Affiliation:
1. the Departments of Pharmacology and Pediatrics, College of Physicians and Surgeons, Columbia University, New York, NY.
Abstract
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 V̇
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.
Publisher
Ovid Technologies (Wolters Kluwer Health)
Subject
Physiology (medical),Cardiology and Cardiovascular Medicine
Reference32 articles.
1. Distribution of M Cells in the Canine Ventricle
2. Antzelevitch C Sicouri S Lukas A Nesterenko VV Liu D-W DiDiego JM. Regional differences in the electrophysiology of ventricular cells: physiological and clinical implications. In: Zipes DP Jalife J eds. Cardiac Electrophysiology: From Cell to Bedside . 2nd ed. New York NY; WB Saunders Co; 1994:228-245.
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