The right pulmonary artery-left atrial axis line. A method for measuring left atrial size on lateral chest radiographs.

Based on the relationships of the aorta, right pulmonary artery (RPA), and left atrium (LA), a method for directly measuring the LA diameter is described. A line drawn downward from the anterior wall of the RPA, parallel to the barium-filled esophagus, will approximate the anterior wall of the LA and enable a direct measurement of the anteroposterior LA diameter. This radiological measurement correlated well with the echocardiographic determination of LA size in 82 adult patients. In some patients, direct measurement of LA size made it possible to detect chamber enlargement before other signs appeared. The major disadvantage of this method is the inability to clearly see the RPA in infants and in approximately 10-15% of adults.