideline on Nuclear Cardiology – 2020 Arq Bras Cardiol. 2020; 114(2):325-429 thus, present variations in sensitivity and specificity. 261 In the SPECT technique, the use of thallium-201 ( 201 Tl) is the established method for evaluating myocardial perfusion and the integrity of the cellular membrane. In comparison with other radiopharmaceuticals and available techniques, this has become the choice for determining viability. 267,268 Additionally and in conjunction with these non-invasive techniques, whose aim is to obtain functional and/ or anatomical information, constituting the basis of the physiopathological approach to underlying HF, multiple detector CT is also appropriate. However, the selection of imaging modalities, whose purpose is to assess CHF and, specifically, viability of a dysfunctional myocardium, depends on the clinical information that is required for adequate patient management (Table 26), with inherent questions and affirmations, such as: • Is the etiological cause of the cardiomyopathy in question ischemic or non-ischemic? • Inpatientsconsidered“ischemic,”theneedforrevascularization should be evaluated with respect to characterization of the quantity of myocardium at risk/viability. • Evolving assessment of LV function and the possibility of remodeling are mandatory elements for analysis within the clinical decision-making process; 269 other elements include secondary mitral regurgitation, 270 implantable devices, such as defibrillators and/or resynchronization therapy. 271 • Moreover, when the etiology of LV dysfunction, considered of the utmost magnitude for therapeutic decision making, is mandatorily under discussion, it has been verified that the majority of patients will have ischemic cardiomyopathy. Data from 24 multicenter studies on CHF, published in high-impact periodicals between 1986 and 2005 and summarized in 2006, including 43,568 individuals, showed a 62% prevalence of CAD. This frequency is probably underestimated to the extent that coronary cineangiography was not performed in all patients. 272 10.2. Morphology It had initially been established that the recovery of ventricular function, when a hibernating myocardium was revascularized, should indicate that structural changes were absent or minimal, as observed in experimental models of stunned myocardium. However, since the beginning of the 1980’s, it has been known that chronically dysfunctional myocardial segments demonstrate distinct morphological changes under microscopy. 273 There is a combination of normal, atrophic, and hypertrophic myocytes, with or without evidence of necrosis. Electron microscopy shows loss and/or disorganization of myofilaments and alterations in sarcoplasmic reticulum and mitochondria. These structural changes may contribute to slow functional recovery following revascularization. 274 10.3. Evaluation of Viable Myocardium The differentiation between the presence and absence of viability is highly relevant in patients under consideration for revascularization. Many patients who demonstrate viability associated with severe LV dysfunction may still be candidates for revascularization, but not for cardiac transplant. 275 10.4. Physiopathology and Definitions • The term viable myocardium, regardless of the contractile state of the myocardium, should be understood differently in the context of CHF and viability study, given that the main objective is to predict improvement in LV function following revascularization. • Persistent contractile dysfunction of the LV may be related to chronic hibernation and/or stunning, and not merely associated with fibrotic tissue. Revascularization may improve function and survival if the dysfunctional myocardium is still viable. Functional improvement will not occur in the presence of fibrosis. • The initial point for discussion of viability implies regional dysfunction, detected by various non-invasive methods, including echocardiography as an initial line of investigation. • Differentiation between hibernation and stunning may be established based on blood flow to the myocardium. While resting flow is chronically diminished in hibernation, it may still be preserved in the stunned myocardium, with a compromised flow reserve however. 19 • Clinically speaking, it may not always be feasible to separate the 2 physiopathological conditions; nor is it always necessary, considering that both entities require revascularization in order for there to be improvements Table 26 – Clinical situations where nuclear cardiology should be considered a preference for assessing myocardial viability, in the following order of choice: PET with 18F-FDG, resting scintigraphy with thallium-201 and reinjection protocol, and resting scintigraphy with Sestamibi - 99m Tc sensitized with oral nitrite • Evaluation of extent/localization of dysfunctional myocardium at risk, through significant hypoperfusion (hibernating myocardium) • Clinical situations in which sensitivity is sought for assessment of viability • Contraindications to the use of MR: patients with pacemakers or cardiac defibrillators incompatible with resonance, cerebral clips, cochlear implants, metallic fragments in their eyes, or renal insufficiency • Conditions which limit image acquisition via MR: claustrophobia, irregular cardiac rhythm, dyspnea with inability to remain in dorsal decubitus for prolonged periods • Availability of the method and local expertise 18 F-FDG: fluorodeoxyglucose labeled with fluorine-18; MR: magnetic resonance; PET: positron emission tomography. 390