The Azygos Lobe

Medical Terminology Daily (MTD) is a blog sponsored by Clinical Anatomy Associates, Inc. as a service to the medical community. We post anatomical, medical or surgical terms, their meaning and usage, as well as biographical notes on anatomists, surgeons, and researchers through the ages. Be warned that some of the images used depict human anatomical specimens.

You are welcome to submit questions and suggestions using our "Contact Us" form. The information on this blog follows the terms on our "Privacy and Security Statement" and cannot be construed as medical guidance or instructions for treatment.

A Moment in History

Jean George Bachmann
(1877 – 1959)

French physician–physiologist whose experimental work in the early twentieth century provided the first clear functional description of a preferential interatrial conduction pathway. This structure, eponymically named “Bachmann’s bundle”, plays a central role in normal atrial activation and in the pathophysiology of interatrial block and atrial arrhythmias.

As a young man, Bachmann served as a merchant sailor, crossing the Atlantic multiple times. He emigrated to the United States in 1902 and earned his medical degree at the top of his class from Jefferson Medical College in Philadelphia in 1907. He stayed at this Medical College as a demonstrator and physiologist. In 1910, he joined Emory University in Atlanta. Between 1917 -1918 he served as a medical officer in the US Army. He retired from Emory in 1947 and continued his private medical practice until his death in 1959.

On the personal side, Bachmann was a man of many talents: a polyglot, he was fluent in German, French, Spanish and English. He was a chef in his own right and occasionally worked as a chef in international hotels. In fact, he paid his tuition at Jefferson Medical College, working both as a chef and as a language tutor.

The intrinsic cardiac conduction system was a major focus of cardiovascular research in the late nineteenth and early twentieth centuries. The atrioventricular (AV) node was discovered and described by Sunao Tawara and Karl Albert Aschoff in 1906, and the sinoatrial node by Arthur Keith and Martin Flack in 1907.

While the connections that distribute the electrical impulse from the AV node to the ventricles were known through the works of Wilhelm His Jr, in 1893 and Jan Evangelista Purkinje in 1839, the mechanism by which electrical impulses spread between the atria remained uncertain.

In 1916 Bachmann published a paper titled “The Inter-Auricular Time Interval” in the American Journal of Physiology. Bachmann measured activation times between the right and left atria and demonstrated that interruption of a distinct anterior interatrial muscular band resulted in delayed left atrial activation. He concluded that this band constituted the principal route for rapid interatrial conduction.

Subsequent anatomical and electrophysiological studies confirmed the importance of the structure described by Bachmann, which came to bear his name. Bachmann’s bundle is now recognized as a key determinant of atrial activation patterns, and its dysfunction is associated with interatrial block, atrial fibrillation, and abnormal P-wave morphology. His work remains foundational in both basic cardiac anatomy and clinical electrophysiology.

Sources and references
1. Bachmann G. “The inter-auricular time interval”. Am J Physiol. 1916;41:309–320.
2. Hurst JW. “Profiles in Cardiology: Jean George Bachmann (1877–1959)”. Clin Cardiol. 1987;10:185–187.
3. Lemery R, Guiraudon G, Veinot JP. “Anatomic description of Bachmann’s bundle and its relation to the atrial septum”. Am J Cardiol. 2003;91:148–152.
4. "Remembering the canonical discoverers of the core components of the mammalian cardiac conduction system: Keith and Flack, Aschoff and Tawara, His, and Purkinje" Icilio Cavero and Henry Holzgrefe Advances in Physiology Education 2022 46:4, 549-579.
5. Knol WG, de Vos CB, Crijns HJGM, et al. “The Bachmann bundle and interatrial conduction” Heart Rhythm. 2019;16:127–133.
6. “Iatrogenic biatrial flutter. The role of the Bachmann’s bundle” Constán E.; García F., Linde, A.. Complejo Hospitalario de Jaén, Jaén. Spain
7. Keith A, Flack M. The form and nature of the muscular connections between the primary divisions of the vertebrate heart. J Anat Physiol 41: 172–189, 1907.

"Clinical Anatomy Associates, Inc., and the contributors of "Medical Terminology Daily" wish to thank all individuals who donate their bodies and tissues for the advancement of education and research”.

Click here for more information

Details: Written by: Prof. Claudio R. Molina, MSc; Published: September 24, 2018; Hits: 6688

This article was written by Claudio Rodrigo Molina, Josefa Catalán Lobo, and Carolina Becker Fehlandt. We thank them for their contribution to Medical Terminology Daily.

Lateral view

The azygos lobe, also commonly referred to as the" accessory lobe of the azygos vein", was first described in 1877 by Heinrich Wrisberg. It is seen in 0,4 % of chest X-rays; 1,5% in high resolution CT, and 1% in anatomical dissections. It is not a true accessory pulmonary lobe as it does not have its own bronchus and does not correspond to a specific bronchopulmonary segment, the “azygos lobe”, is located at the apicomedial portion of the upper right lobe and it separated from the remainder of the upper lobe by a fissure (Denega et al, 2015; Özdemir et al, 2017)

A convex-shaped fissure is created by the course of the vein bearing towards the medial side of the right lung to join with the superior vena cava. Its formation is a result of an unusual embryogenic migration of the posterior cardinal vein; which is a precursor of the azygos vein (Denega et al, 2015).

