Alternate Patterns of Premature Ventricular Excitation During Induced Atrial Bigeminy

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Abstract
Alternate patterns of premature ventricular excitation have been observed during induced atrial bigeminy in 18 subjects, including five normal volunteers. Each study was performed in the cardiac catheterization suite where a transvenous catheter electrode was positioned in the right atrium. Coupled or paired stimuli were delivered to the atrium by an isolated battery-powered source at an adjusted interval which resulted in alternate patterns of ventricular excitation from alternate premature beats ("alternating premature ventricular excitation"). In most instances "alternating premature ventricular excitation" occurred when parameters of preceding cycle length, premature coupling interval, and atrioventricular conduction time were constant. In man also some observations were made of His bundle excitation during premature atrial stimulation, and in the intact dog heart some observations were made of alternating premature ventricular excitation during His bundle stimulation. A tenable explanation for alternating premature ventricular excitation is advanced which rests on three postulates: (1) A long cycle length is followed by a long refractory period. (2) The refractory period of each branch of the specialized conduction system is dependent on its preceding cycle length or recovery period; and (3) the diastolic recovery period of a blocked segment of the specialized conduction system is shorter than the recovery period when blockade does not occur.