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Advances in Arrhythmia and Electrophysiology
Left Atrial Anatomy Revisited
Siew Yen Ho, PhD, FRCPath; José Angel Cabrera, MD; Damian Sanchez-Quintana, MD
R
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ecent decades have seen rapid developments in arrhythmia treatment, especially the use of catheter ablation.
Although the substrates of atrial fibrillation, its initiation and
maintenance, remain to be fully elucidated, catheter ablation
in the left atrium has become a therapeutic option for patients
with this arrhythmia. With ablation techniques, various isolation lines and focal targets are deployed; the majority of
these are anatomic approaches. It has been over a decade
since we published our first article on the anatomy of the left
atrium relevant to interventional electrophysiologists.1 Our
aim then, as now, was to increase awareness of anatomic
structures inside the left atrium. In this review of anatomy, we
revisit the left atrium, inside as well as outside, for a better
understanding of the atrial component parts and the spatial
relationships of specific structures.
thickness. The roof or superior wall is in close proximity to the
bifurcation of the pulmonary trunk and the right pulmonary
artery. The thickness of its muscle component measured transmurally ranges from 3.5 to 6.5 mm (mean, 4.5⫾0.6 mm) in
formalin-fixed heart specimens.4 The thickness of the lateral
wall ranges between 2.5 and 4.9 mm (mean, 3.9⫾0.7 mm). The
anterior wall, related to the aortic root, ranges from 1.5 to
4.8 mm (mean, 3.3⫾1.2 mm) thick, but it can become very thin
at the area near the vestibule (Figure 1B) of the mitral annulus,
diminishing to an average thickness of 2 mm. Importantly, there
is an area of the anterior wall just behind the aorta that is
exceptionally thin, an area noted by McAlpine2 as the “unprotected” area at risk of perforation. The posterior wall of the left
atrium, including its inferior part, is related to the esophagus and
its nerves (vagal nerves), the thoracic aorta, and the coronary
sinus (Figure 1C). Its mean thickness is 4.1⫾0.7 mm (range,
2.5–5.3 mm) which tends to become thinner toward the orifices
of the left and right pulmonary veins.1 Postmortem measurements of the midportions transmurally revealed the area between
the superior pulmonary veins to be the thinnest, 2.3⫾0.9 mm,
with no significant difference between patients with and without
atrial fibrillation, whereas the wall was thinner in the middle and
between the inferior venous orifices in those with atrial
fibrillation.5
The coronary sinus, with its continuation, the great cardiac
vein, runs along the epicardial aspect of the posteroinferior
region (Figure 2A). The venous structure is covered by fatty
tissues of the left atrioventricular groove, but in the majority
of hearts it is not at the same level as the mitral annulus.6
When viewed from the anterior aspect of the chest, the
coronary sinus runs superior to inferior, whereas when
viewed from a left and anterior perspective, the venous
channel runs from posterosuperior to anteroinferior. During
cardiac development, the oblique vein of the left atrium (vein
of Marshall) passes from a superior aspect onto the epicardial
surface of the left atrium in between the left atrial appendage
and the left superior pulmonary vein to descend along the
posterolateral atrial wall to join the coronary sinus (Figure
2B). This is at the junction of the sinus with the great cardiac
vein, which is usually marked by a set of flimsy valves known
as the valve of Vieussens (Figure 2C). In some individuals,
Location and Atrial Walls
Viewed from the frontal aspect of the chest, the left atrium is
the most posteriorly situated of the cardiac chambers. Owing
to the obliquity of the plane of the atrial septum and the
different levels of the orifices of the mitral and tricuspid
valves, the left atrial chamber is more posteriorly and superiorly situated relative to the right atrial chamber. The
pulmonary veins enter the posterior part of the left atrium
with the left veins located more superior than the right veins.
The transverse pericardial sinus lies anterior to the left atrium,
and in front of the sinus is the root of the aorta. The tracheal
bifurcation, the esophagus, and descending thoracic aorta are
immediately behind the pericardium overlying the posterior
wall of the left atrium. Further behind is the vertebral column.
Following the direction of blood flow, the atrial chamber
begins at the pulmonary veno-atrial junctions and terminates at
the fibro-fatty tissue plane that marks the atrioventricular junction at the mitral orifice. The walls of the left atrium are
muscular and can be described as superior, posterior, left lateral,
septal (or medial), and anterior, as suggested by McAlpine,2 who
drew attention to the importance of describing the heart in its
anatomic position in the chest, in the orientation he termed
“attitudinal,” which is the appropriate terminology for cardiac
interventionists.3 The left atrium is relatively smooth-walled on
its internal aspect (Figure 1A), but its walls are not uniform in
Received August 24, 2011; accepted October 17, 2011.
