Classification
In order to confirm a diagnosis post-bronchodilator spirometry is used. If used alone it can underestimate or overestimate disease impact so it should always be combined with symptoms and a differential diagnosis (see other pages).
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Spirometry is a method of assessing lung function by measuring the volume of air that the patient is able to expel from the lungs after a maximal inspiration. It is a reliable method to differentiate between diseases. These include obstructive airways disorders e.g. COPD, asthma and restrictive diseases e.g. cystic fibrosis (where the size of the lungs is reduced).
Spirometry gives three important measures:
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FEV1 (forced expiratory volume in 1 sec): the volume of air that the patient can exhale in the first second of forced expiration.
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FVC (forced vital capacity): the total volume of air that the patient can forcibly exhale in one breath.
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FEV1/FVC: the ratio of FEV1 to FVC.
In COPD the airways are narrowed (as it is an obstructive disease) which will affect the measurements taken during the spirometry test. The FEV1 will be reduced while the FVC will either be normal or slightly reduced, this then affects the overall ratio of FEV1/FVC by reducing it.
To undertake a post-bronchodilator spirometry test the patients should inhale a short-acting beta-2 agonist via a spacer 15-20minutes before the spirometry test. Salbutamol at a dose of 200mcg or terbutaline at a dose of 500mcg may be used.
In order to obtain the results as a percentage the measured values (from the patient) are compared with predicted normal values. The predicted values are shown in tables and are based on a person’s age, height and sex. A simple calculation is then performed by dividing the measured value with the predicted value and multiplying by 100. An example is summarised in the opposite table.
It is important to understand that the results can change depending on the technique and instruction given. Also, if a patient is unable to perform a forced expiration to full exhalation then the slow or relaxed vital capacity (SVC) can be used instead. The SVC is also used if the result is higher than the patients FVC.
A diagnosis of COPD can be confirmed if the FEV1/FVC is less than 0.7. The percentage of FEV1 is then used to classify what stage the patient is at. If FEV1 ≥ 80%, COPD should only be diagnosed if respiratory symptoms such as breathlessness and cough are present. If the person has an exceptionally good response to treatment the spirometry test should be repeated.
NICE states that the severity of COPD should be assessed by the following:
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Reduction in spirometry results
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Degree of breathlessness
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Body Mass Index (BMI) [weight (kg)/ height (m2)] – a BMI<20kg/m2 is associated with increased mortality
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Presence of cor pulmonale (see differential diagnosis for details)
Classifying disease stage
Is spirometry the same as peak flow readings?
No. A peak flow meter is small and more convenient than spirometry. It is often used in the diagnosis and monitoring of asthma as it can detect airways narrowing. It measures the fastest rate of air a patient can exhale unlike spirometry which uses measurements from the first second, and also total volume a patient can exhale and is not reliant on speed. In COPD, peak flow readings can underestimate the severity, although it can be useful as a rough guide for airways narrowing.
Bibliography:
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http://cks.nice.org.uk/chronic-obstructive-pulmonary-disease#!diagnosissub
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CPPE - Chronic Obstructive Pulmonary Disease Book 1