THE use of the angle of clearance as a fluoroscopic method for measuring cardiac size was devised by Wilson1 in 1934. After comparing it with other methods of measuring the size of the heart of patients with rheumatic heart disease, she concluded that the angle of clearance differentiated the normal from the abnormal heart with greater frequency.
A study of the angle of clearance and its reliability as a measurement of cardiac size was made at this clinic by Jackson and his associates2 in 1943. Only normal children were included in this study, and some modifications of Wilson's technic were made. The most significant one was the measurement of two angles instead of one. It was found that the left dorsal border of the cardiac silhouette cleared the spinal column at two points: the first, that at which the cardiac border was separated from the projection of the
McINTOSH CB, JACKSON RL. ANGLES OF CLEARANCE: A Method for Measuring the Cardiac Size of Children with Rheumatic Heart Disease (A Comparison with the Cardiothoracic Index). Am J Dis Child. 1946;71(4):357–364. doi:10.1001/archpedi.1946.02020270027003
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