DOI:10.2214/AJR.07.3127
AJR 2008; 190:1569-1575
© American Roentgen Ray Society
MDCT of the S-Shaped Sinoatrial Node Artery
Farhood Saremi1,
Stephanie Channual1,
Amir Abolhoda2,
Swaminatha V. Gurudevan3,
Jagat Narula3 and
Jeffrey C. Milliken2
1 Department of Radiological Sciences, University of California, Irvine, UCI
Medical Center, 101 The City Dr., Route 140, Orange, CA 92868-3298.
2 Department of Cardiothoracic Surgery, University of California, Irvine, UCI
Medical Center, University of California, Irvine, Orange, CA.
3 Department of Cardiology, University of California, Irvine, UCI Medical
Center, University of California, Irvine, Orange, CA.
Received September 8, 2007;
accepted after revision December 25, 2007.
Address correspondence to F. Saremi
(fsaremi{at}uci.edu).
Abstract
OBJECTIVE. The purpose of this study was to use 64-MDCT to
investigate the anatomic characteristics of the S-shaped variant of the
sinoatrial node (SAN) artery and to describe the clinical implications of the
findings in ablative procedures involving the left atrium.
MATERIALS AND METHODS. Coronary CT angiograms of 250 patients (152
men, 98 women; mean age, 60 ± 12 [SD] years) were retrospectively
analyzed for identification of the origin, number, anatomic course, mode of
termination, and S-shaped variant of the SAN artery.
RESULTS. At least one SAN artery was detected in 244 patients. The
S-shaped variant was seen in 35 (14.3%) of these patients. Thirty-four of the
variants (30.6% of all left SAN arteries) arose from the proximal to middle
portion of the left circumflex artery (mean distance between the ostium of the
left circumflex artery and the origin of S-shaped variant, 28.7 ± 13.1
mm). The other variant (0.7% of all right SAN arteries) originated from the
distal right coronary artery. The S-shaped variant was the only artery
supplying the SAN in 28 (11.4%) of the patients. In patients with two arteries
supplying the SAN, the right SAN artery and the S-shaped variant of the left
SAN artery were seen together in seven patients. The S-shaped SAN artery (mean
distance from atrial wall, 2.43 ± 0.992 mm) had a predictable proximal
course, lying in the posterior aspect in a groove between the orifices of the
left superior pulmonary vein and the left atrial appendage close to the left
atrial wall. The terminal segment of the artery approached the nodal tissue
posterior to the superior vena cava in 22 patients, anterior to the vena cava
in 10 patients, and through branches surrounding the vena cava in two
patients.
CONCLUSION. The S-shaped variation of the SAN artery is common and
has a characteristic anatomic course. MDCT can be used to plan surgical and
catheter-based left atrial interventions in which this artery is at risk of
injury.
Keywords: ablation • coronary artery • coronary CT angiography • left atrium • sinoatrial node artery
Introduction
Reports of cadaveric dissections and angiographic studies of the
human heart describe the S-shaped sinoatrial node (SAN) artery, an anatomic
variant of the left SAN artery, as a relatively large vessel arising from the
left circumflex (LCX) artery and coursing posteriorly between the left atrial
appendage (LAA) and the ostium of the left superior pulmonary vein (LSPV) and
then anteriorly close to the anterior wall of the left atrium
[1-7].
The unusual anatomic course and proximity to the left atrial wall predispose
this vessel to injury during cardiac interventions. Accurate anatomic imaging
of this variant blood vessel can influence planning for surgical and
catheter-based ablation of the left atrium.
Coronary CT angiography has been found useful for evaluation of coronary
artery stenosis, anomalous coronary anatomic features, and the arterial supply
of the cardiac conduction system
[8-13].
Newer-generation 64-MDCT scanners provide enough temporal and spatial
resolution for evaluation of epicardial coronary arteries and their smaller
branch vessels [14,
15]. We have described our
experience with MDCT for detailing the anatomic features of the arterial blood
supply of the sinoatrial and atrioventricular nodes
[13]. The purpose of this
study was to evaluate our extended results with 64-MDCT for imaging of this
variant of the SAN artery.
