www.kardiologiapolska.pl
Kardiologia Polska 2018; 76, 3: 499–509; DOI: 10.5603/KP.a2017.0248 ISSN 0022–9032
ARTYKUŁ SPECJALNY / STATE-OF-THE-ART REVIEW
Address for correspondence:
Wiesława Klimek-Piotrowska, MD, PhD, Department of Anatomy, Jagiellonian University, Medical College, ul. Kopernika 12, 31–034 Kraków, Poland,
e-mail: heart@cm-uj.krakow.pl
Received: 14.11.2017 Accepted: 15.11.2017 Available as AoP: 08.12.2017
Kardiologia Polska Copyright © Polskie Towarzystwo Kardiologiczne 2018
Clinical anatomy of human heart atria and
interatrial septum — anatomical basis for
interventional cardiologists and electrocardiologists.
Part 1: right atrium and interatrial septum
Iwona Kucybała, Katarzyna Ciuk, Wiesława Klimek-Piotrowska
Department of Anatomy, Jagiellonian University, Medical College, Krakow, Poland
INTRODUCTION
Interventional cardiology together with interventional elec-
trocardiology are nowadays one of the fastest developing
branches of medicine and latterly, indications for transcatheter
interventions have been extended to more and more folded
cases. In recent decades, rapid progression in treatment of
various types of atrial arrhythmias, particularly atrial brillation
and atrial utter, has been observed. Ablation within the cavo-
tricuspid and other parts of the right atrium, as well as cardiac
resynchronisation therapy, has become a standard approach.
The right atrium (RA) and the interatrial septum are not only
the direct targets of various interventions but also enable ac-
cess to left heart chambers. The RA consists of many unique
anatomical structures whose presence and morphology not
only may trigger the abnormal electric activity of the heart, but
also hinder the course of procedures. Precise understanding of
heart anatomy and the most frequently observed anatomical
variants of atrial structures seems to be crucial for achieving
satisfying results, and minimising or avoiding complications
during interventional procedures.
This comprehensive summary presents in a thorough
but uncomplicated way a detailed macroscopic morphology
of RA and interatrial septum. It also provides the anatomical
background for the most common atrial arrhythmias and
invasive cardiological procedures.
RIGHT ATRIUM
Overview
The RA of the human heart is located behind the right ventri-
cle in a rightward direction. It consists of the following parts,
namely a venous component, an appendage, and a vestibule.
The main body of the RA has an irregular ellipsoid shape with
a triangular protrusion of the right atrial appendage from the
anterolateral part. All those parts have disparate embryologic
origin [1]. The interatrial septum is the part common for both
atria, and its detailed anatomy and origin will be described
in the next chapter.
Estimated diameters of the RA differ depending on the
method of measurement. Using transcatheter echocardiog-
raphy, the established normal values are: 3.4–5.3 cm (long
axis) and 2.6–4.4 cm (short axis) in a four-chamber view at the
end-systolic phase of the cardiac cycle along with 4.9–6.1 cm
(long axis) and 4.2–5.3 cm (short axis) in a four-chamber view
in the phase of maximal size of the atrium in cardiac magnetic
resonance imaging. Nonetheless, currently there is a lack of
clearly established standards regarding quantitative evaluation
of the size of the RA, since its precise measurements are not
particularly relevant to clinical practice [2].
Superior vena cava
The superior vena cava (SVC) drains into the RA on its
superior wall, and its orice is localised in the venous com-
ponent of the atrium [3]. The mean diameter of the SVC
orice oscillates between 20.1 ± 3.2 mm in the mediolat-
eral dimension and 19.2 ± 3.1 mm in the anteroposterior
dimension. Previous research provided evidence that the
orice of the SVC is always deprived of any anatomical
obstacles [4]. Hence, the SVC should be considered as the
preferred access way during catheterisation of the RA in
the majority of cases.
Terminal crest
The bromuscular junction between the appendage and
the venous component of the RA is externally marked by
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Iwona Kucybała et al.
500
a terminal groove extending vertically between the superior
and inferior caval veins. This structure corresponds to the
endocardially marked muscular band known as the terminal
crest [5]. The terminal crest stretches from the anteromedial
wall of the RA, then it passes parallel to the anterior border
of the SVC orice, and then it curves in a posterolateral
direction (Fig. 1). Close to the inferior vena cava (IVC) it
bends anteriorly to get past the right border of its orice.
