Lung function tests are used to help establish a diagnosis in lung disease. They are particularly helpful in obstructive and restrictive lung disease where there will be recognisable findings on the tests. This section gives a simple overview of lung function tests. There is a lot more to learn if you are interested.
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 (i.e. salbutamol) prior to repeating the spirometry to see the impact this has on the results.
FEV1 means forced expiratory volume in 1 second. This is the amount of air a person can exhale as fast as they can in 1 second. This is a measure of the ability of air to freely flow out of the lungs. It will be reduced if there is any obstruction to the air flow out of the lungs.
FVC means forced vital capacity. This is the total amount of air a person can exhale after a full inhalation. This is a measure of the total volume of air that the person can take in to their lungs. It will be reduced if there is any restriction on the capacity of their lungs.
Obstructive lung disease can be diagnosed when FEV1 is less than 75% of FVC (FEV1:FVC ratio < 75%). This suggests that there is some obstruction slowing the passage of air getting out of the lungs. The person may have a relatively good lung volume but air is only able to move in and out of the lungs slowly 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 for reversibility of this obstruction by giving a bronchodilator (i.e. salbutamol). The obstructive picture is typically reversible in asthma but not COPD.
If FEV1 and FVC are equally reduced and FEV1:FVC ratio > 75% this suggests restrictive lung disease. Restrictive lung disease is where there is a restriction to the ability of the lungs to expand and take air in.
Causes of restrictive lung disease:
- Interstitial lung disease
- Neurological (i.e. motor neurone disease)
- Scoliosis or chest deformity
“Peak flow” is a measure of the “peak”, or fastest point, of a persons expiratory flow of air. It can be referred to as peak expiratory flow rate (PEFR). It is measured using a peak flow meter. It is a simple way of demonstrating how much obstruction to airflow is present in the patient’s lungs. It is useful in obstructive lung disease, particularly asthma, to gain insight into how well the asthma is controlled and how severe an acute exacerbation is.
The technique is to stand tall, take a deep breath in, make a good seal around device with the lips and blow as fast and hard as possible into the device. Take three attempts and record the best result.
It varies dramatically based on the size and age of the patient. To put the result in to context it is usually recorded as a “percentage of predicted”. The predicted peak flow can be obtained based on sex, height and age using a reference chart.
For example an asthmatic patient with a predicted peak flow of 400 that only manages a score of 200 on their best attempt of 3 currently has a peak flow at 50% of predicted.
Last updated February 2019