Copyright © 1994 by the European Society of Cardiology.
© 1994 The European Society of Cardiology
The influence of different electrode configurations on the inhibitory effect of subthreshold high-frequency stimuli
Department of Medicine, Cardiology Unit Valencia, Spain
*Department of Physiology, University Clinic Hospital Valencia, Spain
Received 25 August 1993; revised 25 May 1994; .
Correspondence: Francisco Javier Chorro, Cardiology Unit, University Clinic Hospital, Avda, Blasco Ibanez, 17. 46010 Valencia, Spain.
Abstract
Subthreshold stimuli trains may inhibit the response to a subsequent suprathreshold stimulus; the inhibitory effect decreases or even disappears when the subthreshold stimuli are delivered apart from the suprathreshold stimulus. The purpose of this study is to analyse, in atrial epicardium, the influence of different electrode configurations upon the inhibitory effect of subthreshold stimuli trains delivered at a short distance from the explored zone.
In 18 anaesthetized dogs, the basal atrial effective refractory period was determined using an epicardial unipolar cathodal electrode. A second determination was performed when a subthreshold stimuli train (1 ms cathodal pulses, 800 Hz, and intensity 10% less than the diastolic threshold of the train) was delivered 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 electrode (Group 3, n=6). A third determination was performed in all experiments delivering both the subthreshold stimuli train and the extrastimulus through the same central test electrode.
Atrial effective refractory period did not vary significantly with respect to the basal values 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 increase 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 delivering 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 refractoriness 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 the subthreshold inhibitory effect within the inner ring; (c) with a different electrode set—multiple discrete electrodes surrounding (4.5 mm) the explored zone—inhibition has not been demonstrated.
Key Words: Subthreshold stimuli trains • inhibitory effects • atrial refractoriness • cardiac electrophysiology