Triangle of Auscultation

The triangle of auscultation is a small triangular region of the posterior thoracic wall where the muscular covering of the thorax is relatively thin. It lies medial to the scapula and is bounded by the trapezius, latissimus dorsi, and the medial border of the scapula. Because fewer thick superficial muscles intervene between the skin and thoracic wall in this region, it provides a useful location for listening to respiratory sounds.

The triangle is most readily identified when the scapula is moved laterally, which enlarges the space between its medial border and the vertebral column. Traditionally, a patient may be asked to fold the arms across the chest and flex the trunk slightly during posterior chest auscultation. This position protracts the scapulae and increases access to the region.^[1][2]

Despite its name, the triangle of auscultation is not an opening through the thoracic wall. Its floor contains muscle, fascia, ribs, and intercostal structures. Its importance comes from the relative reduction in superficial muscular thickness compared with surrounding areas of the upper back.

Location

The triangle of auscultation is located on each side of the upper to middle thoracic back, immediately medial to the scapula. It occupies the interval between the medial scapular border and the superficial muscles extending across the posterior thorax.

In the anatomical position, much of the region is partly obscured by the scapula. Protraction of the scapula moves its medial border laterally and makes the triangle larger and more accessible from the surface.

The triangle overlies the posterior thoracic wall in the region of the lower trapezius and upper latissimus dorsi. Its exact dimensions vary with scapular position and with the size and attachments of the surrounding muscles.

Boundaries

The triangle of auscultation has three principal boundaries formed by two muscles and the scapula.

The boundaries become easier to appreciate when the scapula is protracted. The resulting lateral displacement of the medial scapular border increases the width of the triangle.

Medial Boundary

The medial boundary is formed by the lateral border of the trapezius. Trapezius is a large superficial muscle extending from the posterior skull and midline of the neck and thorax to the clavicle and scapula.

In the lower part of the upper back, the fibers of trapezius pass superolaterally toward the spine of the scapula. The lateral margin of these fibers forms the medial side of the triangle.

Because trapezius is superficial, the triangle marks an area just lateral to its muscular coverage where the posterior thoracic wall is less heavily covered by superficial back muscle.

Inferior Boundary

The inferior boundary is formed by the superior border of latissimus dorsi. Latissimus dorsi is a broad superficial muscle covering much of the lower back and extending superiorly toward the humerus.

Its upper border approaches but does not completely fill the interval between the lower trapezius and medial scapular region. The remaining space contributes to formation of the triangle of auscultation.

The precise superior extent of latissimus dorsi varies, so the dimensions of the triangle are not identical among individuals.

Lateral Boundary

The lateral boundary is formed by the medial border of the scapula. This border extends from the superior angle of the scapula to the inferior angle and provides attachment for several muscles, including the rhomboids and serratus anterior.

Unlike the muscular boundaries, the scapular border is part of a mobile bone. Its position relative to trapezius and latissimus dorsi therefore changes substantially during movements of the shoulder girdle.

Protraction moves the medial border of the scapula away from the vertebral column and enlarges the triangle. Retraction brings the scapula medially and reduces the accessible area.

Floor

The floor of the triangle is formed principally by the rhomboid major, which lies deep to trapezius and extends from the thoracic spinous processes to the medial border of the scapula. Deeper to the rhomboid are the ribs, intercostal muscles, and associated thoracic wall structures.

The rhomboid major is much thinner than the combined superficial muscular layers present over many neighboring regions of the back. Consequently, even though muscle remains between the skin and ribs, the overall muscular covering is relatively thin.

Deep to the muscular floor lie the bony and intercostal components of the posterior thoracic wall. The parietal pleura and lung are located still deeper, internal to the thoracic cage.

Layers

From superficial to deep, the structures in the region can be considered approximately as follows:

1. Skin
2. Superficial fascia
3. Rhomboid major
4. Thoracic wall, including ribs and intercostal muscles
5. Endothoracic fascia
6. Parietal pleura
7. Pleural cavity
8. Visceral pleura and lung

The exact layers encountered depend on the specific point within the triangle. The important surface anatomical feature is the absence of thick overlying trapezius and latissimus dorsi from the central part of the region.

Relationship to the Scapula

The scapula has a particularly important influence on the size and accessibility of the triangle. In a resting position, the scapula covers a substantial portion of the posterior thoracic wall, and the medial border lies relatively close to the vertebral column.

