Andrade, Zilton Araújo; Andrade, Sonia Gumes. Fundação Oswaldo Cruz. Instituto Gonçalo Moniz. “Documento produzido em parceria ou por autor vinculado à Fiocruz, mas não consta à informação no documento”.No. SIP/OB-083 from Conselho National de Desenvolvimento Científico e Tecnológico (CNPq).The conducting tissue of the heart was studied in 25 human cases of Chagas’ myocarditis with a method which employs complete serial sections mounted on continuous transparent plastic tape. The pathological changes were correlated with electrocardiographic findings. The inflammation of the acute phase of Chagas’ myocarditis, as seen in one single case, did not seem to interfere with conduction through the AV system. In chronic Chagas’ myocarditis the conducting tissue showed extensive and variable changes: chronic inflammation, fibrosis, atrophy and fragmentation of specific fibers, extreme dilatation and tortuosity of veins, capillaries and lymphatits, fatty infiltration, and arterial medial and intimal fibrosis. A preferential involvement of the right bundle branch and the anterior fascicles of the left branch was observed and an excellent correlation with electrocardiographic abnormalities was found. There was also evidence presented that bundle branch block may be caused by disease proximal to the bundle branches. Complete AV block seemed to be the final result of the progressive inflammatory and degenerative changes involving the conduction system in chronic Chagas’ myocarditis. Inflammation and fibrosis did also involve the sinoatrial node, Purkinje fibers, intracardiac nervous ganglia, and the contractile myocardium.