Tracheal bifurcation angle

The tracheal bifurcation angle can have a wide range of normal values in patients and can vary significantly in serial radiographs. It is of poor diagnostic value due to the lack of sensitivity and specificity in identifying the underlying pathology. 

Terminology

The interbronchial angle is the angle between the central axis of right and left main stem bronchi ¹. The subcarinal angle is the angle of divergence between the right and left main stem bronchi measured along their inferior borders ¹.

Epidemiology

The angle of tracheal bifurcation is not related to age and gender, however, there is a weak correlation with the shape of the chest ¹. 

Pathology

The angle of tracheal bifurcation may be widened in the following conditions due to the mechanical splaying of the bronchi ^1,2:

• subcarinal masses
• lobar collapse
• left atrial enlargement
• generalized cardiomegaly
• pericardial effusion

Radiographic features

Plain radiograph

The mean angle of tracheal bifurcation is 60° (+/- 10° ), i.e. 95% of the patients have normal values between 40-80 °. The angle decreases by 10°  on expiration. Also, the angle varies, sometimes by 20%, in serial radiographs. Thus, the wide range of normal values is a potential source of misinterpretation ^1-3. 

While an angle of tracheal bifurcation greater than 100°  may predict left atrial enlargement, this is an insensitive and non-specific sign ^4,5.

Thus absolute measurements of the angle of tracheal bifurcation (IBA/SCA) are of little diagnostic value and even grossly widened angles should not be interpreted as abnormal ¹.

Practical points

• the tracheal bifurcation angle may be measured either as the interbronchial angle or the subcarinal angle
• not related to age or gender
• can have a wide range of normal values and its absolute measurement is of little diagnostic value
• widening of the tracheal bifurcation angle is an insensitive and non-specific sign which must be interpreted in correlation with the clinical presentation