Apicoposterior segment of left lung

The apicoposterior segment (S1+2) is a bronchopulmonary segment of the left upper lobe that represents the fusion of the apical (S1) and posterior (S2) segments which exist as two separate segments in the right upper lobe into a single combined segment on the left side.

Anatomical Definition

The apicoposterior segment occupies the superior and posterior portion of the left upper lobe. It is the leftsided homologue of both the apical segment (S1) and the posterior segment (S2) of the right upper lobe. The fusion occurs because the left lung is smaller than the right (due to the cardiac silhouette occupying space on the left), and the left upper lobe lacks a separate middle lobe (the lingula serving as its equivalent). This anatomical constraint results in the merging of the apical and posterior territories into one segment.

Bronchial Anatomy

The segment is ventilated by the apicoposterior segmental bronchus (B1+2), which arises from the left upper lobe bronchus. In the most common branching pattern (~6476% of individuals), B1+2 originates as a common stem of the apical and posterior segmental bronchi before bifurcating into subsegmental branches (B1+2a, B1+2b, and B1+2c). A trifurcation pattern (B1, B2, and B3 arising simultaneously) is the second most common variant (~1723%).

Vascular Supply

The corresponding segmental pulmonary artery, A1+2, typically arises directly off the left main pulmonary artery as a common trunk.

Boundaries and Relations

• Superiorly: extends to the lung apex

• Posteriorly: abuts the costovertebral region and posterior chest wall

• Anteriorly and inferiorly: bordered by the anterior segment (S3) of the left upper lobe

• Inferiorly: related to the superior segment of the lingula (S4) and the superior segment of the left lower lobe (S6)