ABC | Volume 114, Nº2, February 2020

Update Update of the Brazilian Guideline on Nuclear Cardiology – 2020 Arq Bras Cardiol. 2020; 114(2):325-429 in ventricular function. Affected areas are referred to as viable myocardium or myocardium at risk. 276 • The coexistence of normal myocardium, however, and the formation of subendocardial scarring will not result in improved function, which is now considered “non- jeopardized” area of viable myocardium. • Most experience in assessing viability has been obtained with nuclear imaging, utilizing SPECT and PET, with assessment of perfusion and metabolism. Moreover, using SPECT, cellular and mitochondrial membrane integrity may be characterized. • It has been demonstrated that 40% to 50% of dysfunctional segments without contractile reservemay still have preserved perfusion and metabolism, some of which will recover function following revascularization procedures. The loss of contractile reserve is associated with structural damage characterized by greater severity and fibrosis formation. • Several viability standards have been recognized in areas of contractile dysfunction, for instance: I) any region with > 50% of radiopharmaceutical uptake in resting images; II) any perfusion defect with > 10% increase in uptake of late images. 277 It is, however, necessary to emphasize that areas with > 50% uptake often do not improve in function, given that these regions contain mixtures of normal myocardium and non-transmural scarring. • Uptake and retention of tracers (sestamibi and tetrofosmin, labeled with technetium-99m) depends on perfusion, cellular membrane integrity, and mitochondrial function, where radiopharmaceutical uptake of > 50% to 60% in dysfunctional areas is frequently used as a sign of viability, when observed in resting images. 10.5. The Most Frequently Used Protocols 24,278,279 1. Stress and redistribution – two steps or image acquisition series/one injection of 201 Tl: This is a conventional technique with image acquisition between 2 and 10 minutes (a maximum of 15 minutes) following injection of 201 Tl during peak stress ( first step ). These images reflect initial distribution of the radioisotope dependent on blood flow and, thus, regional myocardial flow. Two to four hours after initial intravenous administration of the radioisotope, in the resting condition ( second step ), a new series of images is obtained, representing the “ redistribution phase ,” related to the continuous exchange of 201 Tl throughout the myocardium and extracellular behavior. This protocol has been designed to study ischemia, and it is not sufficient for characterization of viability, given that viable tissues may not exhibit improvement in radiopharmaceutical uptake (reversibility) within conventional time periods for redistribution images, giving the apparent impression of persistent reduced radioisotope uptake or fibrosis. 2. Stress/redistribution and reinjection – three steps or image acquisition series/two injections of 201 Tl (Figure 50B): In addition to the conventional protocol, which contains only 1 injection of 201 Tl during stress (at a dose of up to 3.0–3.5 mCi), this includes the reinjection of 201 Tl (generally at a dose of 1 mCi) immediately after the redistribution phase , with the aim of elevating blood concentration of the radioisotope, with a new image acquisition series that may vary in terms of time (between 6 to 24 hours). There is evidence that up to 50% of regions with perfusion defects that are apparently “fixed or persistent” improve in terms of relative radioisotope uptake. This information is predictive of improvement in regional function following revascularization. Areas with sustained or severe reduced uptake following reinjection show a low probability of recovering ventricular function. Further variations are demonstrated in Figures 50 C and D . 3. Stress/redistribution and late imaging – three steps or image acquisition series/one injection of 201 Tl (Figure 50A): The addition of late imaging 24 hours after injection of thallium-201 during the stress or distribution phase (first step) allows more time for the phenomenon of redistribution to occur and, consequently, for increase in myocardial uptake of the radiopharmaceutical. This technique shows good predictive value for improvement of ventricular function following revascularization, but suboptimal NPV owing to the technical quality of images obtained after this period, as well as to the fact that some patients do not demonstrate redistribution during very prolonged periods. 4. Resting/redistribution – two steps or image acquisition series/one injection of 201 Tl while resting: This protocol eliminates the stress phase, based on knowledge of the physiopathology of temporal variations in coronary flow in hibernating myocardium or in unstable patients, leading to perfusion defects that may occur in resting studies, with redistribution in late images. Qualitative studies using the resting-redistribution protocol to evaluate efficacy of revascularization have shown that the majority of myocardial regions with reversible defects during pre-operative periods presented post-operative normalization in perfusion and/or improved ventricular function. Some viable regions, however, may not present redistribution, even in images at 24 hours, unless 201 Tl blood levels are elevated following reinjection. Practically speaking, most of the clinical information necessary to the medical decision-making process, related to viability study, will have been obtained during the stress-redistribution-reinjection sequence, and late images will generally not be necessary. There is no established consensus regarding the accuracy of Sestamibi - 99m Tc while resting for detection of viability, with some studies showing similar uptake for 201 Tl and Sestamibi - 99m Tc (quantitative assessment) in patients with UA and LV dysfunction. It has also been observed to have satisfactory predictive value for improvement in contractility in the ventricular walls following revascularization, similar to that of 201 Tl. 280 The administration of nitrates before injection of the radiopharmaceutical with the objective of improving resting myocardial flow seems to improve accuracy for detecting myocardial viability. 281 391