ABC | Volume 114, Nº2, February 2020

Update Update of the Brazilian Guideline on Nuclear Cardiology – 2020 Arq Bras Cardiol. 2020; 114(2):325-429 related to tracer distribution in the LV during stress. This analysis and these findings are only possible when comparing resting and stress images. In summary, the doctor interpreting the images should be sure that the stress exam is perfectly normal, in order to dispense with resting image acquisition. Whenever possible, nuclear cardiology should always be performed using exercise as a preferential form of stress instead of pharmacological tests, which serve as alternatives only in patients who are unable to exercise efficiently. In the presence of severe multivessel coronary disease (involving three arteries) and apparent relative homogeneous radiopharmaceutical distribution during stress, it is extremely rare not to observe other high-risk findings during stress testing, in addition to deterioration of LV function induced by stress itself (ischemic myocardial stunning). In summary, in this section, various ways to reduce radiation exposure in patients undergoing nuclear cardiology exams have been covered. By means of relatively simple adjustments to protocols, injected doses may be optimized, guaranteeing image quality and, most of all, reliability of findings in an exam which is of great clinical importance to orient patient management. 13. Evaluation of Cardiac Sympathetic Activity by Scintigraphy with 123 I-MIBG 13.1. Introduction Autonomic cardiac innervation plays a fundamental role on cardiac performance by regulating myocardial blood flow, HR, and myocardial contractility. In several cardiac diseases, cardiac neuronal function is altered and frequently associated with worse evolution. Dysregulation of the autonomic cardiac nervous system increases the risk of potentially lethal arrhythmias and may be a marker of poor prognosis. Scintigraphy imaging of distribution and cardiac neuronal function facilitates the comprehension of the physiopathology of various diseases that affect the heart and may guide treatment, with consequent improvements to clinical results. 322 The autonomic cardiac nervous system encompasses sympathetic and parasympathetic innervation, with its norepinephrine (NE) and acetylcholine neurotransmitters, respectively. These work in equilibrium, and they exert stimulating effects , via adrenergic receptors, and inhibitory effects , via muscarinic receptors, both of which are responsible for electrophysiological and hemodynamic adaptations in the cardiovascular system, in response to bodily demands. 322 In this way, sympathetic stimuli are controlled by cerebral regulatory centers that integrate signals coming from other parts of the brain and receptors in the body. Efferent signals follow descending pathways in the spinal cord and make synapses with preganglionic fibers that emerge at levels T1 to L3. In this sequence, they establish synapses with the paravertebral stellate ganglion and innervate the RV, in addition to the anterior and lateral regions of the LV. In the heart, sympathetic nerves follow the coronary arteries in the subepicardium and then penetrate the myocardium. 322,323 Parasympathetic fibers are scarce in number in comparison with sympathetic fibers, and they originate in the marrow, following the vagus nerve. They begin in the epicardium, crossing the atrioventricular groove and penetrating the myocardium. They are located in the subendocardium, predominantly innervating the atria, but they are less dense in the ventricles, with the exception of the inferior wall. They greatly control the function of sinoatrial and atrioventricular nodes. 322 13.2. Cardiac Scintigraphy with 123 I-MIBG Metaiodobenzylguanidine (MIBG) is a molecule with a structure similar to that of NE, obtained by means of modifications to the molecular structure of guanethidine (a false neurotransmitter, also an NE analogue), which acts selectively on sympathetic nerves, without, however, being metabolized by monoamine oxidase or catechol-O- methyltransferase or exercising a stimulating effect, as NE does. For the objective of diagnosis, MIBG is labeled with iodine-123, forming the radiotracer MIBG- 123 I, 322-325 demonstrating a good correlation between myocardial uptake of MIBG- 123 I and NE content in cardiac tissue. 325 After the radiopharmaceutical is injected via intravenous administration, it spreads throughout synaptic space, and is taken up, concentrated, and stored in presynaptic nerve endings in a manner similar to that of NE. MIBG- 123 I is retained and localized in cardiac sympathetic nerve endings with scintigraphy images obtained by a conventional gamma camera. 322-325 The radioisotope iodine-123 predominantly emits gamma photons with an energy of 159 keV and a physical half-life of 13.2 hours, making image acquisition easy and well tolerated. MIBG- 123 I is widely used in Europe and Japan, and it has recently been approved for cardiac use in the USA. 326 In Brazil, this technique is available in some centers. Cardiac scintigraphy with MIBG- 123 I directly evaluates global and regional sympathetic function of the heart, including uptake, reuptake, storage, and NE release processes in presynaptic nerve endings. 327 Intravenous injection of MIBG- 123 I is administered while resting, at least 30 minutes after oral administration of potassium iodide syrup or an iodine-containing solution, in order to block and protect the thyroid. Medications that may potentially interfere with catecholamine uptake, such as antidepressants, and some calcium channel blockers should be suspended for at least 24 hours before administration of the radiopharmaceutical. On the other hand, betablockers, angiotensin converting enzyme inhibitors (ACEI), and/ or angiotensin receptor blockers (ARB) do not need to be discontinued. 328 Approximately 15 minutes and 4 hours after administration of MIBG- 123 I, static images and tomography images (SPECT) of the thorax are obtained in anterior projection, with the patient in dorsal decubitus, with the left arm raised above the thorax. While tomography images are optional , they help evaluate myocardial sympathetic activity and compare with the perfusion study ( see the subsequent topic “Evaluation of arrhythmias” ). Global cardiac uptake of MIBG- 123 I is evaluated by static imaging of the thorax (Figure 52). The following 2 fundamental parameters are visually and semiquantitatively analyzed: the relation between cardiac and mediastinal uptake (heart to mediastinum ratio [HMR]) 398