Anatomical variations in the Sinoatrial Nodal Artery: a meta-analysis and clinical considerations

Anatomical variations in the Sinoatrial Nodal Artery: a meta-analysis and clinical considerations
Anatomical variations in the Sinoatrial Nodal Artery: a meta-analysis and clinical considerations
Running Head: Sinoatrial Nodal Artery – Meta-analysis
Jens Vikse1*, Brandon Michael Henry1*, Joyeeta Roy1, Piravin Kumar Ramakrishnan1, Wan Chin Hsieh2, Jerzy A. Walocha, M.D.,Ph.D.1, Krzysztof A. Tomaszewski,
M.D.,Ph.D.1, International Evidence-Based Anatomy Working Group
* These authors equally contributed to this manuscript
1 Department of Anatomy, Jagiellonian University Medical College, Krakow, Poland
2 First Faculty of Medicine, Charles University in Prague, Czech Republic
Keywords: Anatomy; Artery; Coronary artery imaging; Statistics; Meta-analysis
Word count: 6121
Corresponding Author:
Acknowledgement: The authors would like to thank Karolina Saganiak for the anatomical drawings used in this manuscript.
Funding Statement: This project was funded using the statutory funds of the Jagiellonian University Medical College, Krakow, Poland.
Conflicts of Interest: None
ABSTRACT

The Sinoatrial Nodal Artery (SANa) is a highly variable vessel which supplies blood to the Sinoatrial Node (SAN). Due to its variability and susceptibility to
iatrogenic injury, our study aimed to determine the normal anatomy of the SANa and the prevalence of its anatomical variations. Sixty-six studies (n=21455 hearts) were
included in the meta-analysis after extensive search of major electronic databases. The SANa most commonly runs as a single vessel, originating from the RCA, and
taking a retrocaval course to reach the SAN. The knowledge of its normal and variant anatomy is imperative to prevent iatrogenic injury during cardiac interventions.
Abstract word count: 100 words

1. INTRODUCTION
The sinoatrial nodal artery (SANa) is a branch of the coronary circulation, which supplies blood to the sinoatrial node (SAN), Bachmann’s bundle, crista terminalis,
and the left and right atrial free walls[1]. Due to the highly variable anatomical characteristics of the artery, a consensus has yet to be reached on its normal
anatomy. Furthermore, detailed anatomical knowledge of the artery and its variations is essential to understand both cardiac disease processes and avoiding iatrogenic
injuries during interventional cardiology or cardiosurgical procedures[2,3].
Previous reports in literature have stated that the SANa most frequently originates from either the right coronary artery (RCA) or the left circumflex branch (LCX) of
the left coronary artery (LCA)[4]. When originating from the RCA (Figure 1A), the SANa tends to arise from the proximal segment, contrary to its origin from the LCX
which can be proximal (Figure 1B) or distal (Figure 1C). Furthermore, origins of the artery directly from the LCA have also been commonly reported[5–22] (Figure 1D).
In rare cases, extra-coronary origins of SANa, including from the aorta[9,12,23–26] (Figure 1E) and the bronchial artery[10,26,27], have been observed as well.
Although only a single SANa is usually present in a human heart, both duplication (Figure 1F), and more rarely triplication of the artery, have also been reported.
Since the artery exists most commonly as a single branch without collateral circulation[3,7,8,10–13,15–59], the SAN, supplied by the SANa, is susceptible to ischemic
damage during vasoocclusive disease of the vessel. However, one study[37] investigating 106 hearts from Japanese cadavers found that 53.8% of the studied hearts had
two or more SANa, which may indicate an anatomical heterogeneity in the SAN circulation among different populations.
The microvascularization of the SAN differs from other atrial tissue by consisting of a highly dense plexus of arterioles and capillaries[11,60]. This rich
vascularization facilitates adequate perfusion of the SAN to meet its high metabolic demands[61]. Studies have suggested a correlation between coronary artery diseases
involving the SANa and supraventricular tachycardias[11,43]. One study found a 50% reduction in capillary density in the SAN among patients with atrial fibrillation
compared to patients with sinus rhythm[62]. As such, tissue ischemia and fibrosis secondary to the SANa occlusion may lead to SAN dysfunction[63], although further
studies are required to establish such correlation. Furthermore, the role of SANa variations in the development of atrial pathologies is yet to be explored.
Variations in the course of the SANa in relation to the superior vena cava have also been reported. The different courses of the SANa after branching is termed by its
trajectory in relation to the superior vena cava (SVC). The artery can run in precaval (anterior to the SVC), retrocaval (posterior to the SVC), or pericaval
(combination of pre- and retrocaval) courses (Figure 2). Furthermore, a rare S-shaped course of the SANa has also been described which is reportedly susceptible to an
increased risk of iatrogenic injury during cardiac procedures.
As the use of percutaneous and surgical therapy for supraventricular arrhythmias is increasing[64], a thorough evidence-based knowledge of the anatomy of the SANa is
essential for clinical practice[35,37,54]. As such, the aim of our study was to determine the normal anatomy of the SANa and the prevalence of its variations in the
population.

