ABC | Volume 110, Nº5, May 2018

Original Article Almeida et al MTWA and malignant arrhythmias in Chagas disease Arq Bras Cardiol. 2018; 110(5):412-417 heterogeneity of ventricular repolarization, which is considered a predisposing condition to the beginning and perpetuation of ventricular arrhythmias. It is worth noting the association of MTWA with malignant arrhythmias in several clinical conditions, but few studies have included patients with CCC. This study was aimed at assessing the possible association between MTWA and malignant ventricular arrhythmias in Chagas disease. Method Study This is an observational, case-control study, approved by the Ethics Committee in Research of the Federal University of Minas Gerais (COEP 7918/12). The patients were recruited between 2011 and 2014. Patients The sample consists of patients diagnosed with CCC being followed up at the Hospital das Clínicas of the Federal University of Minas Gerais (HC-UFMG). The individuals agreed to participate and provided written informed consent. Patients should be older than 18 years, have a positive serology for Chagas disease and meet all the diagnostic criteria for CCC, which include asymptomatic structural heart disease with typical electrocardiographic changes, 18 or heart failure with preserved or reduced left ventricular ejection fraction (LVEF), with current or previous symptoms. The case group consisted of patients with CCC and history of malignant ventricular arrhythmia, with indication for ICD implantation for secondary prophylaxis and authorization issued by the High-Complexity Commission of the Brazilian Unified Health System (SUS), according to the ordinance # 152, of March 8, 2007, 19 updated by the ordinance # 1, of January 2, 2014. 20 Patients with CCC and no previous history of malignant ventricular arrhythmia comprised the control group. According to the ordinance # 152, of March 8, 2007, the major indications for ICD implantation in Brazil are as follows: 19 - Individuals resuscitated from documented cardiac arrest due to tachycardia or ventricular fibrillation of non-reversible cause, with LVEF ≤ 35% or structural heart disease; - Spontaneous, sustained ventricular tachycardia, of non- reversible cause, with LVEF ≤ 35%; - On the electrophysiological study, syncope of undeterminedetiologywith inductionof hemodynamically unstable sustained ventricular tachycardia, or clinically relevant ventricular fibrillation with LVEF ≤ 35% or structural heart disease. Individuals with the following characteristics were excluded from the study: difficulty to walk on the treadmill; NYHA functional class IV heart failure; atrial fibrillation or flutter; pacemaker dependency. In addition, individuals with absolute contraindications to undergo exercise test, such as cardiac arrhythmias leading to hemodynamic instability, decompensated heart failure and acute non-cardiac conditions that could be aggravated by physical exercise, were excluded from the study. 21 Microvolt T-wave alternans test The individuals included in this study underwent a medical interview with a standard questionnaire, physical examination and transthoracic echocardiography. Left ventricular ejection fraction was calculated by use of the Simpson’s method. Later, the patients underwent the MTWA test, at the ergometry sector of the Hospital das Clínicas of the UFMG. For performing the MTWA test, the following items were used: Micro-V Alternans Sensors™ of Cambridge Heart high-resolution electrodes, which minimize noise and artifacts; the Cambridge Heart - HearTwave software for analysis and report; and a treadmill. Chronically used medications were maintained. The MTWA test consists in proper preparation with skin cleansing and removal of the superficial layer of dead cells by use of abrasion, placement of electrodes in the 12 standard electrocardiographic leads and in the 3 orthogonal leads (X, Y and Z). Data from the electrocardiographic tracing were collected at rest, during exertion on the treadmill, and during the recovery phase. During exertion, the patient should reach a heart rate between 100 and 110 beats per minute (bpm) and sustain it for 2 minutes and 30 seconds. Then, heart rate between 110 and 120 bpm should be reached and sustained for 1 minute and 30 seconds. For the test to be considered valid, target heart rate should be maintained for at least 60% of the determined time period. The software provides an analysis with measurement of MTWA, characterizing the test as positive, negative or indeterminate. The positive test consists in T-wave alternans with amplitude ≥ 1.9 μV sustained for at least 1 minute, with an initial heart rate < 110 bpm or at rest, in an orthogonal lead or two adjacent precordial leads. The negative test does not detect any significant T-wave alternans for 1 minute with a heart rate ≥ 105 bpm, if there is no impairment to the tracing due to noise or more than 10% of ectopic beats. 22,23 The tests that do not meet any of those criteria are considered indeterminate. The indeterminate tests attributed to noise were repeated. Then the tests were grouped as negative or non-negative (positive and indeterminate), based on studies about the impact of the indeterminate test on the outcome of ventricular arrhythmias. An indeterminate test due to patient’s factors, such as impossibility to keep heart rate between 105 and 110 bpm, frequent extrasystoles and MTWA not sustained for 1 minute, is associated with the occurrence of ventricular arrhythmias similarly to the way the positive test is. 24 Sample calculation The sample was calculated with the Power and Sample Size Calculations software. 25 Considering that Barbosa et al. 26 have found non-negative results in 81.8% of the Chagas disease patients wearing an ICD, estimating that those without malignant ventricular arrhythmia would have 30% less non‑negative MWTA tests (57%), for a power of 80% and alpha error of 5%, we found 50 patients in each group. 413