Explanations of obstructive and restrictive lung disease and how to identify both
Part of the process of assessing patients who present with breathlessness, cough or other respiratory symptoms involves undertaking breathing tests to determine how well the lungs are functioning.
At its most basic, this involves measuring the amount of air that can be forcefully exhaled from a full breath through a device called a spirometer.
These measurements are compared to values that would be expected for someone of similar height, age and gender to achieve (called predicted values) and, together with clinical history and examination, help to judge whether symptoms are the result of an obstructive or restrictive process.
The volume exhaled in the first second (Forced Expiratory Volume – FEV1) is expressed as a percentage of the total volume exhaled (Forced Vital Capacity – FVC) or FEV1/FVC ratio and is normally above 70%.
Obstructive lung disease
In obstructive lung disease, the airways are narrowed, making it difficult to exhale quickly giving a reduced FEV1/FVC ratio. This may be temporary, such as in acute asthma, when the airways can rapidly constrict in response to a trigger (e.g. pollen, house dust mite or pet dander in someone with sensitivity to those inhalants) and with treatment with inhaled bronchodilators and inhaled corticosteroids may go back to normal.
In chronic obstructive disease such as emphysema, long standing damage to the airways causes permanent and irreversible narrowing which does not respond very well to inhaled therapy, resulting in long term symptoms of breathlessness, which progress over time.
The lungs become enlarged, or hyperinflated, increasing the work of breathing.
Restrictive lung disease
In restrictive lung disease, the total volume of the lungs is reduced; this is seen in pulmonary fibrosis, when scarring in the lung tissue causes ‘stiffening’ of the lungs, typically resulting in progressive and marked breathlessness on exertion. The breathing tests show a reduction in both the FEV1 and FVC (‘small’ lungs) but the FEV1/FVC ratio is normal (above 70%) as there is no narrowing in the airways. Restrictive lung volumes are also seen where the chest wall is unable to expand normally – e.g. obesity, kyphoscoliosis or conditions that result in weak respiratory muscles, such as myasthenia gravis or muscular dystrophy.