From the Cardiac Morphology Unit, Royal Brompton Hospital, London, United Kingdom (S.Y.H.); Hospital Universitario Quirón-Madrid, European
University of Madrid, Madrid, Spain (J.A.C.); and the Department of Anatomy and Cell Biology, Faculty of Medicine, University of Extremadura,
Badajoz, Spain (D.S.-Q.).
The online-only Data Supplement is available at http://circep.ahajournals.org/lookup/suppl/doi:10.1161/CIRCEP.111.962720/-/DC1.
Correspondence to Siew Yen Ho, PhD, FRCPath, Cardiac Morphology Unit, Children’s Services, Royal Brompton Hospital, Sydney St, London SW3
6NP, UK. E-mail [email protected]
(Circ Arrhythm Electrophysiol. 2012;5:220-228.)
© 2011 American Heart Association, Inc.
Circ Arrhythm Electrophysiol is available at http://circep.ahajournals.org
220
DOI: 10.1161/CIRCEP.111.962720
Ho et al
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Figure 1. A, Dissection through the atria with parts of the anterior walls removed and viewed from a left anterior perspective to
show the components of the left atrium and its relatively smooth
endocardial surface. B, This dissection of the atria viewed from
the back shows the close relationship of the aortic root to the
atria, including the atrial septum. C, This specimen viewed from
the left shows the posterior location of the left atrium and its
relationship to cardiac and extracardiac structures. Asterisk
marks the location of the coronary sinus. Eso indicates esophagus; LAA, left atrial appendage; LI, left inferior; LS, left superior;
PV, pulmonary vein; RI, right inferior; RS, right superior; RAA,
right atrial appendage; and Tr, trachea.
the lumen of the oblique vein remains patent, forming the
persistent left superior caval vein, draining into the coronary
sinus. In the majority of individuals, however, this vein
becomes a fibrous strand, the ligament of Marshall. Kim et al
demonstrated multiple myocardial “tracts” present within the
vein/ligament that insert directly into the coronary sinus
musculature or into the posterior free wall of the left atrium.7
The coronary sinus is wrapped by its own muscular coats to
varying extents from near the insertion of the Marshall
vein/ligament to its opening in the right atrium. The sleeve
covers a length of 25–52 mm and increases in thickness
toward the coronary sinus orifice.8 Muscular continuity between venous walls and the posterior and inferior left atrial
wall is common.1,8,9 The free wall of the coronary sinus,
however, is often thin and relatively unprotected.
Pulmonary Veins
Our anatomic study on a series of 35 heart specimens found the
classic arrangement of 4 orifices in 74%, with 31% of these in
the setting of a short vestibule or funnel-like common vein. Five
venous orifices were found in 17%, and the remaining 9% had
a common vein on the left or right side.10 In the classic pattern,
the right superior pulmonary vein passes behind the junction
between the right atrium and the superior caval vein, whereas the
inferior pulmonary vein passes behind the intercaval area (Fig-
Left Atrial Anatomy
221
Figure 2. A, Heart sectioned and viewed from simulated left
anterior perspective to show the course of the coronary
sinus(CS) relative to the left atrium(LA). The area of fibrous continuity between aortic and mitral valves lies between the open
triangles. B, This dissection shows the inferior aspect of the
heart with the epicardium of the left atrium and left ventricle
removed. The insertion of the ligament of Marshall (LoM)
marks the junction between great cardiac vein (gcv) and the
coronary sinus. Note the musculature around the CS and
LoM compared with the mainly bare wall (pale color) of the
gcv. C, This histological section in a similar plane to the heart
in A is stained in Masson trichrome stain, which displays
muscle as red and fibrous tissue as green. The CS has a
muscular sleeve and is separated from left atrial wall by a
narrow space (**) that may be traversed by myocardial
strands. ICV indicates inferior caval vein.
ure 3A). The orifices of the right pulmonary veins are directly
adjacent to the plane of the atrial septum. Improvements in
imaging techniques over the past decade have revealed considerably more variations in the number of venous orifices and their
arrangements.11,12
Sleeves of muscle extend from the left atrium to surround
the outer aspects of the venous walls for markedly varying
lengths toward the lung hilum (Figure 3A and 3B).13,14 These
sleeves are well illustrated in the publication by Keith and
Flack (1907) that documented their discovery of the sinus
node.15 Although electric activity in this region has long been
recognized,16,17 it is only in the recent era that muscular
sleeves have come under scrutiny among cardiac electrophysiologists, owing to their association with focal activity
initiating atrial arrhythmias.18,19 The muscle sleeves are
thickest at the veno-atrial junction (1–1.5 mm), and it then
fades away toward the lungs. Despite the term veno-atrial
junction, the structural border between vein and atrium is
indistinct because it has characteristics of both its neighbors
(Figure 3C). The endocardium of the left atrium continues
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February 2012
Figure 4. A, Left atrial appendage and its junction with the left
atrium are viewed from left and posterior perspective. B, This
view of the endocardial surface is transilluminated to demonstrate the thinness of the walls of the appendage and the atrium
wall in the vicinity of the os. LSPV indicates left superior pulmonary vein; RSPV, right superior pulmonary vein.
