Incomplete inspiration can lead to exaggeration of lung markings and heart size
Lung hyperexpansion is a sign of obstructive lung disease
Chest X-rays are conventionally acquired in the inspiratory phase of the respiratory cycle. The radiographer asks the patient to, 'breathe in and hold your breath!' Patients who are short of breath, or those who are unable to follow the instructions may find this difficult.
When interpreting a chest X-ray it is important to recognise if there has been incomplete inspiration. If the image is acquired in the expiratory phase, the lungs are relatively airless and their density is increased. Also, the raised position of the diaphragm leads to exaggeration of heart size, and obscuration of the lung bases.
To assess the degree of inspiration it is conventional to count ribs down to the diaphragm. The diaphragm should be intersected by the 5th to 7th anterior ribs in the mid-clavicular line. Less is a sign of incomplete inspiration.
Anteriorly the sixth rib intersects the diaphragm at the mid-clavicular line
The lungs are not consolidated
The heart size is clearly normal
Assessing for hyperexpansion
While checking for adequate inspiration you may notice that a patient's lungs are hyperexpanded (>7th anterior rib intersecting the diaphragm at the mid-clavicular line). This is a sign of obstructive airways disease.
It is possible to assess for hyperexpansion by counting ribs, or by checking for flattening of the hemidiaphragms.
This patient has taken a good breath in such that the diaphragm is intersected by the 6th rib in the mid-clavicular line.
The hover over image shows an imaginary dotted line between the costophrenic and cardiophrenic angles. The distance between this line and the diaphragm (green lines) should be greater than 1.5 cm (asterisk) in normal individuals. In practice this is rarely measured and a quick assessment of diaphragm shape is all that is necessary.