Felix, S.M., Ferreira, J.S., Martins, P.B., Branco, M.J. and Soares, C.S. 1981. Alterations of surface electrocardiogram in 35 patients during upper fiberendoscopy. Acta Médica Portuguesa. 3, 4 (Aug. 1981), 281–289. DOI:https://doi.org/10.20344/amp.3891.