Tracheal parallax in the assessment of mediastinal shift.

Tracheal shift is a basic radiological sign in interpreting chest films, and there is an impression that it cannot be assessed when the patient is rotated. From a consideration of parallax, it is shown that although the trachea and medial ends of the clavicle both shift in the direction of the rotation, the trachea, lying behind the medial ends of the clavicle, shifts in an opposite direction in relation to the clavicles. This is confirmed by postero-anterior films of a normal patient. If, therefore, the trachea shifts in relation to the clavicles in the same direction as the patient is rotated, genuine tracheal shift is present. An example illustrates the importance of being able to detect tracheal shift despite rotation, in a patient with proven traumatic rupture of the aortic arch.