Pulmonary hyperinflation

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Lung hyperinflation is a common feature of patients with chronic obstructive pulmonary disease (COPD). It is also linked to aging and other chronic diseases that cause airflow obstruction.

Pathology

The airflow limitation during expiration is produced by two factors:

  • destruction of the lung parenchyma
    • in healthy subjects, the volume reached by the lungs after exhalation is determined by the balance of forces between the inward elastic recoil pressure of the lung and the outward recoil pressure of the chest wall
    • aging or pulmonary parenchymal destruction from other causes (e.g. emphysema) results in a displacement of this point of equilibrium to a higher volume
  • airway defects
    • such as mucosal edema, remodeling of airway mucus or impaction. In these cases, the expiratory airflow limitation is increased during exercise

The diseases that limit expiratory airflow produce lung hyperinflation. The most characteristic is chronic obstructive pulmonary disease (COPD). Other diseases that cause an obstructive airflow pattern are:

Radiographic features

Plain radiograph

Observable features include:

  • flattened hemidiaphragmatic contours
    • considered one of the most sensitive indicators of hyperinflation and interobserver variability is small
    • best seen on the lateral chest radiograph and consists of a loss of height of the convexity of the hemidiaphragm
    • to measure, it is possible to draw a line connecting the sternophrenic angle and the posterior costophrenic angle
      • this arch height should be greater than or equal to 2.5 cm
      • it is considered clearly pathological when measures less than 1.5 cm
    • minor measures correlate well with the functional importance of airflow obstruction 
  • retrosternal space measurement
    • a horizontal line is drawn from a point allocated 3 cm below the junction between the manubrium and sternal body, on the posterior cortex of the sternum, and the ascending aorta point; in cases of increased retrosternal space, this distance is equal or greater than 2.5 cm
  • air trapping: when comparing two radiographs acquired in maximal inspiration and maximal expiration, the vertical movement of the diaphragm is less than 3 cm
  • appearance of the ribs:
    • more than 6 anterior or 10 posterior ribs above the diaphragm level on the midclavicular line
    • horizontalisation of ribs
  • presence of air below the heart
  • increased anteroposterior diameter of the chest, also called barrel chest
  • hyperlucent lungs (i.e. less bronchovascular markings per unit area)
CT

Observable features include:

  • air trapping: best seen in expiration
  • measuring the length of the anterior junctional line
    • this is a measurement of the retrosternal space more reproducible than in the chest radiograph
  • saber-sheath trachea
    • a pathognomonic finding of chronic obstructive pulmonary disease (COPD)
    • it refers to a decrease in the diameter of the trachea in the coronal plane and increase in the sagittal plane
    • it can be also seen on the chest radiograph comparing the posteroanterior and lateral projections
  • vascular changes