7th February 2024

Clinical relevance

Table of comparison of characteristics seen in pulmonary function tests in diseases

FEV1 can also be used in reversibility tests in the diagnosis of asthma. 

Different patterns of flow-volume loops in different diseases. 

Note: Flow-volume loop is produced by asking the person to fully inspire to total lung capacity from residual volume, then exhale as hard as possible back to residual volume

Normal

• The loop for the inspiratory limb is symmetric and convex.
• The expiratory limb is linear.

Restrictive disease (e.g. interstitial lung disease, neuromuscular disease, chest wall pathology)

• Narrower loop due to reduced forced vital capacity due to reduced lung compliance.
• Airflow is greater than normal during expiration at each lung volume due to increased elastic lung recoil.

Lower airway obstructive disease (e.g. asthma, emphysema)

• Airflow during expiration at all lung volumes is reduced due to increased airway resistance.
• The expiratory limb is prolonged.

Fixed upper airway obstruction (e.g. goitre, tracheal stenosis)

• Inspiratory and expiratory loops are flattened, resulting in a rectangular-looking loop. This is because fixed upper airway obstruction limits airflow during inspiration and expiration equally.

Variable extra thoracic obstruction (e.g. vocal cord paralysis, neoplasm in upper airway)

• The structures involved in pathology move according to pressure gradients across the glottis.
• Using vocal cords as an example:
  • The vocal cord is drawn inwards during forced inspiration, resulting in increased airway resistance and flattening of the inspiratory limb.
  • The vocal cord is pushed open during forced expiration. Hence, the expiratory limb has a normal shape.

Variable intrathoracic obstruction (e.g. tracheomalacia)

• Forced inspiration results in negative intrathoracic pressure that keeps the trachea open and patent. Hence, the normal shape of the inspiratory limb
• Forced expiration results in loss of the negative intrathoracic pressure, keeping the trachea open, narrowing the trachea and increasing airway resistance. Hence, the expiratory limb will have a flattened shape.