2. METHODS
2.1 Search Strategy
In order to identify articles for inclusion in the meta-analysis, we performed a literature search through June 2015 of the major electronic databases PubMed, EMBASE,
Science Direct, Scopus, Web of Science, Cochrane Library, and China National Knowledge Infrastructure (CNKI). To achieve a high sensitivity among the search results,
the search strategy was particularly tailored for each database. No lower date limit or language restrictions were applied. The following search term combination was
used to search the electronic databases: “sinus node artery” OR “Sinus Node Branch” OR “Keith-flack node” OR “SA Node artery” OR “SAN Node branch” OR “SAN artery” OR
“SAN branch” OR “sinus node blood supply” OR “sinoatrial artery” OR “sinoatrial branch” OR “sinoatrial blood supply” OR “sinuatrial artery” OR “sinuatrial branch” OR
“sinuatrial blood supply” OR “Sinus Node vasculature” OR “SA Node vasculature” OR “SA Node blood supply.”
To identify additional articles eligible for the analysis, the references of all articles included in the meta-analysis were extensively searched. Case reports,
conference abstracts, and letters to the editor were searched but not included in the meta-analysis. PRISMA guidelines were strictly followed throughout the search
process and the entire meta-analysis.
2.2 Criteria for Study Selection
Each study was independently assessed by three reviewers (J.V., B.M.H., and H.W.C.) for eligibility in the meta-analysis. Studies were considered eligible for
inclusion if they (1) reported data on the anatomy of the SANa in humans, (2) were a cadaveric or imaging study, and (3) had clearly defined anatomical definitions.
The exclusion criteria for studies included (1) missing or incomplete data, (2) unclear anatomical definitions, or (3) the study included patients with congenital
heart diseases or malformations. All relevant studies that were published in languages not fluently spoken by any of the authors were translated by medical
professionals, fluent in both English and the language of the original article. When necessary, authors of the original study were contacted, if possible, for further
details or data. Any disagreements among the authors during the eligibility assessment process were solved by a consensus among the entire review team.
2.3 Data Extraction
Data was independently extracted from the included studies by two reviewers (J.V. and H.W.C.). The extracted data included study design, modality, country, sample
size, number of SANa (single, duplication, or triplication), origin of the SANa, course of the SANa, the distance from the ostium to the SANa, diameter of the SANa,
and the prevalence of S shaped branches. In the event of any discrepancies in the data, the authors of the original study were contacted, if possible, for further
information.
2.4 Statistical Analysis
Statistical analysis was performed by B.M.H. and P.K.R. using MetaXL version 2.0 by EpiGear International Pty Ltd (Wilston, Queensland, Australia) to calculate the
single or multicategorical pooled prevalence of an anatomical characteristic of the SANa. A random effects model was used to perform all analyses. In order to assess
the heterogeneity among the included studies, the Chi2 test and I2 statistic were calculated. For the Chi2 test, a p-value of <0.10 was considered to indicate
significant heterogeneity between studies [Cochrane Handbook]. For the I2 statistic, the value was interpreted according to the following: 0% to 40% might not be
important; 30% to 60% may represent moderate heterogeneity; 50% to 90% may represent substantial heterogeneity; and 75% to 100% may represent considerable
heterogeneity [Cochrane Handbook].
When appropriate, to explore the sources of heterogeneity, subgroup analysis based on the type of study or geographical origin of the study was performed.
Statistically significant differences between 2 or more groups/subgroups were determined through the use of confidence intervals. If the confidence intervals between
two groups/subgroups overlapped, the differences were considered statistically insignificant. Additionally, to further probe the source of heterogeneity, a sensitivity
analysis was also performed when appropriate by limiting inclusion to studies with = 100 hearts.