Subsequently, it ends its track in the region of the cavotricus-
pid isthmus [6–9]. The mean length of the terminal crest is
51.0 ± 9.0 mm and its thickness at the level of SVC is about
5.5 mm [10]. Occasionally, the terminal crest can be promi-
nent, thus mimicking a right atrial mass like a pseudo mass,
tumour, thrombus, or vegetation. The proximal and intercaval
course of the terminal crest is quite universal in all hearts. The
pattern of the ramication of the distal part of the terminal
crest into the lower part of the RA varies signicantly among
the population, contrary to its initial and central part. Lastly,
a ten-type classication of nal ramications was created.
The main pattern represents one thick bundle of the distal
crest, terminating in the vestibule of the tricuspid valve (type A
— 25.7%), followed by many thinner bundles radiating from
the distal crest beyond the cavotricuspid isthmus (type B
— 15.7%), or obliquely in the cavotricuspid isthmus area
in a fan-like fashion (type C — 14.3%) (Fig. 2) [9]. In about
one-fth of hearts, a second crest may be noticed, which is
the pectinate muscle located medially to the terminal crest,
and it ends in a discrete ridge [11].
The terminal crest plays an important role in typical atrial
utter. It provides a barrier to conduction transversely across
it. The transverse conduction block by the terminal crest is
more likely in thick bundles. On the other hand, fast conduc-
tion velocities can be observed in the longitudinal direction
of the terminal crest [12]. About two-thirds of focal right atrial
tachycardia, seen in the absence of structural heart disease,
arise along the terminal crest. Ablation targeting the terminal
crest has also been used in patients with inappropriate sinus
tachycardia [5, 8]. Final ramications of the crest seem to play
a role in the propagation of impulses, and may have an impact
on the success rate of cavotricuspid isthmus ablation. Conduc-
tion in the cavotricuspid isthmus courses preferentially along
thicker bundles that could become targets for ablation where
the punctual ablation of distal terminal crest ramication may
result in the interruption of pathological conduction [9].
Pectinate muscles and taenia sagittalis
Pectinate muscles emerge from the terminal crest, then they
extend anterolaterally on the walls of the whole right atrial
appendage towards the vestibule of the RA [3]. They may
present huge diversity of anatomical variants, and they can be
categorised according to the classication shown in Table 1 [8].
The largest and the most prominent pectinate muscle is named
taenia sagittalis (Fig. 1). In 55–65% of cases, single taenia sagit-
talis is present, while in 20–25% of cases more than one could
be detected. In 15–20% of specimens, taenia sagittalis was
absent [7, 8]. A mean length of taenia sagittalis is 12.0 mm,
while its mean thickness is 0.4 mm [7].
As for the electrophysiological implications of the mor-
phology of the appendage and its pectinate muscles, the
presence of prominent trabeculations may lead to unbalanced
propagation of excitatory impulses. It may be the predisposing
factor to the initiation of re-entry and the prolongation of life
spans of its waves, which results in severe atrial arrhythmias
that may even occur in hearts with intact myocardium. More-
over, this type of pectinate muscle morphology has additional
consequences for the treatment of atrial arrhythmias. During
radiofrequency catheter ablation, the tip of the catheter
Figure 1. Lateral view of the right atrium, the right atrial
appendage was dissected. Terminal crest (TC) with three
taenia sagittalis (*) is marked
Table 1. Classication of the morphology of pectinate muscles.
Adapted from [8]
Type Prevalence Description
1 40–45% Orientation perpendicular to the terminal
crest with equal spacing
2 15–18% Orientation parallel to the terminal
crest with equal spacing
3 10–11% Combination of type 1 and 2,
more than one common muscular trunk
4 9–10% Branching of pectinate muscles
5 9–20% Chaotic orientation, intermingling
trabeculations
6 5–8% Prominent muscular columns
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Right atrium and interatrial septum clinical anatomy
501
may get stuck under the muscular columns and result in
the perforation of the atrial wall, which may be fatal. Highly
trabeculated appendages may also predispose to thrombus
formation during the ablation procedure [7, 8].
