Anatomy, Thorax, Triangle of Auscultation

Structure and Function

The auscultation triangle lies at the inferior angle of the scapula. This region has inferior, medial, lateral, and anterior boundaries.[2]

• Inferior boundary - superior border of the latissimus dorsi muscle
• Medial boundary - lateral margin of the inferior aspect of the trapezius muscle
• Lateral boundary - lower aspect of the scapula's medial border.
• Anterior boundary (floor) - rhomboid major muscle, fascial sheet, 6th and 7th ribs, and intercostal space

The auscultation triangle can be made prominent by instructing the patient to cross their arms over the chest before bending forward. Upper extremity adduction displaces the scapula and the triangle's muscular boundaries, reducing the sound barriers between the examiner's stethoscope and the thoracic structures under evaluation. Wheezing, rhonchi, rales, and other adventitious respiratory sounds are easier to distinguish in this region.[3][4][5]

Embryology

Skeletal muscle formation occurs through the process of myogenesis.[6] The muscle cells originate from the paraxial mesoderm. This layer forms the somites, which later turn into the dermomyotomes and myotomes. 

Skeletal muscle development is divided into the myoblast, myotubule, and myofiber stages. Myoblasts, the skeletal muscle cell progenitors, form large, multinucleated cells under the influence of fibroblast growth factors. Myoblasts initially divide and fuse continuously, but the secretion of extracellular matrix proteins heralds their maturation into myotubules. Myofibers form from myotubule fusion. Afterward, they stop dividing as fibroblast growth factors diminish. Myosatellite cells are multipotent cells persisting along the basal membrane that can give rise to new muscle tissue when activated.[7][8][9]

Blood Supply and Lymphatics

The triangle's blood supply is the same as that of its bounding structures.

• Thoracodorsal artery: supplies the latissimus dorsi muscle.[10]
• Superficial cervical artery: supplies the trapezius muscle.[11]
• Dorsal scapular artery: supplies the rhomboid major muscle.[12] 

These arteries ensure adequate oxygenation and nutrient supply for the muscles in this region.

Nerves

The auscultation triangle also has the same innervation as that of its bounding structures.

• Thoracodorsal nerve - This nerve is also known as the middle subscapular nerve, and it innervates the latissimus dorsi.
• Accessory nerve (cranial nerve XI) and the C3 to C4 spinal nerves - These nerves supply the trapezius muscle.[13]
• Dorsal scapular nerve: This nerve supplies the rhomboid major.[14]

The triangle of auscultation does not have its own innervation, but the nerves above supply the surrounding structures for sensorimotor function.

Muscles

The auscultation triangle itself lacks significant muscle mass, although muscles form its borders. These muscles include the following: 

• Lattisimus dorsi - This large muscle at the triangle's inferior border originates from the T7 to L5 spinous processes, thoracolumbar fascia, inferior angle of the scapula, lower ribs, and iliac crest. The latissimus dorsi inserts on the floor of the bicipital (intertubercular) groove of the humerus. The muscle adducts, medially rotates, and extends the arm at the glenohumeral joint.
• Trapezius - This muscle at the superior border of the triangle originates from the C7 to T12 spinous processes, occipital protuberance, ligamentum nuchae, and medial superior nuchal line. The trapezius inserts on the lateral clavicle, scapular spine, and acromion. The trapezius muscle is divided into the superior, middle, and inferior divisions. The superior fibers extend the neck and elevate and superiorly rotate the scapula. The middle fibers adduct the scapula. The inferior fibers stabilize and help rotate the scapula.[15]
• Rhomboid major - This muscle originates from the T2 to T5 spinous processes and inserts on the inner scapular border below the rhomboid minor. The rhomboid major retracts and rotates the scapula.[16]

These muscles move the arm, neck, and various regions of the thorax. Bending forward with crossed arms pulls the muscles away from the triangle, making auscultation easier.

Physiologic Variants

Anatomic variations differ based on which lung segment the auscultation triangle projects on from the surface. Cadaver studies reveal that the triangle can project on either the superior segment of the inferior lobe, posterior basal segment, or lateral basal segment. The projection was mirrored bilaterally in half of the cases.[3]

Surgical Considerations

The triangle of auscultation has advantages when used as an access route for various surgical procedures due to the absence of large muscular structures within the area. These procedures include:

• Scapular tumor excision - The triangle provides direct access to tumors originating from the ventral aspect of the scapula, particularly near the inferior angle. Osteochondromas have been excised through this window. The approach minimizes blood loss since the rhomboid major and trapezius are not dissected.
• Thoracotomy - A thoracotomy through the triangle of auscultation permits access to internal thoracic structures such as the lungs, pulmonary vasculature, heart, or aorta. This procedure may be performed emergently to evacuate a pericardial tamponade, control thoracic hemorrhage, administer internal cardiac massage, or cross-clamp the descending thoracic aorta. Access through the auscultation triangle reduces morbidity, as it dispenses the need for a latissimus dorsi incision.[17] 
• Pleurectomy - A pleurectomy with access via the triangle reduces the need for a painful thoracotomy procedure. Recurrent spontaneous pneumothorax is a pleurectomy indication.[18]
• Rhomboid intercostal nerve block - A rhomboid intercostal nerve block can also be administered through this region to alleviate pain from rib fractures or a thoracotomy. Typically, this nerve block is accomplished by injecting a local anesthetic agent into the fascial plane between the upper intercostal portion of the rhomboids.[19]

Additionally, awareness of the triangle of auscultation is essential in patient positioning, as it can serve as a reference point for avoiding unnecessary back pressure or trauma during surgery.

Clinical Significance

The triangle of auscultation offers several advantages when performing thoracic auscultation and surgical procedures. The thinness of the structures in this region allows better acoustics and access to internal thoracic structures, as discussed in the previous sections. Knowledge of the anatomy of this region can help improve diagnostic accuracy and surgical safety.