Atlas Contents
Fluoroscopic anatomy correlation
Fluoroscopy converts the three-dimensional arrangement of the heart into a succession of two-dimensional silhouettes. The image is useful only when the operator mentally restores depth and relates catheter position to the orientation of the chambers, valves, septal structures, vessels, and extracardiac tissues. This reconstruction is strengthened by using attitudinally appropriate terminology and by comparing at least two projections. A catheter that appears to occupy the same point in one view may lie in a different chamber or on a different surface when examined in an orthogonal view. [1-4]
The following account correlates the frontal, right anterior oblique, and left anterior oblique views with anatomy as it is situated in the upright human body. It is an anatomical framework for interpretation, not a substitute for electrograms, angiography, echocardiography, computed tomography, or electroanatomical mapping when these are required.
1 Attitudinal nomenclature
Human anatomy is described with the subject standing upright and facing the observer. Superior and inferior therefore refer to the head-to-foot axis; anterior and posterior to the sternum-to-spine axis; and right and left to the patient’s own sides. Much of the traditional cardiac vocabulary was created as though the heart had been removed from the thorax and placed on its apex. This convention produced familiar but anatomically discordant labels. The artery traditionally called the posterior descending artery, for example, courses within the inferior interventricular groove. Likewise, several sites formerly called anterior, posterior, or septal at the atrioventricular junctions are respectively superior, inferior, or paraseptal when the heart is considered in situ. [1-3]
Attitudinal terminology is more than a linguistic preference. Fluoroscopy, computed tomography, magnetic resonance imaging, and surgical or pathological views must describe the same spatial reality. Consistent terms reduce the mental rotation needed to translate between modalities. Historical names remain embedded in electrocardiography and clinical practice, so a sensible description may give the attitudinal term first and acknowledge the traditional equivalent when needed. The purpose is clarity, not the retrospective renaming of published observations.
Three corrections are particularly important for electrophysiology. First, the right ventricle is predominantly anterior, not simply rightward; the left ventricle is mainly leftward and posterior. Second, the left atrium is the most posterior cardiac chamber, while the right atrium forms much of the right border of the frontal silhouette. Third, most of the area described clinically as septal at the atrioventricular junction is paraseptal. True septal tissue must separate two cardiac cavities; an atrioventricular groove filled with fibrofatty tissue does not meet that definition. [1,2,5]
2 Spatial relationships of the chambers
In a frontal view, the right atrium lies to the right, receiving the superior and inferior caval veins. The left atrium lies behind the right atrium and the aortic root; only the left atrial appendage normally contributes to the left cardiac border. The right ventricle occupies most of the anterior cardiac surface and wraps around the left ventricle. Its inlet is inferior and rightward, its apical trabecular component extends anteriorly, and its outlet ascends superiorly and leftward towards the pulmonary trunk. The left ventricle forms the left border, apex, and much of the inferior surface, with its outflow directed superiorly and rightward towards the aortic root. [1,2]
The ventricular septum is oblique and curved rather than a flat sagittal partition. The right ventricle remains anterior to it, while the left ventricular cavity lies posteriorly and leftward. From its basal region the muscular septum extends towards the apex in an anterior, inferior, and leftward direction. The superior part of the septum is not the most anterior part; the apical septum lies farther anteriorly. This geometry explains why a catheter apparently positioned centrally in a frontal projection may still be markedly anterior or posterior in an oblique view. [2]
The atrial septal plane is also oblique. The true septal component is largely the oval fossa and its immediate muscular rim. Much of the apparently broad partition seen from the right atrium is an infolding between atrial walls rather than a true septum. The fossa is located posteriorly relative to the aortic root and is approached from the right atrium in a direction that depends on caval entry, atrial size, and thoracic rotation. The coronary sinus occupies the inferior atrioventricular groove and its ostium opens into the inferior right atrium between the inferior caval orifice and the septal tricuspid hinge.
Fluoroscopic interpretation is therefore relational. A catheter along the right cardiac border may be in the right atrium, superior caval vein, or lateral tricuspid junction depending on its course and depth. A posterior catheter may lie in the coronary sinus, left atrium, descending aorta, or oesophagus. The expected trajectory from the access vein, the catheter curve, contrast information, and recordings must be integrated rather than assigning location from the tip alone.
3 Fluoroscopic projections
The frontal or anteroposterior projection provides an initial overview of the caval axes, right atrial border, ventricular silhouette, spine, diaphragm, and catheter progression. It is useful for recognising gross right-left displacement and for following entry into the right atrium, right ventricle, pulmonary artery, aorta, or coronary sinus. Its principal limitation is superimposition: anterior and posterior structures can project onto the same line.
