Final answer:
To estimate cardiac size in an AP or portable radiograph, one measures the transverse diameter of the heart and compares it to the thoracic width, using the cardiothoracic ratio, which should normally be less than 0.50. Radiographs can cause distortions, making echocardiograms a more reliable tool for assessing heart size and functions such as SV, which is measured by subtracting ESV from EDV.
Step-by-step explanation:
To estimate cardiac size in an AP (anteroposterior) or portable radiograph requires understanding several visual cues and using them to assess heart dimensions relative to thoracic structures. The size of the cardiac silhouette is usually compared to the interior diameter of the chest. In a typical AP chest X-ray, the heart should occupy less than half of the thoracic width. A common method used to estimate the cardiac size is to measure the transverse diameter of the heart and to compare it to the transverse diameter of the thorax. The transverse thoracic diameter is measured from the inner aspect of one ribcage to the inner aspect of the opposite ribcage at the level of the diaphragm, and the cardiac transverse diameter is the distance between the most laterally projecting points on the right and left sides of the heart.
Physicians may also refer to cardiothoracic ratios to determine cardiac enlargement. A cardiothoracic ratio greater than 0.50 on an AP view may suggest cardiomegaly (enlarged heart), but this should be assessed alongside other clinical information due to potential distortions in AP or portable X-rays. In addition to the cardiothoracic ratio, other features provide clues about heart size, such as the appearance of the cardiac silhouette and the visibility of certain anatomic landmarks.
One should keep in mind that portable radiographs can distort cardiac size due to their angle and distance from the subject. This is why echocardiograms (ECG) are preferred for more precise measurements, as X-rays can at best provide an estimation.