3. RESULTS
3.1 Study Identification
An overview of the study identification process is summarized in the PRISMA flow diagram (Figure 3). A total of 4380 articles were initially identified through
database searching, with an additional 36 articles identified via reference searching. Subsequently, 174 articles were assessed by full-text for eligibility in the
meta-analysis following screening and removal of duplicates. Of the 174 full-text articles, 108 articles were excluded due to specific reasons and 66 were deemed
eligible for inclusion in the meta-analysis.
3.2 Characteristics of Included Studies
The characteristics of included studies are summarized in Table 1. In the total of 66 studies (n=21455 hearts) included in the meta-analysis, there were 42 cadaveric
studies[3,5–8,10,11,13,14,16–22,27–31,35–38,40–42,44,45,48,51,52,54,56,59,65–70] and 24 imaging studies[9,12,15,23–26,32–34,39,43,46,47,49,50,53,55,57,58,71–74]. The
modalities used in these imaging studies were either X-ray angiography (6 studies)[34,39,43,47,72,74] or Computed Tomography (CT) angiography (18 studies)[9,12,15,23–
26,32,33,46,49,50,53,55,57,58,71,73]. The reported characteristics of the patient population varied between the included studies. In the cadaveric studies, the causes
of death amongst the patients were mostly unspecified, but were also due to various accidents or coronary artery diseases (CAD) (5 studies). In the imaging studies,
most of the patients had a suspected or known CAD or other cardiac pathologies (16 studies). A wide geographical distribution was observed in the included studies,
with a majority of studies originating from China (14 studies), USA (8 studies), Turkey (6 studies), Greece (5 studies), France (4 studies), and Brazil (3 studies).
The number of hearts assessed in the included studies also demonstrated a wide variation, ranging from 10 to 3802. Since no lower date limit was set in the meta-
analysis, the dates of the included studies ranged from 1958 to 2014.
3.3 Number of the Sinoatrial Nodal Artery
A total of 53 studies (n=17675 hearts) reported data on the number of the SANa[3,7,8,10–13,15–59,70]. The results are summarized in Table 2. The SANa was most commonly
found to be present as a single vessel with a pooled prevalence of 95.5% (95%CI:93.6-96.9). Rarely, duplication and triplication of the artery were observed with a
pooled prevalence of 4.3% (95%CI:2.8-6.0) and 0.3% (95%CI: 0-0.7), respectively. Subgroup analyses based on geographical distribution and type of study (cadaveric or
imaging), as well as a sensitivity analysis (inclusive of all studies with = 100 hearts), were performed to explore the high level of heterogeneity (l2=95.77%). The
results of subgroup and sensitivity analyses are also summarized in Table 2.
3.4 Origin of the Sinoatrial Nodal Artery
A total of 61 studies (n=20721 hearts) reported data on the origin of the SANa[5–59,66–68,71,73,74]. The results are summarized in Table 3. The SANa was most commonly
found to originate from the RCA with a pooled prevalence of 68.0% (95%CI:55.6-68.9). The second most common origin of the artery was from the LCX with a pooled
prevalence of 22.1% (95%CI:15.0-26.2), followed by origin of the artery from the LCA with a pooled prevalence of 2.7% (95%CI:0.7-5.2). Other types of origin of the
SANa such as from the aorta or the bronchial artery were equally rare with a pooled prevalence of 0.3% (95%CI:0-1.3). Although the prevalence of a double origin of the
SANa was rare, when present, the artery most commonly originated from the RCA and LCX with a pooled prevalence of 2.0% (95%CI:0.3-4.2). The second most common type of
double origin of the SANa was from the RCA and LCA with a pooled prevalence of 0.9% (95%CI:0-2.3). Other types of double origin of the SANa (i.e. from the RCA and
bronchial artery, from the LCX and pulmonary artery, from the LCX and bronchial artery, both from the RCA, both from the LCX, or both from the LCA) were equally rare
with a pooled prevalence of 0.3% (95%CI:0-1.3). The origin of the SANa when it was triplicated (i.e. 2 from the LCX + 1 from RCA, 2 from RCA + 1 from LCA, 2 from RCA +