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Figure 3. A, The epicardium has been removed from this heart.
This view from the back shows the “classic” pattern of 4 pulmonary veins entering the left atrium and the relationship of the
right superior pulmonary vein (RS) to the posterior aspect of the
junction between the superior caval vein (SCV) and the right
atrium. The right inferior pulmonary vein (RI) passes behind the
intercaval area. Note the sleeves of atrial muscle continuing
around the pulmonary veins and the SCV. Bare venous walls
appear pale in color. B, This view from the left and posterior
perspective shows the proximity of the left superior pulmonary
vein (LS) to the left atrial appendage (LAA), and it has a longer
muscle sleeve than the left inferior pulmonary vein (LI). C,
Removal of the posterior wall of the left atrium shows the
entrances of the pulmonary veins without discrete veno-atrial
junctions. D, This histological section in Masson trichrome stain
shows the thicker atrial wall becoming thinner at the entrances
of the veins to form the muscular sleeves, which taper toward
the lungs. Note the interpulmonary “ridge” (arrow) and the epicardial fibro-fatty tissues (*) containing abundant nerve bundles.
ICV indicates inferior caval vein.
into the endothelium of the vein (Figure 3D). The media of
the pulmonary vein contains smooth muscle cells in a
meshwork of fibrous and elastic tissues, and the transition
from the venous media to the subendocardial region of the
left atrium is represented by a gradual decline of smooth
muscle cells. Myocardial sleeves lie external to the venous
media and internal to the epicardium/adventitia. Usually, a
thin zone of fibro-fatty, or loosely arranged fibrous tissue, is
sandwiched between the media-subendocardium junction and
the sleeve. At the veno-atrial junctions, there are abundant
autonomic nerve bundles and intrinsic ganglionated nerve
plexuses situated in epicardial fat pads.20,21
Our studies of the muscular sleeves have shown mainly
circularly arranged myocardial strands that are pierced with
interdigitating longitudinally and obliquely orientated bundles in the sleeves.13 As the myocardial sleeves thin out,
patchy fibrosis is commonly seen.
Structure of the Left Atrium
The left atrium has a distinctive appendage that is a fingerlike pouch extending from the main body of the atrium. The
main body then comprises of the pulmonary venous portion,
the septal portion, and the vestibule, which is the outlet part
of the atrial chamber surrounding the mitral orifice. Apart
from the appendage, which has a fairly well-defined opening
(the os to the appendage), the other component parts do not
have clear anatomic demarcations.
The Appendage
The left atrial appendage is considerably smaller than its counterpart on the right side. It tends to have a small, narrow, tubular
shape with several bends (Figure 4A). A study of postmortem
and explanted hearts revealed the atrial appendage from patients
with atrial fibrillation to have 3 times the volume of those in
sinus rhythm.22 These investigators also reported that the endocardial surface was smoother and associated with more extensive endocardial fibroelastosis in those with atrial fibrillation and
that these features could predispose to thrombus formation. The
tip of the appendage points anterior and cephalad in most cases
and overlaps the base of the pulmonary trunk, the left coronary
artery, or its anterior descending branch and the great cardiac
vein at varying levels.23 In others, the tip can be directed inferior,
posterior, or, infrequently, into the transverse pericardial sinus.
Externally, the appendage shows multiple crenellations, giving
wide variations in number and arrangement of lobes or
branches.24,25 Its endocardial aspect is lined with muscle bundles
of varied thicknesses arranged in whorl-like fashion. The wall of
the appendage is paper-thin between the muscle bundles. Unlike
the right atrium, the left atrium lacks a terminal crest. Instead, the
border between appendage and body of the left atrium is the
oval-shaped os, with a mean long diameter of 17.4⫾4 mm and
short diameter of 10.9⫾4.2 mm measured on heart specimens.23
In some hearts, the endocardial aspect of the atrial body around
the os can be associated with pits and troughs where the wall
becomes remarkably thin (Figure 4B).
