Apical segmental artery of left lung

The apical segmental artery of the left lung (A1) is a branch of the left pulmonary artery that supplies the apicoposterior segment (S1+2) of the left upper lobe. However, a distinctive feature of the left lung is that the apical (A1) and posterior (A2) segmental arteries are most commonly fused into a single common trunk known as the apicoposterior segmental artery (A1+2), which arises directly from the left main pulmonary artery. This fusion mirrors the corresponding bronchial anatomy, where the apical and posterior segmental bronchi also merge into a common apicoposterior bronchus (B1+2).

Anatomical characteristics:

• Origin: The A1+2 trunk arises as one of the first branches of the left main pulmonary artery at the hilum, typically originating from its mediastinal (superior) aspect before the artery enters the interlobar fissure.

• Territory: It supplies the apicoposterior segment (S1+2) of the left upper lobe, which occupies the apex and posterosuperior portion of the left lung. This segment is the functional equivalent of the separate apical (S1) and posterior (S2) segments found in the right upper lobe.

• Subsegmental branches: The A1+2 trunk typically divides into subsegmental branches designated A1+2a, A1+2b, and A1+2c. According to the Yamashita classification, the most common branching pattern (Type A) consists of A1+2a+b as a common trunk and A1+2c as a separate branch. On thin-section CT, A1+2c is regularly identified along the posterior aspect of its corresponding bronchus B1+2c.

• Relationship to bronchi: The segmental artery closely accompanies the segmental bronchus in approximately 90% of pulmonary segments, though in the left upper lobe the posterior apical segment artery may not always be directly accompanied by its bronchus.

Variability: The arterial anatomy of the left upper lobe exhibits the greatest variability in branching patterns among all pulmonary lobes. The left upper lobe is most commonly supplied by four arterial branches from the left pulmonary artery, though three or five branches are also frequently observed.