1 from bronchial artery, or 1 from RCA + 2 from bronchial artery), or when it arose from the coronary sinus (left or right) were also similarly rare with a pooled
prevalence of 0.3% (95%CI:0-1.2).
Due to the significant heterogeneity found in the analysis (l2=98.94%), subgroup analyses were performed dependent on the geographical distribution and type of study.
The results of the subgroup analyses and an additional sensitivity analysis are summarized in Table 3.
3.5 Course of the Sinoatrial Nodal Artery
A total of 19 studies (n=6033 hearts) reported data on the course of the SANa[3,10,17,22,25–28,30,33,46,49–51,53,54,58,65,74]. With a pooled prevalence of 47.1%
(95%CI:36.0-55.5), the retrocaval course of the SANa was the most common course of the artery, followed by the precaval and pericaval courses with a pooled prevalence
of 38.9% (95%CI:28.5-47.6) and 14.0% (95%CI:7.5-21.1), respectively. Subgroup analyses based on the geographical distribution and type of study, and a sensitivity
analysis were performed because of the high level of heterogeneity found (l2=97.66%). The results are summarized in Table 4.
Nine of the studies above also reported data on the course of the SANa originating from the left (i.e. from the LCA or LCX) or the right (i.e. the RCA) side of the
heart. In the nine studies (n=651 hearts) reporting data on the course of the SANa originating from the LCA or LCX[3,10,26–28,51,53,54,58], the retrocaval course of
the SANa was also the most common course of the artery with a pooled prevalence of 46.5% (95%CI:30.4-62.5). On the contrary, analysis on the nine studies (n=893
hearts) reporting the course of the SANa originating from the RCA[3,10,26–28,51,53,54,58] found the precaval course to be the most common course of the SANa with a
pooled prevalence of 43.1% (95%CI:25.3-58.3). Subgroup analyses in regards to the geographical distribution and type of study, as well as a sensitivity analysis were
also performed. The results for the course of the SANa originating from the LCA or LCX and RCA are summarized in Table 4.
3.6 S-Shaped Branches of the Sinoatrial Nodal Artery
Eight studies (n=5031 hearts) reported data on the prevalence of S-shaped branches of the SANa, The pooled prevalence of the S-shaped branched of the artery was merely
7.6% (95%CI:2.9-14.1). Subgroup analyses by the geographical distribution and type of study were also performed, results of which are summarized in Table 5.
3.7 Morphometric Measurements of the Sinoatrial Nodal Artery
A total of 18 studies (n=3313 RCA) measured the distance between the origin of SANa and the ostium of RCA[16,17,25,26,28,29,32,33,45,46,49,52,54–56,58,69,72], while a
total of 14 studies (n=2029 LCX) measured the distance between the origin of SANa and the ostium of LCX[17,25,26,28,32,33,45,46,49,50,52,55,56,58]. The pooled mean
distance from the ostium of RCA to the origin of SANa and the ostium of LCX to the origin of SANa was 15.871 mm (SD=9.098) and 14.032 mm (SD=8.398), respectively.
Subgroup analyses based on the geographical distribution and type of study were also performed. The results are summarized in Table 6.
The diameter of the SANa (originating from the RCA) at its origin and within the SAN were reported by seven (n=587 hearts)[16,25,28,29,45,56,58] and six (n=2315
hearts)[32,33,45,46,69,72] studies, respectively. At its origin, the pooled mean diameter of the SANa originating from the RCA was 1.275 mm (SD=0.480). Within the SAN,
the pooled mean diameter of the SANa originating from the RCA was 1.578 mm (SD=0.300).
Seven (n=381 hearts) [16,25,28,29,45,56,58] and five (n=1753 hearts)[32,33,45,46,72] studies also reported the diameter of the SANa (originating from the LCX) at its
origin and within the SAN, respectively. At its origin, the pooled mean diameter of the SANa originating from the LCX was 1.376 mm (SD=0.346). Within the SAN, the
pooled mean diameter of the SANa originating from the LCX was 1.502 mm (SD=0.279). Subgroup analyses based on the geographical distribution and type of study were also
performed. The results are summarized in Table 7.

