Number Needed to Entrain

Background— Measuring postpacing intervals (PPIs) is the standard maneuver for localizing reentrant tachycardia circuits. However, changes or termination of the tachycardia during entrainment pacing, or difficulties in defining the correct local activity, limit the use of PPIs. Methods and Results— We hypothesized that the number of pacing stimuli needed to entrain (NNE) was useful for mapping intra-atrial reentrant tachycardias. First, 10 patients with typical atrial flutter were studied to characterize the NNE. Next, 317 entrainment attempts in 30 patients with 76 intra-atrial reentrant tachycardias were analyzed to determine the efficacy of the NNE. The NNE was small at sites within the reentrant circuit (median 2) and large at remote sites during typical atrial flutter. The NNE depended on the pacing cycle length and coupling interval of the initial paced beat, where the NNE became smaller at shorter pacing cycle lengths and coupling intervals. The NNE highly correlated with the difference between the PPI and tachycardia cycle length ( r = 0.906; P <0.001). When the pacing cycle length and coupling interval were 16 to 30 ms below the tachycardia cycle length, a NNE ≤2 and >3 predicted a PPI−tachycardia cycle length ≤20 and >20 ms, respectively, with 100% accuracy. Thirty-six (11%) entrainment attempts changed or terminated intra-atrial reentrant tachycardia. Importantly, the NNE remained valid in those cases. Furthermore, the NNE provided additional information in cases with some difficulties with PPI measurements. Conclusions— The NNE is a simple and reliable criterion, which facilitates mapping intra-atrial reentrant tachycardia. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT001747.