Sinus node
The sinus node, the crescent-shaped natural pacemaker of the
heart, is located in the proximal part of the terminal groove
anterolaterally to the superior cavoatrial junction [13]. It is
difcult to dissect the sinus node macroscopically. There is
some evidence that an extensive “paranodal area” (rather
than a well demarcated sinus node) exists in humans. The
node is a more diffuse, elaborate structure, usually extending
down the inferolateral aspect of the crista terminalis in a cigar
shape [13–15]. The sinoatrial node artery is most commonly
reported as a single vessel, originating from the right coronary
artery (68.0%), and taking a retrocaval course to reach the
sinus node [16]. Multiple anatomical features of the sinoatrial
node are responsible for difculties of its ablation (as may be
required in patients with inappropriate sinus tachycardia).
To name just a few, extensive location, close proximity of the
thick crista terminalis, and the cooling effects of the nodal
artery are the main obstacles [14].
Inferior vena cava, Eustachian valve,
and Eustachian ridge
The orice of the IVC is localised in the postero-inferior region
of the venous part of the RA [17]. The mean dimensions of
its orice are: in the mediolateral direction 24.1 ± 5.7 mm
and in the anterolateral direction 24.2 ± 5.5 mm. The mean
area of the IVC orice equals 4.8 ± 2.0 cm
2
[4]. In 1.8% of
hearts, strands in the IVC orice may be visible [4]. Muscular
extensions from the atrial musculature, which encompass
proximal IVC, are typically not present [18].
The Eustachian valve arises from the anterior border of
the IVC orice and typically presents as a crescentic fold of
endocardium (Fig. 3A) [4]. Its embryologic origin is derived
from the combination of the superior portion of the right sinus
valve and the sinus septum [19]. However, in approximately
30% of cases, the valve is absent. The mean height of the
Eustachian valve is about 5 mm, while the mean percent-
age coverage of the IVC orice is 22.9 ± 14.6% [4, 9]. Even
though in about 1.8% of cases, a prominent valve covering the
whole opening of the IVC may be present, it hinders rather
than enables the catheterisation of the RA and coronary sinus
ostium (CSO), by directing the catheter towards the superior
part of the atrium. Rarely, a giant Eustachian valve can cause
obstruction of the IVC or the formation of a thrombus, and it
may be an obstacle during transcatheter occlusion of patent
foramen ovale. In order to minimise the risk of unforeseen
complications connected with the presence of the Eustachian
valve, transoesophageal or intracardiac echocardiography
should be performed before the procedure. If an extensive
valve is present, the use of SVC access is advised. Alternatively,
the valve can be partially ablated or punctured by a needle [4].
The Eustachian ridge, which is always present, is an
extension of the insertion of the Eustachian valve, which
usually continues its track towards the central brous body,
below and anteriorly to the fossa ovalis, but above the CSO
(Fig. 3A, red arrows). The mean length of the Eustachian
ridge is 25.5 ± 4.1 mm and the mean thickness of the ridge
is 3.6 ± 1.9 mm. The thickness of the Eustachian ridge is
inversely correlated with the diameter of the IVC orice. Ex-
cessive thickening of the Eustachian ridge is quite common,
with 47.9% of hearts presenting a prominent Eustachian ridge.
In 9.2% of all hearts, a thick bundle of the distal terminal crest
enters the Eustachian ridge [9]. Signicant thickening of this
structure has important implications not only for the electro-
physiology of the RA, but also for endovascular procedures
involving cannulation of the coronary sinus. An enlarged Eus-
tachian ridge may create a line of xed conduction block dur-
ing typical atrial utter. In such cases, the block of paraseptal
isthmus can be achieved only after complete ablation of the
excessively pronounced ridge [20]. Moreover, the prominent
Eustachian ridge may form an obstacle for a catheter heading
towards the CSO and cavotricuspid isthmus region, from both
IVC and SVC accesses [4].
The Chiari network
The Chiari network is a net-like structure located in close
proximity to the IVC orice and CSO [21]. It is an embryologic
remnant of incomplete resorption of the right sinus valve. It is
visible in approximately 4.6% of hearts (Fig. 3B). Smaller holes
in its structure may prevent the catheter from passing further
into the atrium, while its larger compartments might restrict the
range of movements of the catheter [4]. The Chiari network
can easily be imaged using transthoracic echocardiography
(as a highly mobile, highly reective echo structure) [22].
Coronary sinus ostium and Thebesian valve
The coronary sinus drains into the RA postero-medially into
its venous part, between the IVC and the right atrioventricular
(AV) ostium [11, 23]. The mean dimension of the longest di-
ameter of its oval-shaped ostium uctuates around 9–15 mm
[23, 24]. The excessive diameter of the CSO is considered
as a risk factor of the AV nodal re-entrant tachycardia [25].
