Cardiac Anatomy
Cardiac Anatomy and Dimensions
The average heart weight varies in proportion to height and weight. The male heart weight is 280 to 340 grams and the female is 230 to 280 grams [1]. Generally, the wall thickness for the left ventricle (LV) is 1.1 to 1.4 cm, the right ventricle (RV) is 0.5 to 0.7 cm, the right and left atrium (RA, LA) are 0.05 to 0.35 cm [2]. The end-diastolic chamber volumes are RV: 165 mls, LV: 150 mls, RA: 57 mls, LA: 50 mls. The ejection fraction for the RV is 45-60% and for the LV is 50-65% [3].
Cardiac Vascular Supply
The left coronary artery (LCA), or its alias, the left main coronary (LM) artery, arises from the left (posterior) coronary sinus, is 5-20 mm in length, and runs leftward to pass behind the pulmonary trunk [9,11,18]. It branches into the left anterior descending (LAD) and the left circumflex (LCX) arteries [9]. In 30-37% of patients the left main coronary artery terminates in a trifurcation in which case it also gives rise to the ramus intermedius artery that is directed laterally (generally when the angle between the LAD and the LCX approaches 90 degrees). When the LM is absent (0.4 to 2% of all cases), the LAD and LCX have separate ostia or share a common ostium on the left coronary sinus. The right coronary artery (RCA) arises from the anterior coronary sinus.
Coronary artery "dominance" refers to which coronary artery supplies the LV's diaphragmatic surface by giving rise to the posterior descending artery (PDA) not to which coronary supplies the majority of the left ventricular myocardium. The PDA most commonly arises from the RCA (70-85%)- this is referred to as "right dominant". It may occasionally arise from the LCX (8-10%) ("left dominant"). Codominance with balanced circulation is present in 5-20% when the RCA and the LCX both give rise to PDA vessels [4,5,10,12,21]. The nondominant system is usually noticeably smaller in caliber than the dominant system [12].
The LAD is divided into three segments [21]. The proximal segment runs from the origin of the LAD to the origin of the first septal perforator [21]. The middle segment runs from the first septal perforator to a point halfway from the LV apex [21]. The distal segment runs from this halfway point to the apex [21]. The LAD follows the interventricular sulcus and often crosses the apex to supply a variable portion of the posteroapical and distal posterior septal walls. In about 90% of cases, the LAD reaches or wraps around the apex. The LAD supplies the anterior and anterolateral left ventricular walls (via diagonal branches [usually between 1-5 that run on the epicardial surface of the heart [18]]), the apex (in a majority of patients), and the anterior 2/3's of the interventricular septum (via septal branches [usually between 1-6]), and portions of the anterior right ventricle in close proximity to the septum. The septal branches (which run perpendicular to the LAD [18]) also supply the atrioventricular (AV) bundle and proximal bundle branch [14]. A proximal stenosis in the LAD should produce a defect involving both the septum and anterior wall, while a distal lesion should produce a defect in the anterior wall which spares the septum. LAD diagonal branches supply the anterolateral wall and should produce an anterolateral defect which spares the apex or septum. Duplication of the LAD is a rare anomaly (about 1% of patients) characterized by a short LAD that terminates high in the anterior interventricular groove and a long LAD that has a proximal course outside the interventricular groove, but returns to the groove in its distal course [17].
The ramus intermedius (ramus medianus) is a large vessel which arises between the LAD and the CX when the LM "trifurcates" [4]. The ramus typically acts to supply the lateral and inferior walls in the manner of diagonal or obtuse marginal branches [21]. A defect in this vessel should produce a defect in the posterolateral wall in the distribution of a high obtuse marginal branch.
