Respiratory
Division
Diagnostic Products
Spirometry
Spirometry made Simple
Guide To Spirometry
Primary
care health professionals have become well acquainted with the peak expiratory
flow (PEF) meter as a simple, quick and effective tool to diagnose and
monitor asthma. The pef meter is of particular value when following asthma
over an extended length of time, although a single reading may be misleading.
The PEF meter measures airflow obstruction in the larger airways. With other
diseases, such as emphysema, pathological changes occur predominantly in the
smaller airways. Here the PEF meter can give a significant under-representation
of the severity of airflow obstruction.
The publication of the British Thoracic Society Guidelines on Chronic Obstructive Pulmonary Disease has provided a great stimulus to more accurately measure airflow obstruction using Spirometers. Spirometry is recommended as the measurement of choice in COPD. Not only does it yield information on airflow obstruction, but also enables the diagnosis of a range of other respiratory conditions (restrictive disorders) where lung volumes are reduced, but airways remain patent (open and unblocked).
Recordings are both reproducible and objective and there are well defined normal ranges that allow for the effects of age, race, sex and height. The spirometer is a quick and simple means of measuring airflow obstruction and is useful at all levels of severity of COPD.
Essentially, a spirometer is a device that measures lung volumes, through the forced expiration of air in our lungs. Flow can also be assessed, by measuring the total volume of air that is expired in the first second of expiration.
The standard
indices are:-
Forced Expired Volume in one second (FEV1) – how much air can be exhaled in the first second of expiration
Forced Vital Capacity (FVC) – the maximum volume of air that can be forcibly expired
Ration of FEV1/FVC as % - the proportion of total volume of air that can be expired in the first second of expiration.
Like any test, spirometry results will only be of value if the expirations are performed satisfactorily and consistently. Three such readings are required and there should be at least two readings of FEV1 that are within 100mls or 5% of each other. Measurements must continue until no more air can be exhaled, which in the case of severe COPD, can take up to 15 seconds.
The
degree of severity of COPD can be judged by comparing the patient’s FEV1 reading
with normal predicted values – the BTS Guidelines describe Mild, Moderate
and Severe COPD according to the percentage of normal obtained.
In asthma, the reduction in FEV1 provides a measure of the severity of airflow obstruction, and parallels change in peak expiratory flow. In both COPD and Asthma, FVC is usually much less reduced, and can be normal. Hence a reduced FEV1, and a near normal FVC give a reduced FEV1/FVC ration – the hallmark of airflow obstruction if below 70%.
Obstructive
Narrowed airways decrease the volume of air that can be forcibly exhaled in the first second (FEV1). Note that the FVC is only achieved after a long exhalation. The Fev1/FVC ratio is markedly reduced. Expiration is prolonged with a slow rise in the curve and the plateau is not reached for as long as 15 seconds (in emphysema).
Restrictive
Both FEV1 and FVC are reduced. As the airways are open and unblocked expiration is rapid and completed within 2-3 seconds. The FEV1/FVC ration is normal or increased. A high or normal proportion is exhaled in the first second, resulting in a rapid rise in the curve but with long volumes reduced compared with predicted levels.
Mixed
Expiration is prolonged with a slow rise to plateau levels. The vital capacity is likely to be significantly reduced compared with an obstructive defect. Mixed patterns, if less severe, can be difficult to differentiate from obstructive patterns.
With acknowledgement to Dr. David Bellamy, GP Bournemouth
