944-69 Anisotropic Repolarization in Ventricular Tissue

In the in vivopreparation. the effective refractory period (ERP) of epicardial ventricular cells is significantly influenced by the sequence of activation. Previous in vitrostudies. however, showed the effects of myocardial fiber orientation (MFO) on ERP were only apparent within 2–3mm from the site of baseline stimulation (S1). To determine the importance of the MFO on the repolarization process, we studied 12 blocks of pig RV tissue in vitro. The size of each block was 30mm×30mm×2mm. Transmembrane action potentials (APs) were recorded and ERP were measured from the epicardial surface which has nearly uniform MFO. The tissue was paced at a 500 ms cycle length. and sequential recordings were made at 1 4, 7, 10, 13, and 16mm from S1 along and across the fibers.ResultsThe propagation of depolarization was slower (p<0.01) in the transverse (0.18 m/s)than in the longitudinal direction (0.34 m/s). In the transverse direction, AP duration 1mm away from the S, site was 188±14 ms, and progressively shortened as the recording site was moved further away from the S1 site (p<0.001). In contrast, the AP duration in the longitUdinal direction did not change as the distance between recording site and the S, site increased. The ERP 16mm from the site of stimulation was longer in the transverse direction (280±19 ms) than in the longitudinal direction (252±12 ms, p<0.01), but the difference was less than that between activation times, reflecting opposite effects of MFO on activation and repolarization.ConclusionThe effects of MFO on repolarization was apparent only when propagation was across the fiber. The effects of MFO are present at sites ≥ 16mm from the S1, and could explain the results of in vivostudies