Materials and Methods
A retrospective analysis of ECG-gated MDCT (Aquilion unit, Toshiba)
examinations of the coronary artery was conducted over 4 months (February-June
2007) with 265 consecutively registered subjects, either self-referred
(n = 130) because of personal interest in participating in research,
for which they had given consent, or referred by a physician because of
suspected coronary artery disease (CAD). Two hundred fifty patients were
included in the study. Fifteen patients were excluded because of severe
artifacts produced by low contrast-to-noise ratio (n = 4), motion
(n = 10), or pacemaker leads (n = 1).
Completed clinical history questionnaires were reviewed: 16.1% of the
patients reported a history of atrial fibrillation, 8.6% had structural heart
disease such as heart failure or cardiomyopathy, 6.8% had a history of
coronary artery stent placement, 5.4% had undergone coronary artery bypass
graft (CABG), surgery, and 5.4% had a history of myocardial infarction. The
mean age of the patients was 60 ± 12 (SD) years (range, 27-86 years).
One hundred fifty-two (60.8%) of the patients were men, and 98 (39.2%) were
women. The mean weight of the entire cohort was 176.3 ± 37.3 lb (80.0
± 16.9 kg) (range, 100-280 lb [45.4-127.0 kg]). The study was approved
by the institutional review board at our institution. Informed consent was
waived, and the study was in compliance with HIPAA regulations.
Patient Preparation
ECG was performed and vital signs were obtained for all patients when they
arrived in the imaging suite. When necessary, patients were given oral and IV
metoprolol to achieve a target heart rate less than 65 beats/min. Unless
contraindicated, a sublingual nitroglycerin tablet (0.4-0.8 mg) was given 1
minute before image acquisition. The mean heart rate before data acquisition
was 58 ± 6 beats/min (range, 40-73 beats/min).
Scan Protocol and Image Reconstruction
The 64-MDCT scanner had an individual detector width of 0.5 mm. Contrast
enhancement was achieved with a mean dose of 74.91 ± 3.32 mL (range,
65-92 mL) of iohexol (Omnipaque 350 mg/mL, GE Healthcare) injected at 4-5 mL/s
and followed by an injection of 50 mL of saline solution at 5 mL/s through an
18-gauge catheter into an antecubital vein to allow washout of contrast
material from the right side of the heart and the superior vena cava (SVC).
The scan parameters were collimation, 64 x 0.5 mm; table feed per
rotation, 7.2 mm; gantry rotation time, 400 milliseconds; tube voltage, 120
kVp; tube current, 400 mA. Start delay was defined with bolus tracking in the
descending aorta at the level of tracheal bifurcation, and scan start was
automatically initiated 4 seconds after a threshold of 180 H was reached.
Retrospective ECG-gated volumetric data sets were acquired during a single
breath-hold. The mean scan duration was 9.0 ± 1.3 seconds with a range
of 7.4-15 seconds.
Axial slices were reconstructed and synchronized to the ECG with a
nonsegmented (
65 beats/min) or segmented (> 65 beats/min) image
reconstruction algorithm based on the heart rate throughout the examination.
When necessary, R-wave indicators were manually repositioned to improve the
quality of synchronization. On the basis of a relative delay strategy,
diastolic data sets were reconstructed at 70%, 75%, and 80% of R-R intervals.
In case of persistence of artifacts related to coronary motion at the
atrioventricular groove, a second reconstruction approach was made, and
systolic images were reconstructed with an absolute delay strategy with a 350-
to 400-millisecond delay after the previous R wave. Data sets reconstructed
during the diastolic phase were used for all patients. Axial slices with a
thickness of 0.5 mm (increment, 0.3 mm) and a cardiac CT angiography algorithm
were used for evaluating coronary vessels and conduction system branches. The
data set least affected by cardiac motion was transferred to an off-line 3D
workstation (Vitrea, Vital Images) for further analysis.
CT Data Analysis
A radiologist and a cardiologist with advanced training in cardiovascular
CT interpretation rendered and evaluated multiplanar reformations of the
contrast-enhanced axial images according to training guidelines published by a
task force of the American College of Cardiology Foundation and the American
Heart Association [16].