Septal Component
The anterior part of the septal component is closely related to the
aortic root through the transverse pericardial sinus. Structurally,
this component is more complex than it appears at first glance.26
When viewed from the cavity of the left atrium, the septal
component merges into the roof, the posterior wall, and the
vestibule. There is little to distinguish the true atrial septum from
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Figure 5. A, This longitudinal cut through the left atrium and left
ventricle shows endocardial surface of the wall of the atrial
component indistinguishable from the anterior and posterior
walls other than for the crescent-like margin (open arrow). Note
the location of the coronary sinus relative to the inferior wall. B,
This longitudinal cut through the 4 cardiac chambers shows the
atrial septum in profile. The floor of the oval fossa (open arrow)
is the true septum. Asterisks mark the levels of attachments of
the tricuspid and mitral valves at the septum. The inferior
pyramidal space (small arrow) is covered by the vestibule of
the right atrium. C, This histological section taken through the
short axis of the heart shows the thin flap valve (open arrow)
and the muscular rim of the fossa (small arrows). Note the
uneven thickness of the left atrial wall. D, This view of the septal
component shows a patent foramen ovale (open arrow). Its
opening is behind the anterior wall of the left atrium and the
transverse pericardial sinus. CS indicates coronary sinus; ICV,
inferior caval vein; SCV, superior caval vein; LI, left inferior; LS,
left superior; PV, pulmonary vein; RI, right inferior; and RS, right
superior pulmonary veins.
its surroundings. A small crescent-like edge is usually the only
discernable margin of the flap valve of the oval fossa (or floor of
the fossa) that is developed from the embryonic septum primum
(Figure 5A). The rest of the flap valve blends into the paraseptal
atrial wall. Thus, the extensive fossa valve overlaps the embryonic septum secundum, which is represented by the raised
muscular rim (or limbus) that is situated on the right atrial side
of the septal component (Figure 5B). During fetal life, the fossa
valve allows blood to flow from the right atrium to the left
atrium through the oval foramen (ostium secundum). After birth,
the fossa valve completely adheres to the left atrial margin of the
rim in most hearts, sealing the fossa opening. A recent study that
examined the septal aspect in 94 heart specimens reported
finding a septal pouch in nearly 40%, which is due to inadequate
fusion of the overlapping septum primum to the septum secundum despite having obliterated the oval foramen.27 An inadequate fusion along the caudal part of the overlap at the foramen
results in a pouch on the left atrial side (see online-only Data
Supplement), whereas fusion limited to the cephalad part of the
overlap results in a pouch on the right atrial side. The pouches
Left Atrial Anatomy
223
may become sites for thrombus deposits.27 However, persistent
patency at the foramen, a patent foramen ovale (PFO) is present
in 10 –35% of the population.28 This is the residual free margin
of the fossa valve that is visible as the crescent-like edge on the
endocardial surface of the left atrium.
Appreciating that the muscular rim is an infolding of the atrial
wall is relevant to defining the site of the true septum that should
be the target area for the transseptal puncture.29 It is the thin floor
of the fossa, measuring approximately 1–3 mm in thickness in
normal hearts that allows direct access between the atrial
chambers. It comprises a bilaminar arrangement of myocytes
with variable amounts of fibrous tissue.30,31 The location and
size of the oval fossa varies from case to case, as does the profile
or prominence of the muscular rim.32 Furthermore, abnormalities of the thorax or of the cardiovascular system such as
kyophoscoliosis or marked left ventricular hypertrophy may
result in displacement of the fossa ovalis.33 Cases with patches,
occluder devices, aneurysmal valves of the oval fossa, and
thickly fibrosed septum that may be due to previous transseptal
procedures are particularly challenging to perforate. The aneurysmal fossa valve is detected as a saccular excursion of ⬎1 cm
away from the plane of the atrial septum. In these hearts, the
fossa membrane often is thinner, devoid of muscle cells, and
mainly composed of connective tissue, making it more resistant
to being perforated and yet more likely to be “tented” deep into
the left atrial cavity with the risk of reaching the lateral wall.
Approximately one-third of hearts with aneurysmal fossa are
associated with a PFO. With or without aneurysmal configuration, the PFO is situated at the antero-cepahalad margin of the
fossa. In heart specimens, the PFO ranges from 1 to 10 mm in
diameter, and the length of the tunnel depends on the extent of
overlap between the flap valve and the rim.34 The location, size,
and tunnel-like form of the PFO should be considered if it is to
be used as a portal for transseptal crossing because they can
affect ease of catheter movement and access to reach target areas
such as the right inferior pulmonary vein. Importantly, the
crescentic aperture of the PFO opens immediately behind the
anterior wall of the left atrium (Figure 5D).
