THE INFLUENCE OF DIFFERENT ELECTRODE CONFIGURATIONS ON THE INHIBITORYEFFECT OF SUBTHRESHOLD HIGH-FREQUENCY STIMULI

Subthreshold stimuli trains may inhibit the response to a subsequent s uprathreshold stimulus; the inhibitory effect decreases or even disapp ears when the subthreshold stimuli are delivered apart from the suprat hreshold stimulus. The purpose of this study is to analyse, in atrial epicardium, the influence of different electrode configurations upon t he inhibitory effect of subthreshold stimuli trains delivered at a sho rt distance from the explored zone. In 18 anaesthetized dogs, the basa l atrial effective refractory period was determined using an epicardia l unipolar cathodal electrode. A second determination was performed wh en a subthreshold stimuli train (1 ms cathodal pulses, 800 Hz, and int ensity 10% less than the diastolic threshold of the train) was deliver ed prior to the extrastimulus through: (a) 8 electrodes equidistant (4 .5 mm) from the central test electrode (Group 1, n=5); (b) a 1 mm wide ring electrode, 2.5 mm from the central test electrode (Group 2, n=7) ; and (c) a 3.5 mm wide ring electrode, 2.5 mm from the central test e lectrode (Group 3, n=6). A third determination was performed in all ex periments delivering both the subthreshold stimuli train and the extra stimulus through the same central test electrode. Atrial effective ref ractory period did not vary significantly with respect to the basal va lues in Group 1 (141 +/- 22 vs 142 +/- 25 ms; ns) though it increased in Group 2 (168 +/- 27 vs 142 +/- 18 ms; P<0.01) and in Group 3 (160 /- 20 vs 133 +/- 25 ms; P<0.01). There were no differences in the incr ease in atrial refractory period between Group 2 (26 +/- 16 ms, range 8 to 47 ms) and Group 3 (27 +/- 13 ms, range 8 to 48 ms). When deliver ing both the subthreshold stimuli train and the extrastimulus through the same electrode, the inhibitory effect was more pronounced (316 +/- 48 vs 139 +/- 21 ms, P<0.0001). Conclusions: (a) Atrial refractorines s may be increased by subthreshold stimuli trains delivered through a ring electrode closely surrounding (2.5 mm) the explored zone; (b) the surface area of the ring electrode does not affect the magnitude of t he subthreshold inhibitory effect within the inner ring; (c) with a di fferent electrode set - multiple discrete electrodes surrounding (4.5 mm) the explored zone - inhibition has not been demonstrated.