Abstract
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.
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