Effects of mid-myocardial pacing on transmural dispersion of repolarization and arrhythmogenesis.

Introduction: Epicardial (Epi) activation of the left ventricular (LV) wall increases transmural dispersion of repolarization (TDR), which creates a substrate for the development of ventricular arrhythmia. We hypothesize that pacing from the LV mid-myocardium may decrease the TDR and occurrence of arrhythmias.

Methods And Results: A transmural electrocardiogram and transmembrane action potentials were simultaneously recorded from Epi, mid-myocardial (M), and endocardial (Endo) layers of the arterially perfused canine LV wedge preparations (n= 8). Transmural dispersion of repolarization varied when the preparations were paced at each layer, respectively (Endo pacing, 35.6 ± 6.6 ms; M pacing, 34.9 ± 7.3 ms; Epi pacing, 72.4 ± 4.9 ms; P< 0.001). A significant difference was noted in TDR between M pacing and Epi pacing (P< 0.001), but not between M pacing and Endo pacing (P= 0.831). This result was reproducible in the presence of ischaemia-reperfusion experiments (n= 8). Transmural dispersion of repolarization was amplified as compared with non-ischaemic experiments and differed when preparations were paced at each layer (Endo pacing, 62.8 ± 13.8 ms; M pacing, 63.3 ± 13.3 ms; Epi pacing, 111.1 ± 17.7 ms; P< 0.001). There was again no significant difference between Endo pacing and M pacing (P= 0.948). However, as pacing was shifted from M to Epi, there was a significant increase in TDR (P< 0.001). Ventricular arrhythmias were induced in two of eight ischaemic preparations during Epi pacing, but did not occur in either M or Endo pacing.

Conclusion: Mid-myocardial pacing can significantly decrease the TDR and prevent the occurrence of ventricular arrhythmias as compared with Epi pacing.