Regional differences in electrophysiological properties of epicardium, midmyocardium, and endocardium - In vitro and in vivo correlations

Background Microelectrode studies have described a population of cells within the midmyocardium (M cells) displaying a steep rate dependence of action potential duration (APD) and high V over dot (max) compared with endocardial (Endo) and epicardial (Epi) cells. Methods and Results We studied repolarization in different myocardial layers in vitro and in situ. In addition to confirming the results of earlier studies, we found that after abrupt lengthening of the cycle length (CL), APDs in M cells reached a new steady state faster than in Epi or Endo cells: the time to achieve 90% of the difference in APD (t(90)) was 13.3+/-0.7 minutes in Endo cells, 12.8+/-1.1 minutes in Epi cells, and 2.6+/-0.4 minutes in M cells (P<.05 compared with Epi or Endo) when CL changed from, 400 to 1000 ms. In situ, we registered activation-recovery intervals (ARIs) in bipolar electrograms obtained from different myocardial layers in conditions of AV block and His-bundle pacing. At all CLs from 300 to 2000 ms, ARIs were equal in all myocardial layers from Epi to Endo cells. Steady-state ARIs coincided with APD of M cells registered in vitro in the physiological range of CL from 300 to 700 ms. When CL was changed from 300 to 1000 ms, the ARI followed the rapid time course typical of M cells (t(90)=2.6+/-0.5, 2.2+/-0.4, 2.5+/-0.4, 2.6+/-0.5, and 2.3+/-0.4 minutes for Epi; 3-, 5-, and 7-mm sub-Epi; and Endo cells, respectively). Conclusions In contrast to in vitro results, there is no significant difference in repolarization among myocardial layers in the intact normal canine heart.