Subthreshold electrical stimulation for termination and prevention of reentrant tachycardias.

Subthreshold electrical stimulation (STS) was used to terminate and prevent reentrant supraventricular and ventricular (VT) tachycardia. Of 12 patients with SVT, 8 had atrioventricular nodal (AVN) reentry, and 4 had orthodromic tachycardias. Trains of STS applied close to the AVN area terminated the tachycardias in five of the eight patients with AVN re-entry and two of the patients with orthodromic tachycardia. In 13 patients with recurrent sustained hemodynamically stable VT (mean cycle length 370 +/- 40 ms), trains of STS were delivered at the site of early activity during the tachycardia. Number of train cycles ranged between 3 to 8 pulses and their cycle lengths ranged between 20 and 70 ms. In 7 of the 13 patients VTs were effectively terminated by STS application close to the site of early activity and in the remaining 6 patients it did not. In nine patients the effect of STS applied at the site of early activity on VT induction from the right ventricular apex was examined. In four of the nine patients STS prevented VT induction and in the remaining five patients it did not. These observations suggest that STS applied in proximity to the area critical for initiation and maintenance of reentry can terminate or prevent induction of the tachycardia.