© 1995 American Heart Association, Inc.
Articles |
Optical Mapping Reveals That Repolarization Spreads Anisotropically and Is Guided by Fiber Orientation in Guinea Pig Hearts
From the University of Pittsburgh (Pa) School of Medicine, Department of Cell Biology and Physiology.
Correspondence to Dr Anthony J. Kanai, Department of Pharmacology, Box 3845, Duke University Medical Center, Durham, NC 27710.
Abstract Guinea pig hearts were stained with a
voltage-sensitive dye and imaged on a photodiode array to
record fluorescent action potentials (APs) from 124 sites.
Activation and repolarization patterns were recorded from the
epicardium during stimulation at different loci and correlated with the
underlying fiber architecture. Endocardial APs were recorded by
inserting a light guide into the ventricular cavity or by
dissecting out the ventricular free wall to expose the
endocardium. In hearts paced on the right atrium to simulate sinus
rhythm, activation emerged synchronously over a large area of the
ventricular epicardium and spread laterally in 5 to 7 ms.
The apparent longitudinal and transverse velocities were 2.66±0.11 and
1.65±0.09 m/s (n=12). In contrast, repolarization began near the apex
on the endocardium and spread transmurally in 6±1.3 ms (n=12) and then
anisotropically along the epicardium in 25 to 30 ms with apparent
maximum (0.53±0.11 m/s) and minimum (0.31±0.10 m/s) repolarization
velocities that aligned with the longitudinal and transverse axes of
epicardial fibers. When paced on the epicardium, activation of intact
hearts (n=12) and perfused sheets (n=8) was anisotropic, with
longitudinal (0.85±0.05 m/s) and transverse (0.44±0.04 m/s)
conduction velocities that aligned with the epicardial fiber
orientation. When activation was initiated at different sites on the
epicardium, repolarization always began near the apex and exhibited
patterns similar to those obtained under right atrial pacing, but with
slower longitudinal (0.41±0.09 m/s) and transverse (0.23±0.07 m/s)
repolarization velocities (n=18). In sheets stretched parallel to the
longitudinal axis of surface fibers, AP durations (APDs) increased as a
function of fiber length, from the length at zero developed tension to
120% of the length at maximum developed tension
(Lmax). Spatial distributions of APDs did not change
during stretches along the rising phase of the length-tension
curve. In sheets stretched to 50% of Lmax, APDs
were shorter and more homogeneous on the endocardium (mean
APD, 188 ms; 
APD, 195-186=9 ms) than on the epicardium (mean APD,
204 ms; APD, 212-186=26 ms; n=8). In guinea pig hearts, activation
is rapid; therefore, repolarization depends primarily on intrinsic
spatial heterogeneities of APDs. Consequently, repolarization begins at
endocardial cells with the shortest APDs and spreads transmurally and
then anisotropically on the surface according to the epicardial cell
orientation.
Key Words: myocardial fiber orientation • action potential duration • optical mapping • repolarization patterns • activation patterns