Visualization techniques such as maximum intensity projection and 3D
reconstruction with tissue sculpting depended on individual coronary anatomy
and image quality. Overall image quality was qualitatively evaluated and
classified as excellent, good, or adequate primarily on the basis of common
image-degrading artifacts related to metal, motion, and background noise. The
imaging findings related to CAD were graded as normal, mild (any vessel with
less than 50% stenosis), moderate (any vessel with 50-70% stenosis), or severe
(any vessel with greater than 70% stenosis).
Axial images were reviewed. Coronary vessels were examined to identify the
SAN artery. Images were analyzed for identification of the origin, number,
anatomic course, and anatomic variants of the arteries to the SAN with
specific attention to the S-shaped variant of the SAN artery. The diameter for
all SAN arteries was measured close to the origin of the artery. The distance
between the ostium of the LCX artery and the origin of the S-shaped SAN artery
also was measured. The anatomic course of the S-shaped SAN artery was
assessed, and its closest distance from the left atrial wall in the groove
between the ostium of the LSPV and the mouth of the LAA was measured. The
diameter of the left S-shaped SAN artery in this groove also was measured. The
mode of termination of the S-shaped SAN artery in relation to the SVC was
classified as retrocaval (posterior to the SVC), precaval (anterior to the
SVC), or pericaval (through multiple branches surrounding the SVC). Each set
of images was evaluated for the presence of additional anatomic findings,
including unusual origin of the S-shaped SAN artery, atrial branches, and dual
blood supply to the SAN.
Statistical Analysis
Statistical analysis was performed with SAS software (version 9.1.3, SAS
Institute). Mean ± standard deviation was used to report univariate
statistics for all continuous data. In addition, 95% CI estimates of
percentages were reported. Statistical analysis for bivariate comparisons was
performed. A value of p < 0.05 was considered statistically
significant.
Results
Image Quality and Vessel Visualization
Image quality was classified as excellent in 76% of the cases, good in 20%,
and adequate in 4%. Axial and 3D images were the optimal views for
visualization of the anatomic course of the SAN arteries and their atrial
branches.
S-Shaped Posterior SAN Artery
Among the 250 patients in the study, the SAN artery was not visualized in
six patients. In the other 244 patients, a single SAN artery originated from
the right coronary artery (RCA) in 133 (54.5%) of the patients and from the
LCX artery in 99 (40.6%) of the patients
(Table 1). A dual blood supply
to the SAN from the RCA and LCX arteries was seen in 12 (4.9%) of the
patients. Among 250 coronary CT angiograms, 46.2% were normal, 30.5% had
evidence of mild CAD, 13.3% evidence of moderate CAD, and 10.0% evidence of
severe CAD in any vessel.
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TABLE 1: Incidence and Measurements of Arteries Supplying the Sinoatrial Node and
Their Associated Branches (n = 244)
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An S-shaped SAN artery was seen in 35 (14.3%) of the 244 patients with SAN
arteries. Thirty-four (30.6%) of the 111 left SAN arteries (99 single, 12 part
of a dual supply) originated from the proximal to middle portion of the LCX
artery (mean distance from the LCX ostium, 28.7 ± 13.1 mm; range,
12.2-65.0 mm). One (0.7%) of the 145 right SAN arteries (133 single, 12 part
of a dual supply) originated from the distal RCA. Among the 34 patients with a
left-sided S-shaped SAN artery, 67.6% had normal findings on coronary CT
angiography, whereas 29.4% had mild CAD and 2.94% had moderate CAD in the LCX
artery. There was no relation between the dominance of the coronary system and
the origin of the S-shaped SAN artery. Among 35 S-shaped vessels, 27 were from
a right-dominant, seven from a left-dominant, and one from a balanced coronary
system. Women were less likely than men to have the S-shaped variant (relative
risk, 0.4596; 95% CI, 0.2177-0.9700; chi-square value, 4.56; p =
0.0327).
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Fig. 1A —62-year-old man with chest pain. Anatomic course of S-shaped
SAN artery. Axial CT scan at level of left atrial appendage (LAA) (A)
and left lateral 3D CT scans in two different projections (B and
C) of heart show that on axial images, S-shaped posterior sinoatrial
node (SAN) artery (large arrows, A-C) can easily be identified
where it courses between LAA and left superior pulmonary vein (LSPV). Artery
arises from proximal left circumflex artery and courses along lateral wall of
left atrium. It usually gives off branches to atrial wall (small
arrows, B) before making U-turn toward LAA-LSPV groove. In groove,
SAN artery is very close to atrial wall (arrow, A) and can be
damaged in surgical procedures on LAA or pulmonary vein isolation procedures.
