Alternate patterns of premature ventricular excitation during induced atrial bigeminy.

SUMMARY 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 whiclh 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. A N UNUSUAL form of alternating ventric-Ilular conduction has been observed in 18 patients during right atrial pacing studies. The purpose of this report is to describe the phenomenon and to propose an electrophysio-logical mechanism of alternating patterns of premature ventricular excitation during induced atrial bigeminy. Right bundle-branch block Incomplete bundle-branch block His bundle electrogram Methods Observations were made on 18 subjects, including five normal volunteers by methods of investigation which have been previously described in detail.1 2 In brief, each study was performed in the cardiac catheterization suite with the subject in the nonsedated post-absorptive state and supine position. A bipolar or tripolar catheter electrode was introduced into an antecubital vein, utilizing sterile percutaneous technic and local anesthesia. The catheter was positioned against the lateral wall high in the right atrium under fluoroscopic and electrocardio-graphic control. Coupled or paired stimuli were delivered to the atrium by an isolated battery-power source (Medtronics R-wave, coupled pulse generator) at an adjusted milliamperage which would assure atrial capture. The atrial coupling interval, or the interval between paired pulses,