Mechanism for Site-Dependent Differences in the Shape of the Resetting Response Curve in Fixed Barrier Reentry

Site Dependence of Resetting Response Curves. introduction: We investigated whether the site of stimulation within a reentrant circuit affects the resetting response curve (RRC). RRCs are used to characterize the excitable gap of reentrant circuits, including the duration of the fully excitable gap and the presence of partially excitable tissue. Methods and Results: We reset proximal and distal to a site of interval‐dependent conduction (IDC) in canine in vitro atrial tricuspid rings. Adjustable reentry allowed changes in the cycle length and direction of reentry. In nine preparations we reset 26 tachycardias. In the 16 tachycardias with one site of IDC, RRCs were significantly different when stimulating distal and proximal to a site of interval‐dependent conduction. For the distal curves, the duration of the flat portion was 42 ± 26 msec greater (P < 0.001), the slope of the increasing portion was 0.20 ± 0.17 less (P < 0.02), and the increase in the return cycle was 14 ± 9 msec less (P < 0.001). These differences resulted from early activation of the site of IDC by the antidromic premature impulse when stimulating from distal sites. As a result, the coupling interval of the orthodromic impulse at the site of IDC was the same or greater than at the stimulation site. In 10 tachycardias with multiple sites of IDC, significant differences in the resetting responses did not occur even when the antidromic impulse penetrated one site of IDC. Conclusion: In a fixed anatomic barrier reentrant circuit with one site of IDC, resetting distal to this site misrepresents the properties of the excitable gap of the entire circuit.