Subthreshold conditioning stimuli prolong human ventricular refractoriness

This study tested whether a subthreshold stimulus (S_c) inserted before a premature stimulus prolonged the right ventricular effective refractory period in humans, and whether the degree of effective refractory period lengthening was influenced by heart rate, S_c current intensity or S_c pulse duration. S_c at current intensity 10 mA and pulse width 2 ms prolonged mean effective refractory period from 255 ms to 277 ms (p < 0.001, n = 20). The increase in effective refractory period was similar in 6 patients studied at pacing cycle lengths 600 (259 to 289 ms) and 400 ms (236 to 258 ms). When S_c current intensities were varied at 2, 5 or 10 mA at a constant pulse duration of 100 ms the effective refractory period progressively prolonged by 6, 40 and 81 ms respectively (p < 0.02, n = 6). The pulse duration of S_c at a constant current of 10 mA significantly influenced effective refractory period prolongation. With S_c pulse durations of 2, 10, and 100 ms the effective refractory period prolonged by 16, 33 and 66 ms respectively (p < 0.01, n = 7). Thus, subthreshold impulses prolong the effective refractory period in human right ventricular myocardium. The prolongation of effective refractory period depended on S_c current intensity and pulse duration but was independent of heart rate at the cycle lengths tested. The use of subthreshold stimuli as antiarrhythmic therapy may be feasible in some patients.