Atlas Contents
Extracardiac structures
The heart cannot be approached as an isolated organ. The oesophagus, phrenic nerves, and vagus nerves lie outside the cardiac chambers but enter the anatomical field of catheter ablation, epicardial intervention, surgery, and cross-sectional imaging. Their positions are variable, they may be separated from myocardium by only thin layers of pericardium and fat, and they are not directly visible with routine fluoroscopy. Safe intervention therefore depends on reconstructing their likely three-dimensional course and, when necessary, defining it with complementary imaging or functional testing. [1-4]
1 The esophagus
The oesophagus descends from the neck through the superior and posterior mediastinum before traversing the diaphragm at the oesophageal hiatus. In the upper thorax it lies behind the trachea. Inferiorly it passes behind the left main bronchus and then comes into close relation with the posterior wall of the left atrium. The descending thoracic aorta is usually leftward and posterior, while the vertebral column lies behind the oesophagus. The organ is mobile within the posterior mediastinum and its relationship to the atrium varies between individuals and within the same patient over time. [1,5]
The left atrium is the most posterior cardiac chamber. Its smooth posterior wall is formed between the right and left pulmonary venous junctions, and the oesophagus can be related to the central atrial wall, either pair of pulmonary veins, or the intervening venoatrial junctions. It is therefore misleading to describe a single fixed oesophageal position. Computed tomography and magnetic resonance imaging show that the lumen can shift laterally and that distension changes its shape. A preprocedural image documents one moment rather than a permanently fixed course.
Between the left atrial endocardium and the oesophageal lumen lie the atrial wall, visceral serous pericardium, the oblique sinus or adjacent pericardial reflection, fibrous pericardium, a variable fibrofatty layer, and the oesophageal wall. These layers are not uniform. The posterior atrial myocardium varies in thickness, and the intervening fat may be minimal. Small oesophageal arteries and a peri-oesophageal vagal plexus occupy the connective tissue around the oesophagus. Thus, thermal injury may involve neural and vascular structures as well as the oesophageal wall itself. [1,5]
The oesophageal wall contains mucosa, submucosa, muscular layers, and adventitia; unlike most of the gastrointestinal tract within the abdomen, its thoracic segment has no serosal covering. Blood supply is segmental, with contributions from oesophageal branches of the thoracic aorta and neighbouring arteries. This arrangement and the restricted mediastinal space are relevant to the evolution of thermal injury after left atrial ablation. Damage can progress beyond the procedure, which helps explain why oesophageal ulceration, peri-oesophageal nerve injury, and the rare atrio-oesophageal fistula may present after a delay.
The region at greatest concern is not defined solely by posterior left atrial voltage or by a conventional segment on an electroanatomical map. It is the changing zone where an ablation site, atrial wall, pericardial tissues, oesophageal wall, and peri-oesophageal neurovascular structures align. Temperature monitoring, oesophageal imaging, power and duration strategies, active cooling, or mechanical deviation attempt to manage this relationship, but none changes the underlying anatomical variability. The precise protective strategy belongs to procedural guidance; anatomically, the essential point is that proximity cannot be inferred reliably from fluoroscopy alone. [6]
Imaging the oesophagus also requires care in interpretation. The visible lumen may contain air, fluid, or a temperature probe, but the outer oesophageal wall is the relevant boundary for measuring separation from the atrium. An empty or compressed lumen can underestimate the full extent of the organ. The posterior left atrial wall should therefore be examined together with the complete oesophageal contour and the interposed tissue plane. The close relation may extend over several centimetres rather than being confined to one point behind a pulmonary vein.