From this point, anatomic course of S-shaped SAN artery is similar to that of
left SAN artery, which courses toward superior vena cava along anterior wall
of left atrium. LPV = left pulmonary vein trunk, AA = ascending aorta.
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Fig. 1B —62-year-old man with chest pain. Anatomic course of S-shaped
SAN artery. Axial CT scan at level of left atrial appendage (LAA) (A)
and left lateral 3D CT scans in two different projections (B and
C) of heart show that on axial images, S-shaped posterior sinoatrial
node (SAN) artery (large arrows, A-C) can easily be identified
where it courses between LAA and left superior pulmonary vein (LSPV). Artery
arises from proximal left circumflex artery and courses along lateral wall of
left atrium. It usually gives off branches to atrial wall (small
arrows, B) before making U-turn toward LAA-LSPV groove. In groove,
SAN artery is very close to atrial wall (arrow, A) and can be
damaged in surgical procedures on LAA or pulmonary vein isolation procedures.
From this point, anatomic course of S-shaped SAN artery is similar to that of
left SAN artery, which courses toward superior vena cava along anterior wall
of left atrium. LPV = left pulmonary vein trunk, AA = ascending aorta.
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Fig. 1C —62-year-old man with chest pain. Anatomic course of S-shaped
SAN artery. Axial CT scan at level of left atrial appendage (LAA) (A)
and left lateral 3D CT scans in two different projections (B and
C) of heart show that on axial images, S-shaped posterior sinoatrial
node (SAN) artery (large arrows, A-C) can easily be identified
where it courses between LAA and left superior pulmonary vein (LSPV). Artery
arises from proximal left circumflex artery and courses along lateral wall of
left atrium. It usually gives off branches to atrial wall (small
arrows, B) before making U-turn toward LAA-LSPV groove. In groove,
SAN artery is very close to atrial wall (arrow, A) and can be
damaged in surgical procedures on LAA or pulmonary vein isolation procedures.
From this point, anatomic course of S-shaped SAN artery is similar to that of
left SAN artery, which courses toward superior vena cava along anterior wall
of left atrium. LPV = left pulmonary vein trunk, AA = ascending aorta.
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The S-shaped variant was the only artery supplying the SAN in 28 patients;
27 of these variants were from the LCX artery and one from the RCA. These 28
cases were 11.4% of all 244 SAN arteries, 27.3% of 99 single SAN arteries
arising from the LCX artery, and 0.8% of 133 single SAN arteries arising from
the RCA. In the 12 cases of dual blood supply to the SAN, the right SAN artery
and left SAN artery were seen together in five (41%) of the patients, and the
right SAN artery and the S-shaped variant of the left SAN artery were seen
together in seven (58.3%) of the patients. This dual supply involving the
S-shaped SAN variant was seen in 2.9% of all 244 patients with SAN
arteries.
The S-shaped variant of the left SAN artery appeared larger and longer than
the usual SAN artery. Whereas the mean diameters of the regular right and left
SAN arteries were 1.58 ± 0.287 mm (range, 0.90-2.2 mm) and 1.40
± 0.314 mm (range, 0.70-1.9 mm), respectively, the mean diameter of the
left S-shaped SAN artery was 2.12 ± 0.347 mm (range, 1.3-2.8 mm) close
to its origin. The left S-shaped SAN artery invariably coursed posteriorly in
a groove at the junction between the orifice of the LSPV and the mouth of the
LAA and had a mean diameter of 1.90 ± 0.331 mm (range, 1.4-2.7 mm)
within the groove (Figs. 1A,
1B and
1C). In this groove, the artery
was close (mean distance, 2.43 ± 0.992 mm; range, 1.0-4.8 mm) to the
left atrial wall. The course of the artery continued toward the SVC along the
transverse sinus anterior to the left atrium and interatrial groove.
The terminal segment of the S-shaped variant of the left SAN artery
approached the SAN by three different routes (Figs.
