A spirometry test measures the volume of air blown out against time and gives more specific information about lung function. This is done by a special technician or specialist using a machine, into which the patient’s age, gender, and height have been entered, so that a predicted value has already been made. The patient blows out into a mouthpiece connected to the machine, as fully and as long as possible, after a deep breath in. Various readings can then be taken and compared with the predicted values.

It is the single best diagnostic test for patients with airflow limitation. All new diagnoses of COPD have to be tested to fulfil scores in the new GP General Medical Services contract.

What does the spirometer measure?

The most common measurements used are:

FEV1 - Forced Expiratory Volume in one second. This is the amount of air that can be blown out within one second. With normal lungs and airways, most of the air from the lungs can be blown out within one second.

FVC - Forced Vital Capacity. The total amount of air that can be blown out in one breath.

FEV1 divided by FVC (FEV1/FVC). Of the total amount of air that can be blown out in one breath, this is the proportion that can be blown out in one second.  

What can the measurements show?

A spirometry reading usually shows one of four main patterns:

• Normal
• An obstructive pattern
• A restrictive pattern
• A combined obstructive / restrictive pattern

Normal

Normal readings vary, depending on age, size, and sex. The range of normal readings is published.

 An obstructive pattern

This is typical of diseases that cause narrowed airways.

If the airways are narrowed, then the amount of air that  can be blown out quickly is reduced. So, the FEV1 is reduced and the ratio FEV1/FVC is lower than normal. As a rule, a disease that causes narrowed airways is likely if:

•  The FEV1 is less than 80% of the predicted value for the age, sex and size of the patient, or
•  The FEV1/FVC ratio is 0.7 or less

However, with narrowed airways, the total capacity of the lungs is often normal or only mildly reduced. So, with an obstructive pattern the FVC is often normal or near normal.

The main conditions that cause narrowing of the airways and an obstructive pattern of spirometry are asthma and chronic obstructive pulmonary disease (COPD). So, spirometry can help to diagnose these conditions. Spirometry can also help to assess if treatment (inhalers etc) 'open up' the airways, as the readings will improve if the narrowed airways become wider.

A restrictive pattern

This is typical of certain lung diseases.

With a restrictive spirometry pattern the FVC is less than the predicted value for the age, sex and size of the patient. This is caused by various conditions that affect the lung tissue itself, or affect the capacity of the lungs to expand and hold a normal amount of air. For example, conditions that cause fibrosis or scarring of the lung such as pneumoconiosis. Or, a physical deformity that restricts the expansion of the lungs. The FEV1 is also reduced but this is in proportion to the reduced FVC. So, with a restrictive pattern the ratio of FEV1/FVC is normal.

A combined obstructive / restrictive pattern

With this the patient may have two conditions, for example, asthma plus another lung disorder. Also, some lung conditions have features of both an obstructive and restrictive pattern. For example, with cystic fibrosis there is a lot of mucus in the airways, which causes narrowed airways, and damage to the lung tissue may also occur.

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