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) =
(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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