2 The phrenic nerves
Each phrenic nerve arises mainly from cervical spinal roots C3 to C5 and descends through the neck on the anterior scalene muscle. After entering the thorax, both nerves pass anterior to the roots of the lungs and descend between the fibrous pericardium and mediastinal pleura, accompanied by the pericardiophrenic vessels. They provide the principal motor supply to the diaphragm and sensory fibres to the fibrous pericardium, mediastinal pleura, and central diaphragmatic region. Injury can therefore produce diaphragmatic weakness or paralysis. [2,3]
The right phrenic nerve enters the thorax near the right brachiocephalic vein and superior caval vein. It usually descends along the right side of the superior caval vein and lateral right atrium before reaching the diaphragm near the inferior caval vein. Its close relationship to the superior cavoatrial junction and right superior pulmonary vein is especially important during superior caval vein isolation, right atrial ablation, and ablation near the right pulmonary venous antra. The nerve remains extracardiac, but only pericardium and a variable amount of fat may separate it from an endocardial energy source. [1,2]
The left phrenic nerve crosses the aortic arch region and descends over the fibrous pericardium related to the left atrial appendage and left ventricular surface. In anatomical studies it frequently overlies the appendage before continuing along the lateral or anterolateral left ventricle to the diaphragm near the cardiac apex. Its course is more variable than a simple straight line and may intersect potential targets for epicardial ventricular tachycardia ablation, left atrial appendage intervention, or surgical access. [1,2]
The nerves are not outlined by routine fluoroscopy. During epicardial procedures, high-output pacing from the intended ablation site can identify functional capture of the phrenic nerve. In endocardial procedures, diaphragmatic contraction during pacing from the superior caval vein or adjacent atrium can similarly define risk. Absence of capture is informative only under adequate pacing conditions and at the tested location. Respiratory movement, catheter displacement, and local tissue thickness must be considered.
Anatomical separation may be created during selected epicardial procedures by introducing fluid, air, or a mechanical barrier into the pericardial space, but displacement is not uniform and must be confirmed functionally and anatomically. The right and left nerves should also be distinguished from the vagus nerves: phrenic nerves run anterior to the lung roots on the lateral pericardium, whereas the vagus nerves pass posterior to the lung roots and contribute branches to the cardiac, pulmonary, and oesophageal plexuses.
Loss of phrenic function is not always immediately obvious from the cardiac field. Unilateral injury may be recognised by reduced diaphragmatic excursion, an elevated hemidiaphragm, dyspnoea, or an abnormal sniff test. Because the nerve course is variable, a previously safe lesion at one level does not establish safety along the remainder of an ablation line. Repeated localisation is anatomically justified whenever the catheter moves towards the caval veins, atrial appendages, pulmonary venous junctions, or lateral ventricular surfaces.
3 The vagus nerve
The vagus nerves carry parasympathetic preganglionic fibres, visceral afferents, and additional fibre populations between the brainstem, thorax, and abdomen. In the superior mediastinum they give cervical and thoracic cardiac branches that join sympathetic cardiac nerves within the superficial and deep cardiac plexuses around the aortic arch, pulmonary trunk, and tracheal bifurcation. The resulting fibres enter the heart along the great vessels and pulmonary veins rather than as a single identifiable cardiac nerve. [3,4,7]
The right vagus nerve enters the thorax beside the trachea and passes posterior to the right lung root. The left vagus crosses the aortic arch, gives rise to the left recurrent laryngeal nerve near the ligamentum arteriosum, and then passes behind the left lung root. Both vagus nerves divide into pulmonary and oesophageal branches. Around the oesophagus these branches form a plexus that reorganises inferiorly into anterior and posterior vagal trunks before entering the abdomen through the oesophageal hiatus.
