Coupled pacing and coupled pacing with concealed conduction. Report of a case describing a new observation.

SINCE their introduction by Lopez and coworkers,' paired electrical stimulation and coupled pacing of the heart have been investigated intensively and have been shown to be useful in slowing the effective mechanical rate of the heart or in causing sustained potentiation of cardiac contraction or for both. A recent symposium on this subject provides an excellent review.2 The techniques, in essence, consist of electrically stimulating the heart shortly after the termination of the effective absolute refractory period of each previous spontaneously or electrically driven beat. These stimuli cause an artificial bigeminal rhythm, the second member of each pair being mechanically ineffective. In paired stimulation (PS) both the mechanically effective beat and the coupled extrasystole are pacemaker induced, while in coupled pacing (CP) a spontaneous depolarization is permitted to occur and only the extrasystole, triggered by the preceding R wave, is artificially induced.3-5 Each extrasystole produced by either of these methods possesses a refractory period of its own and thus causes a doubling of the myocardial refractory period to other stimuli. This lengthened refractory period can be utilized to control tachycardias.' 6 In addition, the introduction of such extrasystoles causes postextrasystolic potentiation (PESP) of the next mechanically effective beat. By repetitive coupling of extrasys-