Retrograde His bundle deflection in bundle-branch re-entry.

Atrial echo beats resulting from a reciprocating mechanism involving the bundle-branches were produced by premature atrial impulses in a patient with an A-V nodal bypass tract. The mechanism of the arrhythmia was suggested by the presence of a retrograde His bundle deflection which appeared 'sandwiched' in between a QRS complex with complete right bundle-branch morphology and a negative P wave. Though at a shorter cycle length the His bundle was still activated retrogradely echo beats were not seen because the retrograde H deflection occurred too early, when both bypass tract and A-V node were still effectively refractory. At the faster driven rate concealed retrograde activation of the right branch (by the premature impulse) was responsible for the right bundle-branch block patterns shown by the post-premature driven beat.