The peri-oesophageal vagal plexus is clinically relevant because it lies immediately behind the posterior left atrium within the same tissue plane at risk from thermal spread. Injury may impair gastric motility and has been associated with acute gastric hypomotility, pyloric spasm, and delayed gastric emptying after atrial fibrillation ablation. These manifestations reflect damage to extracardiac parasympathetic fibres rather than injury to atrial myocardium. Their occurrence reinforces the need to regard the oesophagus and its nerves as one neurovascular anatomical complex. [1,5,6]
Once cardiac branches reach the heart, vagal and sympathetic fibres intermingle within epicardial fat pads and the intrinsic cardiac nervous system. Ganglionated plexuses are concentrated around the posterior and superior atria, particularly near pulmonary venous junctions, the interatrial groove, the superior cavoatrial region, and the inferior paraseptal area. They contain efferent neurons, afferent elements, and local circuit neurons. Their effects are regional but not exclusive: interconnected plexuses influence sinus nodal function, atrioventricular conduction, atrial refractoriness, and ventricular electrophysiology. [7-9]
The gross appearance of epicardial fat does not reliably identify the microscopic distribution or functional role of a ganglionated plexus. Likewise, a vagal response during ablation does not prove complete or selective modification of one anatomical plexus. Human studies show considerable variation in the number, size, and interconnections of epicardial ganglia. Terminology based on a small set of schematic points can therefore oversimplify a distributed neural network. [8,9]
For electrophysiology, the vagus nerve has two distinct anatomical meanings. First, the extracardiac vagal trunks and peri-oesophageal plexus are structures to protect during left atrial intervention. Second, their cardiac branches merge with sympathetic and intrinsic neural elements that may contribute to arrhythmogenesis and are potential targets for neuromodulation or cardioneuroablation. These concepts should not be conflated: modifying an intrinsic cardiac plexus is not equivalent to ablating the thoracic vagus nerve.
Key Anatomical Messages
- The oesophagus has a variable and mobile relationship to the posterior left atrium and pulmonary venous junctions.
- The intervening atrio-oesophageal plane contains pericardium, fat, vessels, and vagal fibres.
- The right phrenic nerve is closely related to the superior caval vein, right atrium, and right pulmonary veins.
- The left phrenic nerve commonly crosses the left atrial appendage and the lateral left ventricular region.
- Phrenic nerves pass anterior to the lung roots whereas vagus nerves pass posterior to them.
- Cardiac vagal branches merge with sympathetic fibres and the intrinsic cardiac nervous system.
- Ganglionated plexuses form a variable interconnected network rather than isolated anatomical points.
References
- 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, Cabrera JA, Climent V, Farré J, Weiglein A, Ho SY. How close are the phrenic nerves to cardiac structures? Implications for cardiac interventionalists. J Cardiovasc Electrophysiol. 2005;16:309-313. doi:10.1046/j.1540-8167.2005.40759.x.
- Standring S, ed. Gray’s Anatomy. 42nd ed. Elsevier; 2020.
- Anderson RH, Spicer DE, Hlavacek AM, Cook AC, Backer CL. Wilcox’s Surgical Anatomy of the Heart. 4th ed. Cambridge University Press; 2013.
- Sánchez-Quintana D, Cabrera JA, Climent V, Farré J, Mendonça MC, Ho SY. Anatomic relations between the esophagus and left atrium and relevance for ablation of atrial fibrillation. Circulation. 2005;112:1400-1405. doi:10.1161/CIRCULATIONAHA.105.551291.
- Calkins H, Hindricks G, Cappato R, et al. 2017 HRS EHRA ECAS APHRS SOLAECE expert consensus statement on catheter and surgical ablation of atrial fibrillation. Heart Rhythm. 2017;14:e275-e444. doi:10.1016/j.hrthm.2017.05.012.
- Aksu T, Gopinathannair R, Gupta D, Pauza DH. Intrinsic cardiac autonomic nervous system: what do clinical electrophysiologists need to know about the heart brain? J Cardiovasc Electrophysiol. 2021;32:1737-1747. doi:10.1111/jce.15058.
- Armour JA, Murphy DA, Yuan BX, Macdonald S, Hopkins DA. Gross and microscopic anatomy of the human intrinsic cardiac nervous system. Anat Rec. 1997;247:289-298. doi:10.1002/(SICI)1097-0185(199702)247:2<289::AID-AR15>3.0.CO;2-L.
- Pauza DH, Skripka V, Pauziene N, Stropus R. Morphology distribution and variability of the epicardiac neural ganglionated subplexuses in the human heart. Anat Rec. 2000;259:353-382. doi:10.1002/1097-0185(20000801)259:4<353::AID-AR10>3.0.CO;2-R.