4. DISCUSSION
A normal heart rhythm refers to a regular rhythm with signal initiation from the SAN at an adequate heart rate. The crescent-shaped SAN is a collection of specialized
myocytes located in the posterior wall of the right atrium, near the junction of the crista terminalis with the superior vena cava[65]. These myocytes are
characterized by automaticity, which facilitates spontaneous depolarization of their cell membrane. Several studies have investigated the anatomical features of the
SANa, which is the major blood supply to the SAN. The aim of our study was to pool data from all relevant studies to provide comprehensive, evidence-based anatomical
data on the SANa with clinical implications for interventional cardiologists and cardiosurgeons.
Although rare, duplication and triplication of the SANa can occur in the human population[7,10,13,18,19,21,23–30,32–35,37,40–42,44–56,58,70] and should be taken into
account by surgeons. The presence of such anatomical variations should be carefully evaluated before open heart surgery to prevent iatrogenic injury to one of the
branches. Our analysis showed that SANa is represented by a single branch in 95.5% of cases, and the pooled prevalence of duplication and triplication was 4.3% and
0.3% respectively. Subgroup analysis according to geography showed that a duplicated SANa has a higher prevalence in the African population (18.8%) compared to the
European (3.2%), Asian (4.9%), North American (3.9%) and South American (2.6%) populations. There has been speculation in literature on ethnical variations in SANa
anatomy[45], however, due to the small sample sizes of the African studies in this meta-analysis (n=88), larger studies will be needed to establish such a geographical
variation. Further subgroup analysis according to type of study revealed that cadaveric studies reported a slightly higher pooled prevalence of duplicated (4.6%) and
triplicated (0.5%) SANa when compared to imaging studies (3.6% and 0.1%, respectively), although the difference was not statistically significant.
The SANa was found to originate most commonly from the RCA (68.0%) with the LCX being the second most common point of origin (22.1%). Sensitivity analysis on the SANa
origin yielded similar results for origin from the RCA and from the LCX with 70.2% and 23.7%, respectively. This is in agreement with previous reports in the
literature[5,7,9,12,13,23,24,27–31,36,44,49,54,55,68,71] reported a predominance of left-sided origin of SANa. Among the included studies, only two[17,56] reported
predominance of left-sided origin of SANa. However, both these studies had a small sample size. A single extracoronary origin of SANa was found in 0.6% of cases,
represented by aortic (0.3%) and bronchial artery (0.3%) origin. Sinoatrial nodal artery originating from the bronchial artery presents an interesting hemodynamic
situation where the SANa is filled during systole, which in theory, would put these patients at risk of SAN ischemia, especially in cases with concomitant cardiac
hypertrophy. In cases of a duplicated SANa, the most common anatomical pattern was one branch arising from the RCA and the other from the LCX (2.0%). It is reasonable
to believe that such bi-coronary blood supply of SAN would prevent ischemia in case of vasoocclusive disease of one of the coronary arteries.
Knowledge about the course of SANa has clinical implications as the precaval course is considered to carry a lower risk of iatrogenic SANa injury during surgical
incisions using the superior septal approach[2,3]. Overall, the most common course of SANa was found to be retrocaval (47.1%), followed by precaval (38.9%) and
pericaval (14.0%). Similar pooled prevalences were found during subgroup analysis on SANa originating from the LCA and the LCX. Interestingly, when SANa originated
from the RCA, the precaval course was found to be the most common (43.1%) followed by retrocaval (38.7%) and pericaval (18.2%) courses. One explanation for this
discrepancy is the anatomical location of the RCA, as it usually branches off the ventral aspect of the aorta. In order for the SANa of the RCA to take the retrocaval
or pericaval course to the SAN, it would require a nearly 180° turn to course posteriorly between the aorta and the SVC (see fig 1). However, further subgroup analysis
revealed this precaval tendency was primarily noted in the cadaveric studies, whereas the imaging studies reported that a SANa from the RCA most commonly followed the
retrocaval course (54.1%). This difference however, was not statistically significant. A retrocaval course was most prevalent in all populations, except North
Americans, which showed a predominance of the precaval course (52.4%). The pericaval course was more often found in Asians (16.1%) than in Europeans (10.9%) and North
Americans (8.7%), and was more often described in imaging than cadaveric studies, both for SANa from the RCA (22.6% vs. 15.0%) and from the LCX (23.8% vs. 12.0%).
Our data shows that S-shaped branches of SANa have a pooled prevalence of only 7.6%, which is lower than what has been reported in previous studies[39,50].
Geographical subgroup analysis showed a pooled prevalence of 10.4% in the European population and 9.5% in the North American population, while their prevalence in the
Asian population was significantly lower (1.3%). Such S-shaped branches are of clinical significance as their long course makes them susceptible to iatrogenic damage
during Cox maze operation for atrial fibrillation[50,75]. As such, cardiac surgeons must be aware of their existence and take special care in their presence to avoid
iatrogenic injury.
The mean diameter of SANa arising from RCA was 1.275 mm at its origin, and 1.578 mm within SAN, the latter being larger than what has been reported previously[45]. In
cases of SANa arising from LCX, the pooled mean diameter was larger (1.376 mm), but this was not statistically significant.
This meta-analysis provides solid, evidence-based description of the anatomical characteristics and variations of the SANa, which are essential to avoid iatrogenic
injury during interventional cardiology (including ablation) and cardiosurgery (particularly open heart surgeries through the right atrium) procedures. Iatrogenic SANa
injury, such as intra-operative damage during mitral valve surgery via the superior septal approach[76], may lead to additional postoperative arrhythmias[2,3].
Our meta-analysis was limited by the poor quality of some of the included studies, the high heterogeneity between studies, and the lack of an available proper quality
assessments for anatomical meta-analysis. Furthermore, due to the lack of an available proper measure for multi-categorical prevalence, no publication bias assessment
was performed. In an attempt to minimize bias in our analysis, we tried to contact authors when possible in order to address questions regarding the data of the
included studies.
Despite subgroup analysis by type of study and geographical distribution, as well as a sensitivity analysis, high heterogeneity persisted throughout the meta-analysis
without any clear identifiable source. We suspect that the primary source of the heterogeneity is the highly variable anatomical nature of the SANa itself. As such, we
highly recommend physicians take care to identify the location and anatomical characteristics of the SANa prior to or during procedures, to reduce the risk of
iatrogenic damage to the vessel.
With the anatomical basis from this meta-analysis, further studies should investigate the role of anatomical variations of SANa in different pathologies, including
supraventricular tachycardias and sick sinus syndrome. Furthermore, studies should also investigate if certain SANa variations makes patients with hypoxemia (i.e.
COPD, pneumonia) more susceptible to development of atrial fibrillation.