When the scapula is protracted, its medial border moves laterally around the thoracic cage. This increases the interval between the scapula and the midline musculature and exposes a larger portion of the triangle.

Protraction is produced primarily by serratus anterior, with pectoralis minor contributing to scapular movement. The position can also be achieved during examination by bringing the shoulders forward or crossing the arms in front of the chest.

Relationship to Rhomboid Major

The rhomboid major extends from the spinous processes of the upper thoracic vertebrae to the medial border of the scapula. It lies deep to trapezius and occupies much of the muscular plane forming the floor of the triangle.

Rhomboid major retracts the scapula and contributes to downward rotation and stabilization of the scapula against the thoracic wall. During active scapular protraction, the muscle lengthens as the medial border moves away from the vertebral column.

Although rhomboid major forms part of the floor, its relatively limited thickness means that respiratory sounds can still be transmitted through this region effectively.

Relationship to the Thoracic Wall

Deep to the muscular floor are the posterior portions of the ribs and the intercostal spaces. The intercostal spaces contain the intercostal muscles and neurovascular structures associated with the thoracic wall.

Internal to the ribs and intercostal muscles lies the endothoracic fascia, followed by the parietal pleura. The pleural cavity separates the parietal pleura from the visceral pleura covering the lung.

The close surface relationship between the triangle and the thoracic wall accounts for its traditional role in examination of breath sounds. The triangle does not provide direct access to the lung, but it reduces the amount of superficial muscle through which sound must be transmitted.

Surface Anatomy

The triangle is not usually visible as a sharply defined triangular depression. Instead, its position is estimated from surrounding landmarks and muscular borders.

The spine of the scapula and medial border of the scapula can be palpated first. The examiner can then identify the region medial to the scapula and between the lower fibers of trapezius and upper border of latissimus dorsi.

Asking the individual to protract the scapulae makes the medial borders move laterally and enlarges the accessible region. Slight flexion of the thoracic spine can further separate the scapulae and facilitate examination of the posterior thorax.

Effect of Scapular Movement

The dimensions of the triangle change dynamically with scapular movement.

This mobility distinguishes the triangle from anatomical spaces whose boundaries are formed entirely by relatively fixed structures.

Anatomical Variation

The size and shape of the triangle of auscultation vary between individuals. Differences in the width and attachments of trapezius and latissimus dorsi, scapular dimensions, resting scapular position, muscular development, and body habitus can all affect its boundaries.

The region may be relatively broad in one person and much narrower in another. The triangle also changes size within the same individual as the scapula moves.

These variations do not alter the fundamental anatomical definition of the triangle but explain why it may be easier to identify in some individuals than in others.

Clinical Significance

The main clinical importance of the triangle of auscultation is its use as a region for posterior thoracic auscultation. Its relatively thin muscular covering can facilitate transmission of respiratory sounds from the thorax to a stethoscope placed against the back.

Lung Auscultation

During examination of the posterior chest, moving the scapula laterally exposes more of the underlying thoracic wall. This can be achieved by asking the patient to bring the shoulders forward or cross the arms in front of the chest.

A stethoscope placed over the region can be used to assess breath sounds transmitted from the underlying lung. In practice, posterior lung examination is not restricted to the triangle, and multiple areas of the posterior and lateral chest are auscultated for comparison.

Posterior Thoracic Examination

The triangle also provides a useful anatomical reference when describing findings on the posterior thoracic wall. Its boundaries link palpable scapular anatomy with the superficial muscular layers of the back and the deeper thoracic cage.

Because the scapula moves extensively, the position of the upper limb should be considered when identifying the triangle during examination.

Surgical Anatomy

The triangle of auscultation has also been described as a potential route for selected approaches to the posterior thoracic wall because the superficial muscular covering is reduced in this region. The suitability and extent of such an approach depend on the specific procedure and the deeper structures that must be accessed.

The triangle of auscultation is therefore best understood as a relatively thin muscular window of the posterior thoracic wall. Trapezius forms its medial boundary, latissimus dorsi its inferior boundary, and the medial border of the scapula its lateral boundary. Rhomboid major forms most of its muscular floor, while scapular protraction enlarges the region and improves access to the underlying thoracic wall.