A companion study shows that both impairing and facilitatory effects contribute to rate-induced changes in AV nodal function assessed with a premature protocol performed at different basic rates. We assessed merits and limits of different currently used premature protocols in exposing and characterizing these opposite effects of rate. The experiments were performed in same 6 rabbit heart preparations than the companion study in which we had varied both basic cycle length (BCL) and prepremature

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... ycle length (PPCL, last cycle before test cycle) to independently characterize impairing and facilitatory effects on test beat, respectively. In present study, a standard premature protocol not distinguishing BCL and PPCL, was performed at control and at 3 fast basic rates (50%, 75% and 100% cycle length shortening in 1:1 nodal conduction range). The protocols were repeated while allowing 20beats, 1-min or 5-min toward a steady state (SS-duration) at the new rate before testing premature beats. Increasing basic rate significantly prolonged the minimum nodal conduction time and effective refractory period but did not affect the functional refractory period. Increasing SS-duration significantly prolonged the minimum nodal conduction time in an interactive manner with the fast rate. However, the effects of SS-duration on nodal refractory periods varied markedly within and between preparations, and were not statistically significant. In conclusions, standard premature protocols performed at different basic rates failed, regardless of SS-duration, to expose genuine rate dependent changes in AV nodal function otherwise observed at same rates in same preparations. Currently used premature protocols are suboptimal in assessing rate-dependent AV nodal function. The AV nodal function is often assessed with premature protocols performed at different basic rates. Resulting responses typically show opposite effects on effective and functional refractory period, great variability and frequent inconsistencies. We tested the hypothesis that this complexity arises from opposite effects of rate. To dissociate impairing and facilitatory effects affecting test beats, we independently varied basic cycle length (BCL) and prepremature cycle length (PPCL, last cycle before test cycle) in 6 rabbit heart preparations. Four BCL (control and 50%, 75%, and 100% shortening in 1:1 conduction range) and 4 equivalent PPCL were tested in different combinations. Because the AV node has a short (<1 cycle) facilitatory memory and a longer impairing memory (>1 min), facilitatory and impairing effects measured at test beat were those associated to PPCL and BCL, respectively. Nodal conduction and refractoriness consistently increased with BCL shortening and decreased with PPCL shortening. The relative BCL effects were similar at different PPCL and vice versa. Each effect altered the baseline from which the other occurred but did not affect its magnitude. When BCL and PPCL were shortened together as in standard protocols, net changes in nodal variables were small and inconsistent. When facilitatory effects were dissipated with a long cycle, BCL shortening only impaired nodal function. Moreover, similar results were obtained using either His-atrial interval or atrial interval to assess nodal recovery time. In conclusions, BCL impairing effects prolong nodal conduction and refractoriness at test beat. PPCL facilitatory effects shortened nodal conduction and refractoriness at test beat. These opposite effects account for the muddle of net changes observed with standard premature protocol. Impairing effects measured at test beat were those associated to PPCL and BCL, respectively. Nodal conduction and refractoriness consistently increased with BCL shortening and decreased with PPCL shortening. Introduction: slow pathway (SP) ablation in AVN RT can be complicated by IIº-IIIº AV block. We assessed the usefulness of pacemapping of Koch's triangle (PMKT) in preventing this complication. Methods: 778 patients with AVN RT were analized. 459 (Group 1) had a conventional SP ablation; 319 (Group 2) had the ablation guided by PMKT, which localized the anterogradely conducting fast pathway (AFP) on the basis of shortest St-H interval obtained stimulating the anteroseptal, midseptal and posteroseptal aspect of Koch's triangle. Results: in Group 2, AFP was anteroseptal in 290 (91%), midseptal in 24 (7%), posteroseptal or absent in 5 (2%). In 23/24 patients with midseptal AFP, SP ablation was strictly performed in the posteroseptal area. In 4/5 patients with posteroseptal/no AFP, retrograde FP was ablated. 2 patients refused ablation. Permanent IIº-IIIº AV block was created in 7/459 of Group 1 vs 0/319 of Group 2 (p=0.046). Ablation was successfull in all cases. Conclusions: PMKT identifies patients with AFP abnormally close to the SP or without AFP. In these cases, guiding ablation, it allows to avoid AV block.