Fj. Chorro et al., THE INFLUENCE OF DIFFERENT ELECTRODE CONFIGURATIONS ON THE INHIBITORYEFFECT OF SUBTHRESHOLD HIGH-FREQUENCY STIMULI, European heart journal, 15(12), 1994, pp. 1681-1688
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.