2A,
2B,
2C and
2D): posterior to the SVC
(retrocaval) in 22 (64.7%) of the patients, anterior to the SVC (precaval) in
10 (29.4%), and through multiple branches surrounding the SVC (pericaval) in
two (5.9%) of the patients. The retrocaval mode of termination was therefore
the most common pattern, and there was a significant difference between this
mode and the precaval mode of termination (chi-square value, 4.50; p
= 0.0339). At the level of the interatrial groove, the S-shaped SAN artery
typically sent multiple branches to supply the interatrial Bachmann's bundle.
In the retrocaval mode of termination, the S-shaped SAN artery penetrated the
interatrial bundle before reaching the SAN in the lateral aspect of the
superior cavoatrial junction.
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Fig. 2A —Terminal anatomic course of S-shaped sinoatrial node (SAN)
artery. 85-year-old man with retrocaval (A and B) and
61-year-old woman with precaval (C and D) mode of termination of
artery. Sculptured 3D CT images (A and C) and corresponding
axial images (B and D) at level of superior cavoatrial junction
show S-shaped SAN arteries (short arrows, A and C).
Proximal courses of artery are similar in most cases. Arising from proximal
left circumflex artery, S-shaped SAN artery turns posteriorly and courses in
groove between left atrial appendage (LAA) and left superior pulmonary vein
(LSPV) orifices (short arrows, B and D). We found that
in most cases distal artery coursed close to interatrial groove, penetrated
interatrial muscle bundle, and followed its course behind superior vena cava
(SVC) (retrocaval) to reach SAN area on lateral aspect of cavoatrial junction
(long arrows, A and B). This variant is prone to injury
in superior septal approach to mitral valve repair. In precaval mode of
termination (long arrows, C and D), artery courses away
from interatrial groove to reach anterior margin of superior cavoatrial
junction and is less susceptible to injury. AA = ascending aorta, RSPV = right
superior pulmonary vein.
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Fig. 2B —Terminal anatomic course of S-shaped sinoatrial node (SAN)
artery. 85-year-old man with retrocaval (A and B) and
61-year-old woman with precaval (C and D) mode of termination of
artery. Sculptured 3D CT images (A and C) and corresponding
axial images (B and D) at level of superior cavoatrial junction
show S-shaped SAN arteries (short arrows, A and C).
Proximal courses of artery are similar in most cases. Arising from proximal
left circumflex artery, S-shaped SAN artery turns posteriorly and courses in
groove between left atrial appendage (LAA) and left superior pulmonary vein
(LSPV) orifices (short arrows, B and D). We found that
in most cases distal artery coursed close to interatrial groove, penetrated
interatrial muscle bundle, and followed its course behind superior vena cava
(SVC) (retrocaval) to reach SAN area on lateral aspect of cavoatrial junction
(long arrows, A and B). This variant is prone to injury
in superior septal approach to mitral valve repair. In precaval mode of
termination (long arrows, C and D), artery courses away
from interatrial groove to reach anterior margin of superior cavoatrial
junction and is less susceptible to injury. AA = ascending aorta, RSPV = right
superior pulmonary vein.
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Fig. 2C —Terminal anatomic course of S-shaped sinoatrial node (SAN)
artery. 85-year-old man with retrocaval (A and B) and
61-year-old woman with precaval (C and D) mode of termination of
artery. Sculptured 3D CT images (A and C) and corresponding
axial images (B and D) at level of superior cavoatrial junction
show S-shaped SAN arteries (short arrows, A and C).
Proximal courses of artery are similar in most cases. Arising from proximal
left circumflex artery, S-shaped SAN artery turns posteriorly and courses in
groove between left atrial appendage (LAA) and left superior pulmonary vein
(LSPV) orifices (short arrows, B and D). We found that
in most cases distal artery coursed close to interatrial groove, penetrated
interatrial muscle bundle, and followed its course behind superior vena cava
(SVC) (retrocaval) to reach SAN area on lateral aspect of cavoatrial junction
(long arrows, A and B). This variant is prone to injury
in superior septal approach to mitral valve repair. In precaval mode of
termination (long arrows, C and D), artery courses away
from interatrial groove to reach anterior margin of superior cavoatrial
junction and is less susceptible to injury. AA = ascending aorta, RSPV = right
superior pulmonary vein.
