[Cardiothoracic ratio].

Cardiothoracic ratio has been a useful index of cardiac dilatation, and a value of 50% is generally considered to indicate the upper limit of normal cardiac size. However, this value is not always correct and increases the number of false-positive results, especially in obese or older subjects who may have a misdiagnosis of cardiac enlargement. The maximal transverse diameter of the cardiac shadow of chest X-ray film consists mainly of the diameters of the left ventricle and the right atrium as shown by X-ray computed tomography, but this ratio is influenced by many factors; not only left ventricular dilatation or hypertrophy, but also dilatation of the other cardiac chambers and the aorta, rotation and shift of the heart, respiratory phase, body posture, and measurement errors. In contrast, echocardiography is an accurate method for the diagnosis of cardiac dilatation. This study correlated left ventricular dimension with cardiothoracic ratio in 80 consecutive subjects with normal physical, ECG and Doppler echocardiography examinations. There was no relationship between left ventricular end-diastolic dimension and the cardiothoracic ratio. Chest X-ray is mandatory for initial cardiac examination. However, we must be careful about roentgenographic diagnosis of cardiac dilatation. Cardiac enlargement should be diagnosed by echocardiography, but follow-up and management can be based on chest X-ray films.