Instead of sliding over the lung medially, the vein invaginates into the parenchyma of the lung and becomes enveloped by layer of pleural folds, forming a mesentery-like structure, also called “mesoazygos”. Further migration into the lung as it passes towards the right hilum creates a convex semicircular fissure with the vein located at the base of the fissure. This fissure can be identified in an X-ray chest image as a coma-shaped (“Teardrop sign”) or curved linear shadow in the paramediastinal region of the right lung; it terminates at the level of second costal cartilage (Akhtar et al, 2018; Caceres et al, 1993). The lower portion of the azygos fissure is teardrop-shaped and its contains the azygos vein (Özdemir et al, 2017; Kotovet al, 2017).

Instead of sliding over the lung medially, the vein invaginates into the parenchyma of the lung and becomes enveloped by layer of pleural folds, forming a mesentery-like structure, also called “mesoazygos”. Further migration into the lung as it passes towards the right hilum creates a convex semicircular fissure with the vein located at the base of the fissure. This fissure can be identified in an X-ray chest image as a coma-shaped (“Teardrop sign”) or curved linear shadow in the paramediastinal region of the right lung; it terminates at the level of second costal cartilage (Akhtar et al, 2018; Caceres et al, 1993). The lower portion of the azygos fissure is teardrop-shaped and its contains the azygos vein (Özdemir et al, 2017; Kotovet al, 2017).

Images provided by the authors.
Click on the image for a larger depiction

The pathway of the vein within the lung is subject to individual variation, and this defines the position of the fissure within the apex of the upper lobe. The most superior portion of the fissure adopts a triangular form, called the trigone. The localization of the trigone determinates the size of the azygos lobe (Caceres et al, 1993; Fuad & Mubarak, 2016 ). A Left azygos lobe has been reported, but it is extremely rare (Özdemir et al, 2017)

Diagnosis of the azygos lobe may be complicated by morphologic variants of the fissure, physiological changes in the size of vein, and the projection of additional shadows within the lobe which may be misinterpreted as scar tissue, a calcified area of a post-infection process, or a malignant tissue or nodule.

It usually has no clinical implications and is an incidental finding in images but the azygos vein may undergo physiological variations, reflected by changes in the size of its shadow and its position in the imaging studies. Expiration, the Valsalva maneuver, or the upright position effect on the venous return to the heart, may enhance the size of the vein and its shadow. Changes in intrathoracic pressure may result in the “empty azygos fissure” phenomenon, in which the medial displacement of the azygos vein occurs after the reexpansion of the collapsed lung, secondary to pneumothorax or pleural effusion as well as a shortened mesoazygos.

In rare cases the azygos veins may undergo variceal changes that are usually located in the arc of the vein. They remained asymptomatic or may be accompanied by a “pressure like” or “tightness” sensation within the chest, recurrent hemoptysis with bright red blood dry cough and dyspnea. The initial differential diagnosis includes acute myocardial infarction, aortic dissection and myocarditis. On the chest X-ray it may present as a round or oval paratracheal shadow with a smooth surface or outline. Untreated, it may predispose the patient to the risk of rupture, thrombosis, or pulmonary embolism. Azygos thrombosis is extremely rare and most cases in the literature had an undelaying azygos dilation or some prothrombotic status like malignancy. In all lung tissue some pathological process can originate in the azygos lobe as bullous, bronchiectatic changes, pneumonia and tuberculosis. (Kotov et al, 2018; Denega et al, 2015)

Otherwise, it seems the mesoazygos fold serve as a barrier to dissemination of the infection or malignant cells.

For thoracoscopic procedures, recognition of the azygos lobe is particularly important as it can cause partial obstruction of the surgical site view during thoracoscopic sympathectomy. In the literature, two cases have been reported where the phrenic nerve was coursing within the azygos fissure (Kauffman et al, 2010; Pradhan, 2017; Özdemir et al, 2017; Paul, Siba & James, 2018)

Thoracic surgeons as well as treating physicians need to be aware of this rare anatomical variation.

NOTE: For an explanation of the etymology of the word "azygos" click here.

Sources:

1. Akhtar, Jamal & Lal, Amos & B. Martin, Kevin & Popkin, Joel. (2018). Azygos lobe: A rare cause of right paratracheal opacity. Respiratory Medicine Case Reports. 23. 10.1016/j.rmcr.2018.02.001.
2. Caceres, Jennelyn & Mata, Jonathan & Alegret, X & Palmer, J & Franquet, T. (1993). Increased density of the azygos lobe on frontal chest radiographs simulating disease: CT findings in seven patients. AJR. American journal of roentgenology. 160. 245-8. 10.2214/ajr.160.2.8424325.
3. Denega T, Alkul S, Islam E, Alalawi R. (2015). Recurrent hemoptysis - a complication associated with an azygos lobe. The Southwest Respiratory and Critical Care Chronicles, [S.l.], v. 3, n. 11, p. 44-47. ISSN 2325-9205.
4. Fuad, A.R., & Mubarak (2016). Two Cases of Azygos Lobe with Normal and Aneurysmal Azygos Vein on Computed Tomography. Int J Anat Res 2016, Vol 4(1):1843-45. ISSN 2321-4287
5. Kauffman, Paulo & Wolosker, Nelson & De Campos, José Ribas & Yazbek, Guilherme & Biscegli Jatene, Fábio. (2010). Azygos Lobe: A Difficulty in Video-Assisted Thoracic Sympathectomy. The Annals of thoracic surgery. 89. e57-9. 10.1016/j.athoracsur.2010.03.030.
6.Kotov G, Dimitrova I N, Iliev A, et al. (2018). A Rare Case of an Azygos Lobe in the Right Lung of a 40-year-old Male. Cureus 10(6): e2780. doi:10.7759/cureus.2780

The Azygos Lobe

Recent