Ventricular Fusion Beats during Electric Stimulation in Man; Application to Conduction Velocity and Anomalous Av Excitation.

VENTRICULAR fusion beats are observed in a variety of circumstances whenever two impulses depolarize different parts of the ventricles simultaneously.' The resulting electrocardiographic complex is a mixture, or fusion, of the two responses. Special opportunities for quantitative studies of ventricular fusion beats were found in patients with Stokes-Adams disease after electric pacemakers were implanted in the left ventricle.2 The two impulses producing fusion came from either two implanted electric pacemakers or an electric pacemaker and a conducted sinoatrial beat. In both circumstances , the important information could be precisely determined regarding the locations of the two pacemakers and the time when each impulse reached the ventricles. In a previous report of studies in 19 patients we presented an analysis of fusion in the ventricle between conducted beats and an electric pacemaker.3 Ventricular fusion beats of the same type were also studied in a single patient by Nunfiez-Dey et al.4 and by Katz and Pick,5 and in four patients by Gerbaux and Lenegre.6 These authors all recognized the important point that such studies could give measures of conduction times between different parts of the heart. They differed considerably, however, in their interpretations of the pathways involved, and their data were too limited to permit a full analysis. Some quantitative studies of fusion between electrically stimulated and conducted beats have also been carried out in animals. Butterworth and Poindexter7 and Ohnell8 demonstrated that the mechanism of fusion accounted for the features of anomalous AV excitation (Wolff-Parkinson-White phenomenon), but they did not use their data to derive conduction times. It is the purpose of this paper to present more extensive quantitative data on the fusion phenomenon in man, to derive measurements of conduction velocities in Purkinje and myocardial tissues, and to discuss the significance of these observations in the assessment of the mechanism of anomalous AV excitation. Patients and Methods Fusion beats have been studied in 40 of our 99 patients with Stokes-Adams disease in whom electric pacemaker-electrode systems were implanted. In nine patients in whom new systems had to be implanted, observations of competition were made in the operating room during the short period when both pacemakers were functioning. In 32 patients* in whom atrioventricu-lar conduction returned at least intermittently after implantation, competition was observed between the independent sinoatrial node and the electric pacemaker. The distance between the two electrode sites was measured by the surgeon in six patients.t Similarly, the distances …