title: [Letter: Quadrigeminy in concealed bigeminy].
mesh_terms: D001146:Arrhythmia, Sinus / Q000503:physiopathology*; D006352:Heart Ventricles / Q000503:physiopathology; D006801:Humans; D012032:Refractory Period, Electrophysiological; D012849:Sinoatrial Node / Q000503:physiopathology