5. CONCLUSION
Although the anatomical characteristics of the SANa are highly variable, our comprehensive evidence-based assessment found that the most common form of the artery can
be described as a single vessel, originating from the RCA, and taking a retrocaval course to reach the SAN. However, due to its high variability, physicians should be
cautious in identifying the anatomy of the SANa in individual patients before or during interventional cardiology or cardiosurgical procedures. Furthermore, special
attention should be paid to the relatively common S-shaped branch of the SANa which exposes it to a particularly high risk of iatrogenic injury during such invasive
procedures.

6. ACKNOWLEDGEMENTS
This project was funded using the statutory funds of the Jagiellonian University Medical College, Krakow, Poland.
The authors would like to thank Karolina Saganiak for the anatomical drawings used in this manuscript.

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TABLE LEGENDS
Table 1. Characteristics of the included studies.
Table 2. Number of the sinoatrial nodal artery (SANa) in different population subgroups.
Table 3. Origin of the sinoatrial nodal artery (SANa) in different population subgroups.
Table 4. Course of the sinoatrial nodal artery (SANa) in different population subgroups.
Table 5. S-shaped branches of the sinoatrial nodal artery (SANa) in different population subgroups.
Table 6. Distance between the origin of the sinoatrial nodal artery (SANa) and the ostia of the right coronary artery (RCA) and the left circumflex artery (LCX) in
different population subgroups.
Table 7. Diameter of the sinoatrial nodal artery (SANa) in different population subgroups.
FIGURE LEGENDS
Figure 1. The various origins of the sinoatrial nodal artery (SANa).
A. From the Right Coronary Artery; B. From the Left Circumflex Artery (proximal); C. From the Left Circumflex Artery (distal); D. From the Left Coronary Artery; E.
From the Aorta; F. Dual origin from the Right Coronary Artery and the Left Circumflex Artery.
LA – Left Atrium; RA – Right Atrium; SVC – Superior Vena Cava; Ao – Aorta; P – Pulmonary Trunk; RCA – Right Coronary Artery; LCA – Left Coronary Artery; LCX – Left
Circumflex Artery; LAD – Left Anterior Descending Artery; SANa – Sinoatrial Nodal Artery.
Figure 2. The various courses of the sinoatrial nodal artery (SANa).
A. Retrocaval course; B. Pericaval course; C. Precaval course.
LA – Left Atrium; RA – Right Atrium; SVC – Superior Vena Cava; Ao – Aorta; P – Pulmonary Trunk; RCA – Right Coronary Artery; LCA – Left Coronary Artery; LCX – Left
Circumflex Artery; LAD – Left Anterior Descending Artery; SANa – Sinoatrial Nodal Artery.

Figure 3. Flow chart of study identification, evaluation and inclusion in the meta-analysis.

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