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Fig. 2D —Terminal anatomic course of S-shaped sinoatrial node (SAN)
artery. 85-year-old man with retrocaval (A and B) and
61-year-old woman with precaval (C and D) mode of termination of
artery. Sculptured 3D CT images (A and C) and corresponding
axial images (B and D) at level of superior cavoatrial junction
show S-shaped SAN arteries (short arrows, A and C).
Proximal courses of artery are similar in most cases. Arising from proximal
left circumflex artery, S-shaped SAN artery turns posteriorly and courses in
groove between left atrial appendage (LAA) and left superior pulmonary vein
(LSPV) orifices (short arrows, B and D). We found that
in most cases distal artery coursed close to interatrial groove, penetrated
interatrial muscle bundle, and followed its course behind superior vena cava
(SVC) (retrocaval) to reach SAN area on lateral aspect of cavoatrial junction
(long arrows, A and B). This variant is prone to injury
in superior septal approach to mitral valve repair. In precaval mode of
termination (long arrows, C and D), artery courses away
from interatrial groove to reach anterior margin of superior cavoatrial
junction and is less susceptible to injury. AA = ascending aorta, RSPV = right
superior pulmonary vein.
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An unusual variant of the S-shaped SAN artery originating from the RCA
distal to the origin of the posterolateral artery was identified in one (0.4%)
of the 244 patients with SAN arteries. This artery coursed posteriorly around
the posterior aspect of the coronary sinus and left atrium and then anteriorly
toward the ostium of the left inferior pulmonary vein and then the LSPV (Fig.
3). The course was 1.2 mm from
the left atrial wall in the groove between the LAA and the LSPV and along the
transverse sinus, terminating precavally to supply the SAN.
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Fig. 3 —45-year-old woman with S-shaped sinoatrial node artery
(short black arrows, C) arising from right coronary artery
(RCA).
A-C, Axial CT images at level of left atrial appendage (LAA)
(A) and coronary sinus (B) and 3D posterior CT scan (C)
of heart show rare variant found in one patient. S-shaped sinoatrial node
artery originates from terminal branches of dominant right coronary artery
posterior to coronary sinus (CS) (B and C) and courses along
posterolateral wall of left atrium toward groove between LAA and left superior
pulmonary vein (LSPV). IVC = inferior vena cava, LIPV = left inferior
pulmonary vein, PLA = posterolateral artery (long black arrows,
C), RA = right atrium. White arrows indicate areas of A and
B that correspond to C.
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An atrial branch from the S-shaped posterior SAN artery was found in 6.0%
of all 250 cases reviewed. The proximal portion of this branch was commonly
seen supplying the posterior wall of the left atrium (Figs.
1A,
1B and
1C). In two patients, an atrial
branch was seen arising from the proximal LCX artery, which had a course
similar to that of the S-shaped artery between the LAA and the LSPV but did
not reach the SAN area. The SAN was supplied by the right SAN artery in one of
these patients and by the left SAN artery in the other (Figs.
4A,
4B,
4C and
4D).
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Fig. 4A —Findings to be differentiated from S-shaped sinoatrial node
artery. S-shaped sinoatrial node artery invariably courses in groove between
left atrial appendage and left superior pulmonary vein and is best localized
on axial images. 65-year-old man with a history of coronary artery disease and
chest pain. CT scan shows independent small atrial branch arising from left
circumflex artery (not shown) and coursing in groove between left atrial
appendage and left superior pulmonary vein and behind left atrium (white
arrows) but not reaching sinoatrial node area. True sinoatrial node
artery is left-sided artery arising from left circumflex artery (black
arrows).
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Fig. 4B —Findings to be differentiated from S-shaped sinoatrial node
artery. S-shaped sinoatrial node artery invariably courses in groove between
left atrial appendage and left superior pulmonary vein and is best localized
on axial images. 56-year-old woman with recanalized ligament of Marshall. CT
scan shows ligament of Marshall (arrow) is recanalized because left
brachiocephalic vein is occluded (not shown).
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Fig. 4C —Findings to be differentiated from S-shaped sinoatrial node
artery. S-shaped sinoatrial node artery invariably courses in groove between
left atrial appendage and left superior pulmonary vein and is best localized
on axial images. 61-year-old man with persistent left superior vena cava. CT
scan shows superior vena cava (arrow) partially filled by collateral
vessels. Cardiac chambers are poorly filled with contrast material.