The LCx has only two segments-the proximal segment runs from the
LCx
origin to the origin of the first obtuse marginal branch, and the
distal segment lies beyond this point [21]. The LCx supplies the
posterolateral LV wall and the LA. LCX runs in the left
(posterior) atrioventricular groove and gives rise to obtuse
marginal
branches
that supply the lateral wall of the LV (usually 3 marginal
branches of
which the second is usually the largest). A LCX stenosis should
produce
a defect in the posterolateral wall in a non-dominant vessel and
posterolateral and
inferior defects in a dominant vessel. The sinoatrial node artery
arises from the left circumflex in about 40% of individuals. About
1/3
of the sinoatrial nodal arteries that arise from the left
circumflex
may have an "s-shaped" anatomic variant in which the vessel
courses
posteriorly between the left atrial appendage and the ostium of
the
left superior pulmonary vein, and then anteriorly close to the
anterior
wall of the left atrium [16]. The unusual anatomic course and
proximity
to the left atrial wall predispose this vessel to injury during
cardiac
interventions [16]. The LCx artery may also supply branches to the
atrioventricular node [21].
The RCA arises from the right aortic sinus. It travels to the right and passes between the right ventricular outflow tract (posterior to the pulmonary artery) and the right atrial appendage and then runs inferiorly in the right (anterior) atrioventricular sulcus [9,10,12]. The right coronary artery supplies the posterior 1/3d of the septum, the right atrium and right ventricle. The RCA is divided into proximal, mid, and distal portions- the proximal portion extends from its origin to the acute marginal origins, the mid portion is from the acute marginal origin to the horizontal portion in the posterior right atrioventricular groove, and the distal portion extends beyond that [18]. The first branch of the RCA in 50-60% of individuals is the conus artery (it can sometimes- up to 30-50% of cases [12,14]- have an origin directly from the right coronary sinus) [10,21]. The conus branch always courses anteriorly to supply the pulmonary outflow tract [12]. The conus artery also forms the circle of Vieussens- an anastamosis with the LAD arterial circulation [14]. Vieussens' arterial ring is a collateral artery between the conus branches of right coronary artery and the LAD that is found in 48% of the population [20]. It can provide a source of collateral blood flow in proximal coronary artery occlusions [20].
The sinoatrial node sits near the junction of the upper right
atrium
and the SVC and acts as the primary pacemaker in normal sinus
rhythm
[21]. The sinoatrial node artery arises from the proximal RCA in
55-66%
of individuals within a few millimeters of the RCA origin (in the
remaining individuals it arises from the left circumflex artery or
there is a dual blood supply via the RCA and LCx [about 6% of
patients]) [10,15]. The sinoatrial node artery usually courses
along
the anterior interatrial groove toward the superior cavoatrial
junction
[13]. At the cavoatrial junction the artery's course becomes
variable-
either circling anteriorly (precaval) or posteriorly (retrocaval)
to
enter the node [13]. Next, several anterior branches supply the
free
wall of the right ventricle [10]. Acute marginal branches to the
anterior free wall of the right ventricle arise from the mid to
distal
RCA in the atrioventricular groove [10,11].
The AV node is a small bundle of tissue located at the center of
the
Koch triangle- a triangle enclosed by the septal leaflet of the
tricuspid valve, the coronary sinus, and the membranous part of
the
interatrial septum [21]. The AV node is important to the
electrophysiologic activity of the heart because it delays
conduction
of the electric impulse from the atria to the ventricles, allowing
time
for the additional ventricular preload caused by atrial
contraction
[21].The
RCA supplies the atrioventricular nodal artery 88-90% of the time
(it
arises from the distal LCx in about 11% of patients and has a dual
supply via the RCA and LCx in about 2% of patients) [15]. The AV
nodal
artery originates from the apex of the U-turn of the distal RCA
and
penetrates the base of the posterior interatrial septum at the
crux of
the heart in 80-87% of patients [13]. In the remaining patients it
originates from the terminal portion of the LCx (8-13% of cases),
or
less commonly from both the RCA and LCx arteries (2-10%) [13].
The right superior septal perforator is a variant that is noted
in
about 3% of patients at cardiac cath [21].
Like septal perforators arising from the proximal LAD, the right
superior septal perforator supplies the anterior septum, but
arises
from the proximal RCA or directly from the right sinus of Valsalva
[21]. This vessel can serve as a potential collateral supply to
the LAD
[21].
The RCA, when dominant (85% of cases), terminates in the
posterior
descending artery (which supplies the inferior/posterior LV wall
and
inferoseptum [12]) and posterior left ventricular branches [10].
The
PDA can extend around the apex to supply one-third of the anterior
interventricular septum if the LAD is small [10]. When the left
system
is dominant, the RCA is typically diminutive compared with the LCx
artery [14].