In the right anterior oblique projection, rotation separates anterior from posterior relationships while retaining a useful superior-inferior axis. This view profiles the long axes of the atria and ventricles and helps distinguish an anterior right ventricular position from a posterior coronary sinus or left atrial position. It displays the triangle of Koch, the inferior right atrial isthmus, and the course of the coronary sinus in a manner that is readily correlated with sagittal anatomy. The tricuspid and mitral junctions are seen more nearly in profile than in the left anterior oblique projection. [2]
The left anterior oblique projection rotates the heart so that the right and left atrioventricular junctions are viewed more nearly en face and the ventricular septum is approached tangentially. It is particularly useful for assigning a catheter to a sector of the tricuspid or mitral circumference, assessing whether it lies rightward or leftward of the septal plane, and following the coronary sinus around the left atrioventricular groove. The inferior caval vein, tricuspid junction, and right ventricular inflow can overlap, but the projection clarifies circumferential position. [2]
No fixed angle is anatomically universal. Rotations around 30 to 45 degrees are commonly used, but cardiac axis, scoliosis, diaphragmatic level, chamber enlargement, and congenital malposition alter the view. Cranial or caudal angulation can reduce overlap for specific targets, yet the labels RAO and LAO describe the direction of the X-ray beam rather than a guaranteed anatomical plane. The operator should judge what structures have actually been separated in the individual patient.
Two projections provide parallax and are safer than a single view for three-dimensional localisation. Movement from RAO to LAO can reveal whether a tip is anterior or posterior, rightward or leftward, or on the atrial or ventricular aspect of a junction. Modern electroanatomical mapping reduces radiation and adds geometry, but its reconstructed surfaces are only as accurate as point acquisition and chamber assignment. Fluoroscopy and cross-sectional imaging remain valuable independent checks.
4 The right and left atrioventricular junctions
The right atrioventricular junction is the muscular and fibrous boundary supporting the tricuspid valve. It is not a circular ring. The hinge line of the tricuspid leaflets varies in level, and the septal leaflet is displaced apically relative to the mitral hinge. In the left anterior oblique view, the junction is displayed around much of its circumference. Attitudinal sectors progress through right superior, superoanterior, anterior, anteroinferior, and right inferior regions, with paraseptal regions at its medial limits. Sites traditionally called right anterior are often superior, whereas those called right lateral are largely rightward and anterior. [2,4]
The left atrioventricular junction supports the mitral valve and is discontinuous where the aortic and mitral leaflets are in fibrous continuity. Potential atrioventricular myocardial connections are therefore mainly related to the mural mitral leaflet. In attitudinal orientation, the mural junction extends through left superior, posterosuperior, posterior, posteroinferior, and left inferior sectors. The old term left lateral corresponds principally to a posterior location. The coronary sinus and great cardiac vein follow the epicardial aspect of the left junction but their distance from the hinge and ventricular myocardium varies.
The septal and paraseptal junctions demand particular precision. The superior paraseptal region includes peri-Hisian and adjacent cristal connections. The mid-paraseptal region lies beside the triangle of Koch on the right and the subaortic septal area on the left. Right inferior paraseptal connections are approached from the inferior right atrial aspect outside the coronary sinus; left inferior paraseptal connections occupy the corresponding left-sided junction. Connections approached within the coronary sinus or a tributary, usually the middle cardiac vein, are appropriately termed inferior paraseptal. These categories describe anatomical relationships and should not be collapsed into the traditional label posteroseptal. [2,6]
Fluoroscopic sector assignment requires both oblique views. The left anterior oblique projection estimates circumferential position around each junction. The right anterior oblique view establishes whether the catheter lies superiorly or inferiorly and anteriorly or posteriorly, and whether it is following an atrial, ventricular, or venous course. Intracardiac electrograms identify local myocardium and conduction tissue, while coronary angiography may be required when energy is delivered close to an epicardial coronary artery.
5 The inferior paraseptal region and pyramidal space
The inferior paraseptal region is not a discrete point and is not a true atrioventricular septum. It is a three-dimensional junctional complex centred on the inferior atrioventricular groove, where the right and left atrial vestibules, the hinges of the septal tricuspid and inferior mitral leaflets, the coronary sinus ostium, the middle cardiac vein, the ventricular septal summit, and the central fibrous body come into close spatial relation. Its extracardiac core is the inferior pyramidal space. [2,5,6]
The pyramidal space is a wedge of fibrofatty tissue that extends from the inferior atrioventricular groove towards the central fibrous body. Its base is related to the crux of the heart and contains the coronary sinus and proximal portions of its tributaries, including the middle cardiac vein. Its rightward wall is formed by the inferior right atrial vestibule and triangle of Koch; its leftward relation is the inferior left atrial vestibule and mitral junction; and its deeper boundary abuts the crest or summit of the muscular ventricular septum. The space penetrates between the basal ventricular masses, sometimes described morphologically as the buttresses of the ventricles. It contains fat, vessels, nerves, and atrial or transitional myocardial extensions, rather than a muscular septum separating right atrium from left ventricle. [2,6]
This distinction resolves a recurrent misconception. In the triangle of Koch, right atrial myocardium overlies fibrofatty tissue, which in turn overlies the ventricular septal crest. The arrangement is an atrioventricular sandwich. A direct passage from the right atrial cavity towards the left ventricular cavity through this region would leave the cardiac cavities and traverse the fibrofatty pyramidal space. Consequently, pathways in this vicinity are generally paraseptal, even when their atrial and ventricular insertions appear aligned in a fluoroscopic projection.
