Spirometry

A spirometer is a device used to measure timed expired volumes, and from these it is possible to calculate how effectively and how quickly the lungs can be emptied. Two important measurements that are made are vital capacity (VC) and forced expired volume in one second (FEV₁). VC is the maximum volume of air that can be exhaled during either a forced (FVC) or a slow (VC) manoeuvre. FEV₁ is the volume expired in the first second of maximal expiration after a maximal inspiration and is one measure of airway calibre.¹

Spirometry is the method of choice, as the measurement of peak expiratory flow (PEF) with conventional peak flow meters has significant limitations.^{3, 4,<LE IV> 5 <LE IV>}

Most adults, and children over 7 years of age can perform spirometry.³

Best practice guidelines recommend that all doctors managing asthma should have access to and use a spirometer to assess, diagnose and monitor airway disease.

How useful is spirometry in the management of asthma? Current evidence.

There appear to be no published studies conducted specifically to examine the outcomes of measuring spirometry in patients with asthma. However, in a study of patients with acute asthma in an Emergency Department, physicians were found to underestimate the degree of airflow limitation based purely on clinical examination and subsequent knowledge of spirometry results directly altered management in 20.4% of patients.^{6 <LE IV>} In asthma, there are often large disparities between symptoms, measurement of spirometry and PEF variability. Therefore it is important to measure spirometric function in these patients.⁷

Modified from: King, Gregory and Johns, David P. The use of spirometry in the management of asthma. A paper prepared for the Australian Government Department of Health and Ageing’s General Practice Spirometry Sub-group, October 2002.

How do I get the best results?

Explain clearly what the test involves and demonstrate the correct technique to the patient. It is important to ensure a good seal around the mouthpiece, and ensure that the patient’s posture is correct, i.e. they are seated upright. Explain that maximum inspiration, followed by maximum forced expiration until no more can be exhaled (or for at least 6 seconds if possible, but in children 3 seconds is usually sufficient) is required.³

Expiration must be rapid and complete with maximum effort maintained during expiration. Repeat three times to ensure the best result is obtained, with a minute’s rest between. The aim is to obtain three reproducible (FVC within 200 ml) and acceptable (good start, maintenance of forced expiration, no cough) measurements. The best FEV₁ and FVC result from any of these curves is recorded. No more than eight attempts should be undertaken as more are unlikely to be successful due to patient fatigue.³

Repeat spirometry about 10 minutes after giving a bronchodilator via a large volume spacer. A spacer is more efficient than a metered dose inhaler and it gives you the opportunity to explain the use and benefits of a spacer.^{3, 8,<LE III-2> 9 <LE III-2>}

An increase in FEV₁ of at least 200 ml and 15% is significant. Values should be expressed as absolute figures and also as a percentage of predicted based on the patient’s age, height, weight and sex.³

The absence of reversible airflow obstruction does not exclude the diagnosis of asthma. Repeated measurements, perhaps combined with home measurement of PEF, are sometimes necessary to document the presence of asthma.³