Light gives a simple calculation to estimate the size of a pneumothorax.


Measures on an upright expiratory chest radiograph:

(1) average diameter of the hemithorax in cm

(2) average diameter of the lung in cm


diameter of the lung in cm =

= (average diameter of the hemithorax in cm) – (distance from edge of lung markings to chest wall in cm) =

= (average diameter of the hemithorax in cm) – (width of the pneumothorax in cm) =


Technical issues:

(1) The medial aspect of the scapula and large emphysematous bullae should not be mistaken for a pneumothorax.

(2) Measurements from a supine radiograph should not be used since the lung markings may extend to the chest wall (as the trapped air moves anteriorly).


lung expansion in percent =

= ((diameter of lung)^3) / ((diameter of hemithorax in cm)^3) * 100%


percent pneumothorax =

= 100% - (lung expansion in percent)



• The calculation makes sense if you assume each element is a sphere. The shape could be better described by a shape such as an ellipsoid.


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