The compact atrioventricular node lies near the apex of the triangle of Koch and gives rise to the penetrating atrioventricular bundle at the central fibrous body. Inferior nodal extensions can descend towards the coronary sinus vestibule. The atrioventricular nodal artery commonly courses through the pyramidal space and may pass close to the coronary sinus ostium and right atrial endocardium. In the anatomical study by Sánchez-Quintana and colleagues, it lay 0.5 to 5 mm from the coronary sinus mouth in 75% of specimens, and the mean distance from the right atrial endocardium at the base of Koch’s triangle was 3.5 plus or minus 1.5 mm. These relationships explain why ablation near the base of the triangle can endanger both conduction tissue and its arterial supply. [6]
On fluoroscopy, the coronary sinus ostium and a catheter within the middle cardiac vein act as venous markers of the inferior paraseptal region, but neither identifies the precise location of an accessory connection. The right anterior oblique projection shows the inferior and posterior depth of the venous course relative to the tricuspid hinge and septal profile. The left anterior oblique projection helps distinguish right-sided, intravascular, and left-sided approaches around the crux. A catheter inside the middle cardiac vein can be separated from one on the endocardial right or left junction only by integrating both views, contrast venography when indicated, electrograms, and the response to pacing.
The terms right inferior paraseptal, inferior paraseptal, and left inferior paraseptal should therefore be retained as distinct anatomical descriptions. The first indicates a right-sided insertion outside the coronary sinus; the second is appropriate for connections approached within the coronary sinus or middle cardiac vein; and the third denotes the left-sided counterpart. The designation posteroseptal obscures both the inferior location and the fact that much of the region is extracardiac and paraseptal.
Finally, the pyramidal space should not be conflated with the inferoseptal recess or with the inferoseptal process of the left ventricle. These names refer to related but different morphological concepts. The inferoseptal recess is a left ventricular recess between the aortic leaflet of the mitral valve and the septal surface of the left ventricle; an area of mitral-to-tricuspid fibrous continuity forms its roof. The inferoseptal process is the adjacent basal left ventricular myocardial prominence. By contrast, the inferior pyramidal space is the extracardiac fibrofatty wedge within the inferior atrioventricular junctional complex. Their proximity makes careful definitions essential, particularly when correlating sectional imaging, fluoroscopy, and specimens. [7]
Key Anatomical Messages
- Describe cardiac position using the anatomical position of the intact body.
- The right ventricle is predominantly anterior and the left atrium predominantly posterior.
- RAO and LAO projections provide complementary rather than interchangeable information.
- Most clinically named septal atrioventricular sites are anatomically paraseptal.
- The inferior pyramidal space is extracardiac fibrofatty tissue and not a muscular atrioventricular septum.
- Right inferior paraseptal inferior paraseptal and left inferior paraseptal pathways describe distinct relationships.
- The AV node nodal extensions nodal artery coronary sinus and middle cardiac vein are all relevant near the pyramidal space.
References
- Anderson RH, Spicer DE, Hlavacek AM, Hill A, Loukas M. Describing the cardiac components: attitudinally appropriate nomenclature. J Cardiovasc Transl Res. 2013;6:118-123. doi:10.1007/s12265-012-9434-z.
- Farré J, Anderson RH, Cabrera JA, et al. Cardiac anatomy for the interventional arrhythmologist: I. Terminology and fluoroscopic projections. Pacing Clin Electrophysiol. 2010;33:497-507. doi:10.1111/j.1540-8159.2009.02644.x.
- Cosío FG, Anderson RH, Kuck KH, et al. Living anatomy of the atrioventricular junctions. A guide to electrophysiologic mapping. Circulation. 1999;100:e31-e37. doi:10.1161/01.CIR.100.5.e31.
- Anderson RH, Ho SY. Anatomy of the atrioventricular junctions with regard to ventricular preexcitation. Pacing Clin Electrophysiol. 1997;20:2072-2076. doi:10.1111/j.1540-8159.1997.tb03631.x.
- Cabrera JA, Sánchez-Quintana D. Cardiac anatomy: what the electrophysiologist needs to know. Heart. 2013;99:417-431. doi:10.1136/heartjnl-2011-301154.
- Sánchez-Quintana D, Ho SY, Cabrera JA, Farré J, Anderson RH. Topographic anatomy of the inferior pyramidal space: relevance to radiofrequency catheter ablation. J Cardiovasc Electrophysiol. 2001;12:210-217. doi:10.1046/j.1540-8167.2001.00210.x.
- Cabrera JA, Anderson RH, Porta-Sánchez A, et al. The atrioventricular conduction axis and its implications for permanent pacing. Arrhythm Electrophysiol Rev. 2021;10:181-189. doi:10.15420/aer.2021.32.
