"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."
"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."