title: The enigmatic sixth wave of the electrocardiogram: the U wave.
abstract: The U wave is the last, inconstant, smallest, rounded and upward deflection of the electrocardiogram. Controversial in origin, it is sometimes seen following the T wave with the TU junction along the baseline or fused with it and before P of the following cycle on the TP segment. In this review we will study its temporal location related to monophasic action potential, cardiac cycle and heart sounds, polarity, voltage or amplitude, frequency and shapecontour. We will analyze the clinical significance of negative, alternant, prominent U wave, and the difference between T wave with two peaks (T1-T2) and true U wave. Finally we will analyze the four main hypotheses about the source of U wave: repolarization of the intraventricular conducting system or Purkinje fibers system, delayed repolarization of the papillary muscles, afterpotentials caused by mechanoelectrical hypothesis or mechanoelectrical feedback, and the prolonged repolarization in the cells of the mid-myocardium ("M-cells").
authors.affiliation: International VCG Investigators, ABC Faculty of Medicine, Foundation of ABC, Santo André, Brazil. riera@uol.com.br
authors.forename: Andrés Ricardo
authors.initials: AR
authors.lastname: Pérez Riera
authors.forename: Celso
authors.initials: C
authors.lastname: Ferreira
authors.forename: Celso Ferreira
authors.initials: CF
authors.lastname: Filho
authors.forename: Marcelo
authors.initials: M
authors.lastname: Ferreira
authors.forename: Adriano
authors.initials: A
authors.lastname: Meneghini
authors.forename: Augusto Hiroshi
authors.initials: AH
authors.lastname: Uchida
authors.forename: Edgardo
authors.initials: E
authors.lastname: Schapachnik
authors.forename: Sergio
authors.initials: S
authors.lastname: Dubner
authors.forename: Li
authors.initials: L
authors.lastname: Zhang
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D004562:Electrocardiography; D006321:Heart / Q000502:physiology*; D006801:Humans; D010210:Papillary Muscles / Q000502:physiology; D011690:Purkinje Fibers / Q000502:physiology*; D012032:Refractory Period, Electrophysiological / Q000502:physiology*