His bundle rhythms with retrograde conduction to the atria.

His bundle electrograms were repeatedly recorded in two patients with complex arrhythmias. In the first case the pacemaker was located in the lower part of the left bundle-branch, retrogradely activating the His bundle. Atrial standstill was associated with this mechanism. Afterfour days, His bundle tachycardia occurred, with constant retrograde activation of the atria, and anterograde block. In the second patient, who had progressive degeneration of the conducting system, His bundle rhythm activated the ventricles with a 2:I block. There was synchronization between the pacemaker located in the His bundle, and the atria. As the disease progressed, dissociation between atrial, His bundle, and ventricular rhythms developed. Concealed His bundle automatic activity may explain some complex arrhythmias which would remain unsolved if the His bundle potentials were not recorded.