Collaterals: Since coronary arteries are end vessels, collaterals provide the only alternative circulation pathway when stenoses are present. Intracoronary: filling of the distal portion of an occluded vessel from its proximal portion. Intercoronary: filling of the distal portion of an occluded vessel from one of the other two coronary arteries or their branches. Intersegmental: filling of the distal portion of an occluded vessel from a different branch of the same coronary artery. Conus artery: in 50% of cases, this artery either originates from the anterior Sinus of Valsalva or the RCA. The meeting of this vessel with an analogous left conus branch forms the "arc of Vieussens" [6]. Kugel's artery [7] is an anastomotic vessel that arises from either the proximal RCA or LCx and, courses in the plane of the AV valve ring along the anterior atrial wall behind the aorta to reach the interatrial septum (near the junction of the interventricular septum) where it passes posteriorly to the crux to supply the posterior circulation [15]. During this passage it may give collaterals to the sinoatrial and to the AV nodal arteries. This collateral channel between the RCA and LCx reconstitutes the distal RCA circulation after proximal occlusion of a dominant RCA; or the distal LCx circulation after proximal occlusion of a dominant LCx.
Reporting of coronary artery disease:
The LAD is divided into proximal, middle, and distal portions [14]. The LAD proximal segment extends from the left main bifurcation to the origin of (and includes) the first major septal perforator [12,14]. The middle LAD segment is immediately distal to the origin of the first major septal perforator and extends to the point where the LAD forms an acute angle on the RAO view (close to the origin of the second diagonal or second septal perforator) [12,14]. If this angle or the second diagonal cannot be identified, the middle segment ends one-half the distance from the first major septal perforator to the apex [12,14]. The apical LAD is the portion distal to the middle segment [12]. The LCx is divided into proximal and distal segments [14]. The proximal LCx segment extends from its origin to (and includes) the origin of the major obtuse marginal branch [12,18]. The distal segment is beyond the origin of the margin obtuse marginal [12]. The RCA is also divided into 3 segments- proximal, mid, and distal [14]. The proximal RCA extends from the ostium to a point halfway to the acute margin of the heart [14]. The mid-RCA represents the other half of that distance and the distal RCA courses along the posterior AV groove, from the acute angle of the heart to the origin of the PDA [14]. However, RCA anatomy has also been described as the proximal portion extending from its origin to the acute marginal origins, the mid portion is from the acute marginal origin to the horizontal portion in the posterior right atrioventricular groove, and the distal portion extending beyond that [18].
Coronary Venous Anatomy:
The coronary veins run in the interventricular and atrioventricular grooves with the coronary arteries [21]. The anterior interventricular vein runs along with the LAD in the interventricular groove from the apex of the heart towards the base [14, 21,22]. Once the anterior interventricular vein reaches the LAD bifurcation, it turns to descend in the left atrioventricular groove with the LCx artery and it is then called the great cardiac vein [14,21,22]. The great cardiac vein receives small tributaries (typically the left marginal vein from the lateral wall or a left posterior vein draining the inferolateral wall) and then drains into the coronary sinus at the base of the heart [21]. The vein of Marshall (left atrial oblique vein of Marshall) is a small vein draining the left atrium and it is a remnant of the left SVC- the point at which it enters the coronary sinus anatomically divides the great cardiac vein and the coronary sinus [21,22]. Internally, the transition from GVC to CS is defined by the valve of Vieussens (which is usually incomplete) [22].
The
coronary sinus is the wide vein that courses in the posterior AV
groove
accompanying the LCx artery- it drains into the RA and receives
the
great cardiac vein proximally and the middle cardiac vein distally
[12]. The coronary sinus is approximately 45 mm long and 10-12mm
in
diameter. The coronary sinus ostium is guarded by the thebesian
valve,
which usually consists of a thin semilunar fold in the
anteroinferior
rim of the ostium [13,22]. The thebesian valve is present in
73-86% of
autopsied hearts [22].
The middle cardiac vein (also known as the posterior
interventricular vein) runs in the posterior interventricular
groove
with the PDA and also drains into the coronary sinus [12,14,21].
