Biomedical Engineering Reference
In-Depth Information
Fig. 6.8
(
Top
) Action potentials in epicardial (Epi), midmyocardial (Mid), and endocardial
(Endo) myocytes (from [
633
] with permission). (
Bottom
) Corresponding cell shortening.
6.5.4
Repolarization
Following depolarization and ventricular contraction, repolarization initiates cardiac
relaxation. Ventricular repolarization takes a longer time in the heart apex than base,
and near endocardium than near epicardium. Endocardial myocytes have smaller
rates of repolarization than epicardial myocytes, because of a lower density of
voltage-gated potassium channels (K
V
4; transient outward K
+
flux) in endocardial
and septal myocardia than in epicardial myocardium.
65
A space gradient in voltage-
gated K
+
channels is required for coordinated cardiac repolarization.
6.5.5
Electrophysiology and Contraction Kinetics
Differences in electrophysiological characteristics among cardiomyocytes are
associated with variations in shortening kinetics [
633
](Fig.
6.8
). Features of epi-,
endo-, and midmyocardial myocytes are summarized in Table
6.18
.
65
Loss in repolarization heterogeneity can lead to polymorphic ventricular tachycardia.
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