Inhibition by trains of subthreshold high-frequency stimuli: an experimental study in spatial limitation

The aim of this study was to determine whether the spatial limitation of the inhibitory effect of subthreshold conditioning stimuli trains (TSc) could be overcome by their simultaneous emission through several electrodes surrounding the area where the suprathreshold extrastimuli (S2) are delivered. In seven anesthaetized open chest dogs the effective ventricular refractory period was determined before and after the introduction of unipolar cathodal TSc, using epicardial electrodes. TSc pulse frequencies tested were 100 Hz, 200 Hz, 400 Hz, 600 Hz and 800 Hz, and train intensity was 10% lower than the train diastolic threshold for every pulse frequency. S2 and TSc were delivered: (a) by the same electrode; (b) by two different electrodes 3 mm apart; and (c) TSc through six peripheral electrodes surrounding the central electrode that delivered S2 (heptapolar electrode). Trains of 400 Hz showed the highest diastolic threshold, permitting the use of the highest train intensities. When TSc and S2 were delivered through the same electrode the ventricle remained unexcitable during the entire cardiac cycle in six of the seven dogs. In turn, when TSc and S2 were delivered by two different electrodes, the effective ventricular refractory period (EVRP) could only be increased by greater than or equal to 10 ms in three of the seven dogs (18 ms, 62 ms and 10 ms). When TSc was delivered simultaneously through six peripheral electrodes the increments were higher (118 ms, 88 ms and 75 ms) in these three dogs, and there was one additional dog with 12 ms increments of of EVRP.(ABSTRACT TRUNCATED AT 250 WORDS)