Prevention of reentrant tachycardia by single beat or repetitive stimulation

Eleven patients with an atrioventricular accessory pathway were studied by programmed electrical stimulation to determine if reentrant tachycardia could be prevented by delivery of either a single atrial extrastimulus, applied at a critical time after the tachycardia-initiating stimulus (or stimuli), or a train of stimuli. In all 11 patients, reentrant tachycardia was reproducibly induced from the high right atrium with a single premature beat, and in all patients initiation of tachycardia was prevented from the same site by a second premature beat. This second extrastimulus was effective if delivered within a zone which began 10 ms outside the effective refractory period of the tachycardia-initiating stimulus and averaged 61 ms in width. This was termed the 'preventive zone'. In 7 patients the effect of train stimulation to the high right atrium was studied. In all 7, the results were concordant with those obtained by single-beat stimulation. Any train which achieved single atrial capture within the preventive zone was effective in preventing tachycardia. Prevention was always possible with a single atrial extrastimulus or with single-capture train stimulation, whereas termination of an ongoing tachycardia required at least 2 atrial extrastimuli, or stimulation from the right ventricle.