Because the interatrial groove (also known as the Waterston
groove to the cardiac surgeons) marks the epicardial aspect of
the infolded muscle rim, it is not a true septal structure. It can
become quite thick, especially in its superior, posterior, and
inferior margins. In some patients, the epicardial fat may
increase the thickness of the infolding to 1–2 cm in the normal
heart. Thickness of more than 2 cm on noninvasive imaging is
increasingly reported as indicative of lipomatous hypertrophy,
with incidence up to 8%. On cross-sectional echocardiography,
the septum appears like a “dumbbell.”35 Even in hearts without
so-called “septal hypertrophy,” transgression through the rim
can hinder needle penetration, and, being a thicker structure, can
restrict maneuverability after crossing and also increase the risk
of exiting the heart, dissecting into the fatty tissue plane, and
causing hemopericardium. At particular risk is the anterior rim
of the fossa, which is in close anatomic relationship with the
aortic mound. The latter is seen as a protuberance into the right
atrial cavity. Puncture in this area is likely to allow the needle to
enter the transverse pericardial sinus, resulting in a high risk of
aortic perforation. Notably, it is not uncommon to find small pits
and crevices in this region that may lodge the tip of a perforating
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February 2012
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Figure 6. A, This is a view of the roof and posterior wall of the
left atrium with transillumination to demonstrate the thinner
parts of the walls. The epicardium has been removed to show
the arrangement of the myocardial strands (broken arrows) in
the superficial parts of the walls. B, This view of the posterior
and inferior walls shows abrupt changes in orientation of the
myocardial strands (broken arrows). An interatrial muscle bundle is present in this heart (double-headed arrow). C, This view
of the left side shows the myocardial strands in the region
between the left superior (LS) and left inferior (LI) pulmonary
veins (arrow). D, Muscle bridges (arrows) between the superior
and inferior pulmonary veins connect obliquely or directly
superior-inferiorly.
device and give the false impression of having “tented” the
septum. Anteroinferiorly, the rim and its continuation into the
atrial vestibules on both sides of the septal component overlie
the myocardial masses of the ventricles from which they are
separated by the fat-filled inferior pyramidal space, through
which passes the artery supplying the atrioventricular node
(Figure 5B).36 On the right atrial side, the anteroinferior rim
is continuous with the Eustachian ridge (sinus septum) and
the vestibule leading to the tricuspid valve. Owing to the
more apical attachment of the mitral compared with the
tricuspid valves at the septum, the vestibule of the right
atrium overlies the crest of the muscular ventricular septum.
Consequently, the compact atrioventricular node sited on the slope
of the crest is within a millimeter or so of the endocardial surface of
the right atrium at the apex of the triangle of Koch. The inferior
part of the compact node bifurcates into nodal extensions
leftward and rightward toward the mitral and tricuspid annuli,
respectively.37
Pulmonary Venous Component and Left
Lateral Ridge
The posterior part of the left atrium receiving the pulmonary
veins is its venous component. Mainly, the wall is smooth on the
endocardial surface, without clear distinction between venous
and atrial walls, especially where the terminal parts of the veins
are funnel-shaped. As mentioned previously, the posterior left
atrial wall is not uniform in thickness (Figure 6A). The area
between the ipsilateral pulmonary venous orifices has a very
scanty content of fibro-fatty tissue. Transmurally, the musculature of the wall shows changes in orientation of the myocardial
strands.1,26 The subendocardial strands at the veno-atrial junctions are mainly loop-like extensions of the longitudinal and
oblique strands from the atrial wall. Abrupt changes in orientation of the strands in the subendocardium may be observed in the
interpulmonary areas, where obliquely or circumferentially
aligned myocardial strands meet with predominantly longitudinally aligned strands (Figure 6A and 6B). Usually the area of
abrupt change is accompanied by a change in wall thickness,
thicker toward the septum and thinner laterally.
Viewed from within the atrial cavity, the endocardial surface
has the appearance of ridges in between the superior and inferior
venous orifices. In addition, there is a ridge-like structure
between the entrance of the left superior pulmonary vein and the
os of the left atrial appendage. The interpulmonary carinas (or
isthmuses) may have transmural myocardial thicknesses up to
3.2 mm. Intervenous muscular connections are commonly located toward the epicardial surface (Figure 6C).38 These cross
from the anterior quadrant of the superior venous sleeve to the
posterior quadrant of the inferior venous sleeve and vice versa
(Figure 6D). Direct connections between superior and inferior
venous sleeves are also common.
