Lung function tests (spirometry) help diagnose various lung conditions, such as asthma, chronic obstructive pulmonary disease (COPD) and restrictive lung diseases. They can monitor the severity and progression of the disease and guide management.
Outcome Measures
Spirometry is the test used to establish objective measures of lung function. It involves different breathing exercises into a machine that measures volumes of air and flow rates and produces a report. Reversibility testing involves giving a bronchodilator (e.g., salbutamol) before repeating the spirometry to see if this impacts the results.
Forced expiratory volume in 1 second (FEV1) is the air a person can forcefully exhale in 1 second. This measures how easily air can flow out of the lungs. It is reduced with airflow obstruction.
Forced vital capacity (FVC) is the total air a person can exhale after a full inhalation. This measures the total volume of air that the person can take into their lungs. It is reduced with restricted lung capacity.
Obstructive Disease
Obstructive lung disease is diagnosed when the FEV1 is less than 70% of the FVC, meaning a FEV1:FVC ratio of less than 70%. This suggests that obstruction is slowing the air passage out of the lungs. The person may have a relatively good lung volume, but air can only move slowly in and out of the lungs due to obstruction.
In asthma, the obstruction is a narrowed airway due to bronchoconstriction. In COPD, there is chronic airway and lung damage, causing obstruction. You can test the reversibility of this obstruction by giving a bronchodilator (e.g., salbutamol). The obstructive picture is typically reversible in asthma but less so in COPD.
Restrictive Disease
In restrictive lung disease:
- FEV1 and FVC are equally reduced
- FEV1:FVC ratio greater than 70%
Restrictive lung disease limits the ability of the lungs to expand and fill with air. The lungs are restricted from effectively expanding. This is different from obstructive lung disease, where there is obstructed airflow.
Restriction of lung expansion leads to inadequate ventilation of the alveoli and insufficient blood oxygenation.
The FEV1:FVC ratio is normal or raised in restrictive lung disease without obstructive pathology affecting airflow through the airways. The FVC is reduced due to the restriction of lung expansion and capacity.
Restrictive lung disease includes conditions that limit how well the chest wall and lungs can expand, for example:
- Interstitial lung disease, such as idiopathic pulmonary fibrosis
- Sarcoidosis
- Obesity
- Motor neurone disease
- Scoliosis
TOM TIP: It is worth remembering that a low FVC indicates restrictive lung disease, and a low FEV1:FVC ratio (under 70%) indicates obstructive lung disease. A low FVC and a low FEV1:FVC ratio indicate a combination of obstructive and restrictive lung disease. This is a common data interpretation question in exams.
Peak Flow
Peak flow measures the “peak”, or fastest point, of the expiratory flow of air. It is also called peak expiratory flow rate (PEFR) and is measured using a peak flow meter. It is a simple way of demonstrating how much obstruction to airflow is present in the lungs. It helps gauge how well the condition is controlled and the severity of an acute exacerbation in obstructive lung disease, particularly asthma.
The technique is to stand tall, take a deep breath in, make a good seal around the device with the lips and blow as fast and hard as possible into the device. Take three attempts and record the best result.
An average peak flow varies dramatically between individuals. The predicted peak flow is based on sex, height and age using a reference chart or calculator. The result can be recorded as a percentage of the predicted.
For example, an asthmatic patient with a predicted peak flow of 400 that manages a score of 200 on their best attempt has a peak flow of 50% of the predicted result (actual / predicted x 100).
Last updated May 2023