Clinically, the CSO is utilised as a passage to the left atrial and
left ventricular epicardium during cardiac resynchronisation
therapy, catheter ablation of cardiac arrhythmias, debrilla-
tion, perfusion therapy, mitral valve annuloplasty, targeted
drug delivery, or retrograde cardioplegia administration [23,
26]. Three elements of the CSO are related to its successful
cannulation: the size of the ostium, its barriers into the RA, and
the presence of a valve that may cover the ostium [4, 23, 27].
The CSO is usually covered by the Thebesian valve, which
is present in 62–85% of hearts [11, 23, 28]. The valve is the
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Iwona Kucybała et al.
502
embryologic remnant of the inferior portion of the right sinus
valve and usually has the form of an endocardial fold attached
to the right border of the CSO, inferiorly to the Eustachian
ridge [17, 19, 23]. The mean coverage of the CSO by the
Thebesian valve is 48.3 ± 36.6% [4]. The size of the CSO is
closely tied to the presence and size of the Thebesian valve.
Figure 3. A. Inferior vena cava orice (IVCO) with the Eustachian valve (EusV) present; B. Right atrium view with the Chiari’s
network; CSO — coronary sinus ostium
A
B
Figure 2. Schematic views of the nal ramications of the distal terminal crest muscle bres (orange) into the lower part of the
right atrium; CTI — cavotricuspid isthmus; IVC — inferior vena cava. Adapted from [9]
C
D
A
B
E
H
I
F
G
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Right atrium and interatrial septum clinical anatomy
503
The CSO diameter is signicantly larger when the Thebesian
valve is absent, compared to those accompanied by the
presence of the valve. Additionally, there exists an inverse
correlation between the size of the CSO and the height of the
Thebesian valve. This phenomenon could be the consequence
of increased blood ow in coronary sinuses of larger diameter,
which lead to atrophy of the valve [23]. Moreover, when the
persistent left SVC drains into the coronary sinus, the most
common congenital malformation of thoracic venous return
is present when the Thebesian valve is absent [29].
It has been proven that the shape and size of the Thebe-
sian valve affects the success of coronary sinus cannulation.
Five types of Thebesian valve have been distinguished, and
their detailed characteristics are shown in Figure 4 and Table 2
[23, 30]. Some types of Thebesian valve morphology are
especially prone to obstruction of CSO during clinical proce-
dures. In general, “fold” type valves may signicantly prolong
the duration of the procedure of coronary sinus cannulation.
A particularly obstructive type of Thebesian valve is the one
that covers more than 100% of the ostium; this anatomic
variant is seen in around 2.6% of hearts (Fig. 5). A similar
percentage of failures in coronary sinus catheterisation is at-
tributed to the inability to locate the CSO (2.87%) [31]. In such
cases, because of the large fold of the valve that covers the
Table 2. Classication of the morphology of the Thebesian valve. Adapted from [23] and [30]
Thebesian
valve type
Name Prevalence in cadaver
material
Mean CSO
coverage ± SD
H/D-ratio
threshold
value in MSCT
Description
I Remnant 26% 24 ± 8% 0.00–0.35 Small hem of endocardium
not protruding into CSO
II Semilunar 33% 58 ± 11% 0.35–0.64 Semilunar fold signicantly
protruding into CSO
III Fold 17% 81 ± 6% 0.64–1.00 Large, non-semilunar fold almost
completely covering CSO
IV Cord 14% NA NA Single thick strand, in most
cases located in CSO midline
V Mesh or fenestrated 10% NA NA Net-like valves, multiple cords
or fenestrated valves
of type I, II, and III
CSO — coronary sinus ostium, H/D — height/diameter; MSCT — multi-slice computed tomography, NA — not applicable, SD — standard deviation
Figure 4. Types of Thebesian valve as seen in cadavers and multi-slice computed tomography: remnant (a); semilunar (b); fold
(c); cord (d); mesh or fenestrated (e); CS — coronary sinus; CSO — coronary sinus ostium, RA — right atrium, TV — Thebesian
valve. Adapted from [23] and [30]
![Table 1. Classification of the morphology of pectinate muscles. Adapted from [8]](/figures/table-1-classification-of-the-morphology-of-pectinate-2nvrs3h0.webp)
