Circulation, Vol 84, 841-851, Copyright © 1991 by American Heart Association
DK Racker, PC Ursell and BF Hoffman
BACKGROUND. Little anatomic information is available on the annular
myocardium. This study was conducted to determine the anatomic substrate
for atrial flutter due to circus movement around the tricuspid annulus in
the Y-shaped incision canine model of atrial flutter. METHODS AND RESULTS.
We studied photographs of the annular myocardium serial histological
sections, made in either of three different planes, and compared these with
photographs of the intact and blocked gross heart specimens. We found that
the annulus is the most caudal region of the atrial wall. The epicardial
aspect of the annulus abuts the ventricular septum or the aortic root in
the medial region; in other regions, it is covered by the fat of the
coronary sulcus. Its endocardial aspect is delimited by the tricuspid
leaflets inferiorly and by the pectinate muscle bundles superiorly, except
in the medial region where the pectinate muscle bundles are absent. The
annular myocardium is bilaminated. A continuous subepicardial
circumferential lamina is the most prominent and is robust in the anterior,
lateral, and posterior regions, but it attenuates to a fine muscular
connection in the medial region. Myofibers of its superior border merge
with the pectinate muscle bundles or are admixed in the medial region with
myocardium at the base of the medial atrial wall. Its inferior border makes
little contact with the annulus fibrosus about the ring; however, in the
medial region, these myofibers insert into fibrous tissue superior to the
septal leaflet. A discontinuous, subendocardial perpendicular lamina
contains myofibers that descend from the atrium; most of these myofibers
insert into the annulus fibrosus about the ring, but the lamina is absent
in the anteromedial region. CONCLUSIONS. We conclude that the continuous
circumferential lamina provides the anatomic substrate for circus movement
of excitation in this model.
ARTICLES
Anatomy of the tricuspid annulus. Circumferential myofibers as the structural basis for atrial flutter in a canine model
Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York.
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