The left lateral “ridge” between the appendage and the left
pulmonary veins was first described by Keith in 1907 as the
“left tenia terminalis.”39 It is recognized as a Q-tip sign on
echocardiographic imaging and, when prominent, can be
mistaken for a thrombus or atrial mass. Integral to the
ablation line for isolating left pulmonary veins, this “ridge” is
actually an infolding of the lateral atrial wall. The fold is
narrower at its superior border with the left superior pulmonary vein compared with its border with the inferior pulmonary vein (2.2– 6.3 mm versus 6.2–12.3 mm). In 75% of the
hearts studied, the fold is less than 5 mm wide and its profile
of the fold varies from being flat, round, or pointed (Figure
7A and 7B). The fold has thicker muscle in the anterosuperior
portion. Within the fold runs the remnant of the vein of
Marshall, abundant autonomic nerve bundles, and a small
atrial artery which, in some cases, is the sinus nodal artery
(Figure 7C).7,40 A study on postmortem hearts revealed that
the Marshall remnant is less than 3 mm from the endocardial
surface and that the fold has muscular connections to the
pulmonary veins.40
The Vestibule and the Mitral Isthmus
Surrounding the outlet of the atrium is the vestibule. On the
endocardial aspect, it is a smooth zone. The myocardium of its
distal parts overlaps the atrial surfaces of the mitral leaflets to a
greater or lesser extent, albeit only for a millimeter or so. Owing
to the left ventricular outflow tract interposing between the
ventricular septum and the mitral valve, the portion of the
vestibule overlying the area of normal fibrous continuity between the mitral and aortic valves is distant from ventricular
myocardium (Figure 2A). Although the remaining part is close
to the ventricular myocardium, fibro-fatty tissues at the mitral
hinge line (annulus) serve as an insulating plane between atrial
and ventricular masses. For normal conduction of the cardiac
impulse from atria to ventricles, it is well recognized that there
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Figure 7. A and B, Endocardial perspectives viewing the os of
the left atrial appendage and the orifices of the left pulmonary
veins to show examples of the variations in topography of the
left atrial “ridge” (white arrows). C, This section in similar orientation shows a rounded profile of the fold that forms the “ridge”
(open arrow). There is a small artery (arrow) in the fold. LC
indicates left common; LI, left inferior; and LS, left superior pulmonary veins.
Left Atrial Anatomy
225
Figure 8. A, This cut shows in profile the mitral isthmus
between the mitral annulus and the orifice of the left inferior pulmonary vein (LI). B, This corresponding histological section
shows the irregular thickness of the atrial wall and the relationship to the great cardiac vein.
continues into the coronary sinus. The circumflex artery is also
in the vicinity, and in some hearts its wall is in contact with atrial
myocardium (Figure 8A).
Interatrial Muscular Connections
is only one pathway of muscular continuity, and this is through
the specialized myocytes of the atrioventricular conduction
system originally described by Tawara in 1906.41 From the
perspective of the left atrium, the atrioventricular node and
bundle are related to the vestibular portion that overlies the right
fibrous trigonal area of aortic-mitral fibrous continuity. An
approximate landmark could be the posteromedial commissure
of the mitral valve.
Although the distal border of the vestibule on the endocardial
aspect could be taken as the mitral valve, the proximal border is
unclear, especially in the anterior, septal, and inferior portions.
The quadrant relating to the left atrial appendage, however,
usually has series of pits and troughs in the atrial wall, in stark
contrast to the smooth vestibular wall. The mitral isthmus
extends between the orifice of the left inferior pulmonary vein
and the mitral hinge line (Figure 8). Thus, when traced from the
venous orifice, the isthmus traverses the atrium’s posteroinferior
wall, including the corresponding quadrant of the vestibule.42,43
The length is 35⫾7 mm when measured in a series of normal
heart specimens without history of atrial fibrillation.42 The
average of the maximal myocardial thickness of the wall is
3.8⫾0.9 mm in the midportion; the thickness tapers at either
end. From the annulus, the first 5 mm of the vestibular part is
thin, with a myocardial depth of 1.5⫾0.7 mm. Additionally,
where the endocardial surface of the isthmus contains pits and
troughs, the atrial wall becomes exceptionally thin. Some pits
are “foramina Lannelongue,” draining small cardiac veins.
Passing along the epicardial side is the great cardiac vein that
Apart from muscular continuity at the rim and floor of the
oval fossa, there are multiple muscular bridges between the
atrial chambers.1 Composed of ordinary atrial myocardium,
these bridges are of varying widths, thicknesses, and proximity to the musculature of the true atrial septum.26,31 Muscular bridges between veins and atrial walls are not
uncommon.
