What is spirometry?
Spirometry (lung function tests) measure the FEV1 and FVC (see below for explanations). These variables are dependent on height, age and sex. A spirometer is essentially a box with a tube for blowing into. The technique involves a maximum inspiration followed by a maximum forced expiration (until the patient can exhale no more). Very long forced expiratory times are seen in patients with airflow limitation.
Instruction: Tell the patient to “breathe in as much air as you can and then breathe out as fast and as hard as you can, for as long as you can, until there’s no air left in your lungs”. It can be useful to say “Keep going, keep going, keep going!” towards the end to ensure that patients continue to exhale all the way to lung 'emptiness'.
Obstructive vs restrictive
The FEV1:FVC ratio is around 80% in normal subjects.
In obstructive lung disease there is a decreased ratio because the FEV1 falls more than the FVC
In restrictive lung disease both FEV1 and FVC are reduced, so the ratio remains normal, however it may also be increased due to the FVC falling proportionally more
In emphysema and asthma (obstructive diseases), the total lung capacity (TLC) may increase, but there is usually an overall reduction in the FVC. This is due to the small airways causing airflow obstruction before the normal residual volume (RV) is reached. Trapped air in the lungs leads to an increased RV.
Obstructive disease: Asthma & Chronic Obstructive Pulmonary Disease (COPD)
An abnormal FEV1 with little variability in serial peak-expiratory flow rate (PEFR) (<10% in 24-hours) suggests COPD. Remember that in asthma, diurnal variability is characteristically demonstrated (>15% in 24-hours).
A normal FEV1 tends to excludes the diagnosis of COPD.
Capacities & volumes
Tidal volume (TV): This is the normal volume of air inhaled and exhaled with normal breaths. Usually around 500ml.
Vital capacity (VC): This is the change in volume of air in the lungs from a complete (forced) inspiration to a complete (forced) expiration. In patients with lung disease it can be greater than FVC due to air trapping.
Forced Expiratory Volume in the first second (FEV1): The maximum volume of air expelled from the lungs in the first second of a forced expiration (starting from a full inspiration).
Forced Expiratory Ratio (FER): This is the FEV1 expressed as a percentage of the forced vital capacity (FVC). In normal subjects it should be around 75-80%.
Peak Expiratory Flow Rate (PEFR): The maximum flow rate (not volume) obtained on a forced expiration, starting from a full inspiration. It is measured in L/min using a peak-flow meter (see the PEFR section from the menu on the right).
Forced Expiratory Flow (FEF 25-75%): The overall forced expiratory flow rate of the middle part of the FVC manoeuvre. It gives an indication of what is happening in the lower airways and is expressed in L/s. This measurement is not that commonly used, but can be an early indicator of airways disease.