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Fig. 4D —Findings to be differentiated from S-shaped sinoatrial node
artery. S-shaped sinoatrial node artery invariably courses in groove between
left atrial appendage and left superior pulmonary vein and is best localized
on axial images. 72-year-old man who has undergone coronary artery bypass
graft surgery. CT scan shows typical course of saphenous vein graft to obtuse
marginal artery. Graft usually courses behind left atrial appendage but with
enough distance from groove (white arrow) and should not be mistaken
for S-shaped sinoatrial node artery on axial images. S-shaped variant of
sinoatrial node artery (black arrows) is evident.
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Discussion
Several previously published reports
[1-6,
17,
18] provide anatomic
descriptions of the blood supply to the cardiac conduction system. These
reports, however, are based solely on findings at invasive transarterial
angiography or cadaveric dissection of the human heart. These descriptions
also focus primarily on the main named epicardial vessels supplying the nodal
tissues, such as the left or right SAN arteries. We have reported
[13] our experience with
noninvasive MDCT of the arterial supply to the SAN and the atrioventricular
node. To our knowledge, the current report is the first of noninvasive
anatomic imaging of a common variant of the left SAN artery, namely, the
S-shaped SAN artery, with 64-MDCT (0.5-mm isotropic resolution) of living
human subjects. Our results are in concordance with the previous angiographic
and cadaveric findings. We found that the S-shaped variant of the left SAN
artery accounts for approximately 30% of SAN arteries arising from the LCX
coronary artery and 14.3% of all SAN arteries. Kyriakidis et al.
[19] and Nerantzis and
Avgoustakis [2] have similarly
reported that this variant can be seen in 27% and 21%, respectively, of left
SAN arteries. These angiographic and cadaveric findings confirm the accuracy
of our observations.
McAlpine [1] extensively
described the S-shaped variant as a posterior SAN artery arising from the
proximal 50 mm of the LCX artery coursing over the lateral wall of the left
atrium between the orifices of the LAA and LSPV and then commonly proceeding
intramurally through the interatrial septal bundle to the superior cavoatrial
junction. This artery supplies the SAN and surrounding area, including a large
part of the right atrial free wall and interatrial septum, the dome of the
left atrium, and, on occasion, a portion of the atrioventricular nodal area
[2,
3]. McAlpine's descriptions of
the S-shaped variant in cadavers mirror our noninvasive imaging findings. It
is of interest that the S-shaped SAN artery follows a course similar to that
of the left superior cardinal vein of the fetus and the oblique vein of
Marshall in adults. Therefore, structures such as a persistent left SVC or a
recanalized Marshall ligament should be considered in the differential
diagnosis of an enhancing structure passing between the LAA and the LSPV
(Figs. 4A,
4B,
4C and
4D). In addition, although
saphenous vein grafts usually lie behind the LAA in patients who have
undergone CABG graft surgery, the proximity of the graft to the LSPV-LAA
groove can cause the graft to be mistaken for the S-shaped SAN artery (Figs.
4A,
4B,
4C and
4D).
Potential clinical implications of our imaging findings in patients
undergoing invasive procedures on the left and right atria are beyond anatomic
curiosity. For instance, the long, aberrant, yet predictable course of the
S-shaped variant of the SAN artery can expose this vessel to injury during
epicardial or endocardial ablation performed in proximity to the
posterosuperior left atrial wall and the base of the LAA. These procedures
include epicardial pulmonary vein isolation and ligation of the LAA as part of
any modified Cox maze operation for correction of atrial fibrillation
[20-26].
The consequence of such injury, especially if the vessel is the sole blood
supply to the SAN, can be sinus node dysfunction and even junctional escape
rhythm, negating the potential therapeutic benefit of the ablation procedure.
Therefore, atrial fibrillation ablation procedures should be care fully
planned with this anatomic variant in mind.
Imaging information on the mode of termination of the SAN artery, the
S-shaped variant in particular, can be of clinical relevance
[27]. For example, in the
popular transseptal superior dome approach to the mitral valve apparatus,
cardiac surgeons deliberately extend the septal incision toward the roof the
left atrium and the LAA [28].