The
posterior vein (also called the posterolateral vein) of the LV
drains
the lateral LV wall and connects to either the great cardiac vein
or
the coronary sinus [14].
The left
marginal vein courses along the lateral border of the left
ventricle
(usually adjacent to the obtuse marginal artery)
and connects to the great cardiac vein (or directly into the
coronary
sinus) [13,22]. Both the
posterolateral and left marginal veins are targets for
biventricular
pacing [13]. The small cardiac vein runs in the sulcus between the
right atrium and ventricle and usually drains into the RA [21].
Pulmonary venous anatomy:
Electrophysiologic ablation requires knowledge of the pulmonary venous anatomy [13]. Prior to AF ablation, electrophysiologists need to know [13]:
1- Normal anatomy and anatomic variants of the pulmonary veins.
2- Ostial diameters of each vein and the distance to the first order branch.
3- The presence of accessory or supernumerary pulmonary veins.
4- The dimensions of the left atrium and the presence of left atrial appendage thrombus.
5- Anatomic course of the esophagus relative to the posterior left atrial wall and pulmonary veins.
6- Presence of major anomalies, such as a common ostium to the superior and inferior veins, persistent left SVC, vein of Marshall, or anaomalous pulmonary venous return. In a recent chest CT review, the prevalence of an anomalous pulmonary venous connection was 0.2% [13]. In this study, 79% of patients had an anomalous left upper lobe vein connecting to a persistent left vertical vein; 17% had an anomalous right upper lobe vein draining into the SVC; and 3% had an anomalous right lower lobe vein draining into the suprahepatic IVC [13].
Approximately 80% of the population has 4 pulmonary veins [19]. The normal pulmonary venous anatomy consists of two right pulmonary veins and two left pulmonary veins with separate ostia, however, anatomic variants are common [13]. The superior pulmonary vein ostia are larger (19-20 mm) than the inferior vein ostia (16-17 mm) [13]. The pulmonary venous trunk is defined as the distance from the ostium to the first order branch [13]. The superior pulmonary veins tend to have a longer trunck (21.6 +/- 7.5 mm) than do the inferior pulmonary veins (14.0 +/- 6.2 mm) [13]. Common anomalies include a conjoined (common) left or right pulmonary vein- seen in 25% of individuals [13]. Conjoined pulmonary vein is seen more frequently on the left side [13]. Supernumerary veins are also frequently seen- the most common of which s a separate right middle pulmonary vein that drains the right middle lobe [13]. Early branching (less than 1 cm) is also frequently observed [13].
Coronary Artery Stenosis
Significant obstructive CAD lesions are predominantly located in
the
proximal and mid coronary artery segments [19]. The prevalence of
ostial involvement is low (an ostial stenosis of 50% or more is
found
in less than 3% of patients) [19] The severity of stenosis on
coronary
CTA is generally described as a diameter stensosis (not an area
stenosis) [19].
Coronary artery diameter and cross sectional area (CSA):.
50%
diameter reduction =
70% CSA reduction, 70% diameter reduction = 90% CSA reduction, 90%
diameter reduction =
99% CSA reduction [5]. A "critical stenosis" generally
occurs
with a 90%
CSA reduction and produces ischemia in the tissues subtended by
the
artery. A 50% LM
diameter stenosis and 90% LAD, LCX, or RCA diameter stenoses at
rest
(70% diameter
stenosis during exercise) are considered hemodynamically
significant.
In general, there is
excellent interobserver agreement for minimal (<50%) and severe
(>80%) stenoses when
viewed angiographically. It is important to note that there is as
much
as a ?20%
interobserver variability in the visual quantitation of moderate
(40%
to 80%) stenoses
[8].
Right Atrium:
The right atrium measures 3.4-5.3 cm in the long axis and 2.6-4.4 cm in the short axis [23]. The sinoatrial node (the dominant pacemaker of the heart) is located in the myocardium between the crista terminalis and the superior vena cava [23]. The atrioventricular node is bounded by the coronary sinus ostium, the septal leaflet of the tricupid valve, and the tendon of Todaro (a fibrous band connecting the eustachian and thesbesian veins (these three anatomic landmarks form a triangular area referred to as the triangle of Koch [23].REFERENCES:
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