Superior segment of left lung

The superior segment of the left lower lobe (S6) is one of the bronchopulmonary segments of the left lower lobe of left lung discrete, functionally independent units of lung parenchyma, each supplied by its own segmental bronchus (B6), segmental artery (A6), and drained by its own segmental vein (V6).

Anatomical Definition and Boundaries

The superior segment (S6) occupies the posterosuperior portion of the left lower lobe, situated immediately below the oblique (major) fissure and posterior to the hilum. It is bounded:

• Superiorly and anteriorly by the oblique fissure, which separates it from the upper lobe.

• Inferiorly by the basal segments of the lower lobe (S8, S9, S10; note that S7 on the left is typically diminutive or absent due to the cardiac silhouette).

• Medially by the mediastinum, where it relates to the descending aorta and the esophagus.

• Posteriorly by the costal surface abutting the posterior chest wall.

Bronchovascular Anatomy

The segmental bronchus B6 arises as the first branch off the left lower lobe bronchus, directed posteriorly. It typically divides into two subsegmental branches: B6a (superior subsegment) and B6b (lateral subsegment). The corresponding segmental artery A6 accompanies B6, and the segmental vein V6 drains the segment, with V6 tributaries often being double. The left B6 bronchus tends to be larger than its right counterpart, making the "B6 sign" (a visible air bronchogram within the superior segment) more commonly seen on left-sided chest radiographs.

Anatomical Variants

• A subsuperior segment (S*) may be present between S6 and the basal segments in approximately 24% of individuals, supplied by its own bronchus B and artery A.

• A posterior pulmonary lobe, defined by an aberrant horizontal fissure on the costal surface of the lower lobe, corresponds to S6 in about 50% of left-sided cases (incidence ~3% on the left). Anomalous intersegmental vessels communicating with adjacent segments have been reported in up to 37% of such cases, which is surgically relevant during S6 segmentectomy.

• The intersegmental vein between S6 and S is consistently complete, but intersegmental veins of S may be absent in approximately 60% of B cases, which has implications for segmentectomy planning.