The left SAN artery traverses the transverse sinus and reaches the nodal
tissue by a retrocaval route and becomes susceptible to injury. It is
clinically well documented that patients who undergo the superior transseptal
surgical approach to mitral valve surgery often have postoperative
disturbances in sinus node function, but these changes are transient
[29,
30]. Our study revealed that
the termination mode of the S-shaped variant of the SAN artery can differ from
that of normal SAN arteries. We observed that the S-shaped variant most often
(64.7%) has a retrocaval termination, followed by precaval (29.4%) and
pericaval (5.9%) terminations. This pattern of termination is different for
normal left-sided SAN arteries, for which the precaval route is most
common.
Busquet et al. [31]
conducted a cadaveric study on the normal SAN artery and described its
termination as precaval (58%), retrocaval (36%), or encircling (6%). We
[13] reported that the
terminal SAN arteries (right or left) approached the SAN retrocavally in 47.5%
of the patients, precavally in 42.5%, and pericavally in 10%. Berdajs et al.
[27] analyzed 50 human
cadaveric hearts that did not have previous pathologic alterations and found
that the sinus node artery crossed the superior posterior border of the
interatrial septum in 54% of the hearts. Because retrocaval variants course in
proximity to the interatrial groove
[13], the S-shaped posterior
SAN artery may be more prone than normal SAN arteries to surgical trauma
during superior transseptal incisions. Intraoperative damage to the SAN artery
can be avoided if preoperative MDCT angiography accurately depicts the course
and mode of termination of the artery, leading to modification of the surgical
approach as indicated.
Several previous reports [2,
4,
31] have described a blood
supply to the SAN through the common variants of the right and left SAN
arteries. Kyriakidis et al.
[19] reported a dual supply to
the SAN involving the right SAN artery and the S-shaped variant of the left
SAN artery. To our knowledge, our report is the first on the MDCT findings of
this unusual variant of a dual blood supply to the SAN. The presence of a dual
blood supply to the same anatomic cardiac region may serve as a protective
mechanism against ischemia in patients with progressive coronary artery
atherosclerosis. For instance, the collateral arcade between the right SAN
artery and the S-shaped left SAN artery arising from the mid-portion of the
LCX coronary artery may lessen coronary ischemia in the LCX territory in the
case of development of hemodynamically significant proximal LCX stenosis.
Furthermore, a patient with a dual blood supply would be potentially less
vulnerable to ischemia-mediated sick sinus syndrome.
Hutchinson [32] reported
that the S-shaped posterior SAN artery originated from the terminal part of
the RCA and coursed in a clockwise direction around the walls of the right and
left atria to the SAN in one (2.5%) of 40 hearts. We found this variant of the
S-shaped posterior SAN artery branching from the terminal branches of the RCA
in one (0.4%) of 244 patients with SAN arteries. This rare variant also
coursed in a clockwise direction around the posterior wall of the left atrium.
We also identified multiple posterior atrial branches arising from the
S-shaped variant of the left SAN artery in 6% of the patients. In addition, we
found two atrial branches arising directly from the LCX artery and having an
anatomic course similar to that of the S-shaped variant. These atrial branches
should not be confused with the S-shaped SAN artery because they do not extend
to the SAN area (Fig. 4A,
4B,
4C,
4D).
Our study had the expected limitations of a retrospective observational
review, and our data were primarily collected from patients with structurally
normal hearts. The latter limitation may make our findings difficult to
generalize to patients with cardiac anomalies or pathologic heart conditions.
Nevertheless, our results are in accordance with those of historical
angiographic studies and cadaveric dissections of the human heart, emphasizing
the validity of our observations. In addition, the number of patients included
in our study was larger than in past studies of SAN arteries
[3,
7,
27,
31,
32], which may allow greater
generalization of our results.
The S-shaped variant of the SAN artery as imaged with 64-MDCT is a common
variant of the left SAN artery in patients undergoing noninvasive evaluation
for CAD. MDCT is a useful imaging adjunct for precise description of the
course and mode of termination of the S-shaped variant of the SAN artery. This
imaging technique can be used to avoid injury to this vessel during surgical
or catheter-based procedures on the atrial chambers of the human heart.
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Abstract
Introduction