The prevalent interatrial conduction pathway for propagation
of the sinus impulse to the anterior left atrial wall is through the
Bachmann bundle, which is also known as the interauricular
band (Figure 9A, 9B, and 9C). Indeed, it is the most prominent
muscular interatrial bridge in majority of hearts.44,45,46 It is
superficially located, composed of nearly parallel alignment of
myocardial strands that blend into the musculature of the atrial
walls. It branches in its rightward and leftward extensions to
embrace the atrial appendages. Myocardial strands from its
superior rightward arm can be traced toward the location of the
sinus node, the terminal crest, and sagittal bundle. Leftward, the
Bachmann bundle runs along the anterior left atrial wall, where
it merges with the superficial and circumferentially arranged
myocardial strands of the subepicardium, adding to the wall
thickness. After passing around the neck of the left atrial
appendage, the arms rejoin to continue into the musculature of
the lateral and posteroinferior atrial walls.46 The only place
where the Bachmann bundle appears as quite a distinct bundle,
separated by fatty tissues from atrial wall, is at the anterior
interatrial groove (Figure 9B and 9C). A small series studied by
Becker revealed more pronounced fibro-fatty replacement of the
226
Circ Arrhythm Electrophysiol
February 2012
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Figure 9. A, This dissection viewed from the front displays the
Bachmann bundle and its bifurcating branches leftward and
rightward (broken arrows). The dotted shape marks the site of
the sinus node. B, This view from above shows the Bachmann
bundle crossing the interatrial groove as a distinct bundle. C,
This histological section in comparable display to B shows the
Bachmann bundle and its rightward extensions toward the sinus
node (dotted area). D, This dissection of the posterior and inferior parts of the interatrial groove shows multiple muscle bridges
(arrows) connecting the 2 atria. LAA indicates left atrial appendage; ICV, inferior caval vein; SCV, superior caval vein; LI, left
inferior; LS, left superior; PV, pulmonary vein; RI, right inferior;
and RS, right superior pulmonary veins.
myocardium in the terminal crest and in the Bachmann bundle in
hearts from patients with atrial fibrillation than without atrial
fibrillation.47 Conceivably, with more extensive acquired
changes in the Bachmann bundle, other interatrial bridges could
play an increasing role. In some hearts, the Bachmann bundle
may coexist with broad muscular bridges across the posteroinferior
interatrial groove, joining left atrial musculature to the intercaval
area of the right atrium and to the insertion of the inferior caval vein
(Figure 9C).26,44,46 These may provide the potential for inferior
breakthrough of the sinus impulse instead of the anticipated
anterior breakthrough.48
Muscle bridges are observed in some hearts connecting the
muscular sleeves of the right pulmonary veins to the right
atrium, the superior caval vein to the left atrium, and the
coronary sinus to the left atrium. In particular, connections
between the muscular wall of the coronary sinus and the
remnant of the vein of Marshall to the left atrium are
common.1,8,9
The Neighborhood
Parts of the left atrial body, the entrances of the pulmonary veins,
the atrial appendage, and the coronary sinus are in close vicinity
to important structures that may be affected by interventional
maneuvers that are carried out on and within the left atrium. The
epicardial aspect of the left atrium, particularly in the region of
the veno-atrial junctions, the interatrial groove, the roof, and the
tract of the coronary sinus along the inferior wall, is covered by
pads of fatty tissues. Preganglionic parasympathetic and postganglionic sympathetic fibers come together into the fat pads,
and ganglionated plexuses populate the subepicardium.21,49,50
Outside, the left atrium is covered by the fibrous pericardium,
which has a double-layered inner lining, the serous pericardium,
which encloses the pericardial cavity. One layer is fused to the
fibrous pericardium and the other layer lines the outer surface of
the heart as the epicardium and as the visceral pericardium that
ensheaths the veins and great vessels. The aorta and pulmonary
trunk are ensheathed together, whereas the veins are enclosed in
another sheath. The junctions between the two layers form the
pericardial reflections, one of which is the transverse pericardial
sinus located behind the great arteries and in front of the atria.
Another, the oblique sinus, lies behind the posterior wall of the
left atrium and is a large area of continuity between reflections
along the pulmonary and caval veins. However, local variations
of reflections around individual pulmonary veins can affect
access for complete pulmonary venous isolation from the pericardial space.51
The spatial relationship of the anterior atrial wall and plane of
the atrial septum to the root of the aorta is relevant in transseptal
procedures as well as device closure of PFO and septal defects.
The roof of the atrium is related to the bifurcation of the
pulmonary arteries and the left bronchus (Figure 1C). In cases
with dilated left atrial chamber, the pulmonary bifurcation may
be shifted cephalad to become closer to the underside of the
aortic arch, which in turn may compress the recurrent laryngeal
nerve. Pai et al reported a case of transient vocal cord paralysis
thought to have been caused by a stiff electrode catheter pushed
against the atrial roof during ablation.52 The atrial appendage has
important relationships with cardiac and extracardiac structures
that are relevant to ablating within its lumen or closing its
opening. The os is situated above the left atrioventricular groove,
which contains the circumflex artery and the great cardiac vein
together with their branches.23 Our study on cadavers found the
course of the left phrenic nerve and its accompanying pericardiophrenic vessels in the fibrous pericardium that was overlying
the atrial appendage in majority of cases.53
The esophagus, though useful as a portal for echocardiographic imaging, is at risk of damage during left atrial
ablation procedures. Understanding the course of the esophagus is essential to reduce the risk of atrio-esophageal fistula
after catheter ablation on the posterior wall. The esophagus
descends slightly to the left between the trachea and the
vertebral column and continues its descent behind the fibrous
pericardium that overlies the posterior wall of the left atrium,
to the right of the aortic arch, and the right side of the
descending thoracic aorta (Figure 10A and 10B). In many
patients, the descending thoracic aorta also runs in the
vicinity of the posterior wall and in some cases may be
apposing the fibrous pericardium.54
In a study of cadavers, we found the esophagus in virtual
contact with the posterior wall over a distance of 30 –53 mm
(mean, 42⫾7 mm).4 Its course may be more toward the right or
the left pulmonary venous orifices or more centrally. Furthermore, the esophagus curves to lie beneath the posteroinferior
wall as it descends toward the diaphragm and may be only a
couple of millimeters away from the pulmonary venous orifices
and practically in contact with the coronary sinus.55
Ho et al
Left Atrial Anatomy
227
close proximity to the posterolateral and lateral aspects of the
superior caval vein and anterior to the superior and inferior right
pulmonary vein. It is particularly close to the superior pulmonary
vein, with a mean minimal distance of 2.1⫹0.4 mm in a study
made on cadavers.58 The same study found that the distance was
⬍2 mm in a third of the cadavers, suggesting that it could be at
risk of damage during right pulmonary vein isolation. The left
phrenic nerve takes an anterior (18%), lateral (59%), or posteroinferior (23%) course on the fibrous pericardium overlying the
left heart.53 The lateral course passes over the tip of the left atrial
appendage, whereas the posteroinferior course passes over the
roof of the appendage os.
Conclusions
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The left atrium has a distinctive atrial appendage and an atrial
body that comprises component parts that blend into one
another. The patterns of general myocardial arrangement in
the left atrial wall and the presence of interatrial muscle
bundles may provide some anatomic background to atrial and
interatrial conduction. Understanding the structure of the
component parts and their relationship to one another and to
other cardiac structures is relevant to interventional procedures inside and outside of the left atrium.
Disclosures
Figure 10. A and B, Specimens viewed from the tilted right
superior and posterior perspectives, respectively, to show the
courses of the esophagus (eso) and descending aorta relative to
the left atrium (LA). C and D, Right and left views, respectively,
show the courses of the phrenic nerves. RB indicates right
bronchus; RPA, right pulmonary artery; SCV, superior caval
vein; RI, right inferior; RM, right middle; and RS, right superior
pulmonary veins.
The gap between the fibrous pericardium and the anterior
aspect of the esophagus is filled with fibro-fatty tissues
containing lymph nodes as well as esophageal arteries and the
periesophageal plexus from branches of the Vagus nerve.56
The Vagus nerves pass from behind the root of the lungs to
form the right and left posterior pulmonary plexuses. Two
branches from the left pulmonary plexus then pass caudally to
descend on the anterior surface of the esophagus joining with
a branch from the right pulmonary plexus to form the anterior
esophageal plexus that pass in close proximity to the left and
right pulmonary veno-atrial junctions. The esophageal plexuses extend through the diaphragm to become the posterior
and anterior vagal trunks that innervate the pyloric sphincter
and gastric antrum. We found the distance between the
endocardial surface of the left atrium and the esophageal wall
to be ⬍5 mm in 40% of the 15 cadavers. Thus, not only the
esophagus and its arterial supply but also the nerve plexuses
and lymph nodes may be put at risk when ablating the
posterior atrial wall.4,56 Injury to the periesophageal vagal
nerves can result in acute pyloric spasm and gastric
hypomotility.57
Descending bilaterally along the surface of the fibrous pericardium that envelopes the heart, the phrenic nerves and their
accompanying pericardiophrenic artery and vein are in the
neighborhood of the left atrial appendage and right pulmonary
veins (Figure 10C and 10D). The right phrenic nerve courses in
None.
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KEY WORDS: ablation
䡲 fibrillation
䡲
atrioventricular node
䡲
atrium
䡲
conduction
Left Atrial Anatomy Revisited
Siew Yen Ho, José Angel Cabrera and Damian Sanchez-Quintana
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Circ Arrhythm Electrophysiol. 2012;5:220-228
doi: 10.1161/CIRCEP.111.962720
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SUPPLEMENTARY MATERIAL
Fossa
Left Atrium
This view of the septal aspect of the left atrium shows the valve of the oval fossa
transilluminated. There was complete adhesion of the valve to the muscular rim on the
right atrial aspect. Nevertheless, a probe (green rod) can be inserted behind the
crescentic free margin of the fossa into a pouch-like space up to the level of the black
arrow but it was not possible to enter the right atrium.