Spirometry

See Also: Spirometry Calculator

A spirometer is a device to measure timed expired and inspired volumes, and hence indicate how quickly and effectively the lungs can be emptied and filled.
Spirometry differentiates between obstructive and restrictive disease of the airways; is the gold standard for the diagnosis, assessment and monitoring of COPD and can also assist in the diagnosis of asthma. Vital capacity is a predictor of death from respiratory and cardiac disease and abnormal spirometry is associated with an increase in 'all-cause' mortality.¹
National evidence-based guidelines for the management of asthma² and COPD ³ recommending the use of spirometers for diagnosis and monitoring together with the their required use in the quality outcome framework of the GMS contract has led to an increase in the use of spirometry in primary care (now in 70-80% UK practices⁴).
Poorly performed spirometry produces misleading results and there have been some concerns regarding the validity of some primary care spirometry but studies incorporating training have found no differences between test results produced in primary care and in pulmonary function laboratories.⁵ Anyone performing spirometry should be fully trained and undertake regular updates. Quality audits should also be routine.

Many remember the large volume-displacement devices with bellows or water sealed bell beloved of physiology labs but the spirometers most commonly used in primary care are now electronic, flow-sensing devices:

1. Small, hand-held devices that provide digital readings. These are the cheapest options (typically about £500) and will fit into a medical bag but do not provide a graphical display (spirogram) and therefore it may be difficult to judge when an expiration is complete. They also need to be used in combination with predicted charts and a calculator to interpret results.
2. Portable meters with integrated printers. Typically more expensive than (1) but will provide calculations, spirograms to monitor the blow and a printout including a flow volume loop.
3. Systems designed to work with a computer that will display a graph, make calculations of predicted values and reversibility and provide a print-out for records. They also enable tests to be e-mailed for a second opinion and for electronic storage.

BTS guidelines recommend that:

• Spirometers should be regularly calibrated (volume devices weekly and flow devices daily, with a 3 litre syringe).
• Spirometers should generate a hard copy which is sufficiently large to check values manually.
• Volume/time plots are mandatory, flow/volume plots are optional
• Electronic devices without hard copy tracing may lead to underestimation of FEV₁and FVC. ⁶

All medical equipment used with multiple patients should be cleaned and disinfected to prevent cross-infection according to manufacturer's instructions.
Disposable 'one-way' valved mouthpieces reduce the risk of cross infection (but prevent inspiratory flow-volume loops).
Good practice should be to keep a calibration and maintenance log and list of patients tested with the spirometer (in case of unwitting testing of a patient with TB, for example, to enable contact tracing).

• Prior to testing, the patient's condition should be stable (ie at least 6 weeks since the last exacerbation).
• Standing is not mandatory but may provide better results. Sitting is safer for the elderly and infirm - if sitting, sit straight up, and keep head slightly extended.
• Breath in maximally
• Hold mouthpiece between teeth, then apply lips for an airtight seal.
• Breath out as hard and fast as possible. The patient should aim for maximal flow at the moment expiration starts. With handheld devices, watch the vane rotating, and make sure it does not start rotating while the spirometer is brought to the lips, thus avoiding artifacts.
• Keep breathing out until the lungs are 'empty'.
• Some users get the users to practice just emptying their lungs; ie a slow vital capacity (SVC) - before getting them to repeat the same as quickly as possible. This allows comparison of the SVC with the FVC and allows the user to discard poor attempts where the FVC is below the expiratory volume.
• Limit the total number of attempts (practice and recording) to eight.

Three satisfactory blows should be performed and best values taken for interpretation. Criteria for satisfactory blows are:

• The blow should continue until a volume plateau is reached - this may take more than 12 seconds in severe COPD.
• FVC and FEV₁ readings should be within 5% or 100ml.
• The expiratory volume-time graph should be smooth and free from irregularities.

Reversibility testing

• Perform baseline spirometry first.
• Bronchodilator reversibility testing: before undertaking bronchodilator testing, the patient should stop short acting β2 agonists for 6 hours, long acting bronchodilators for 12 hours and theophyllines for 24 hours. Administer bronchodilator (at least 400mcg salbutamol) and repeat spirometry after 15 minutes.
• Steroid reversibility testing: a steroid trial (30-40mg prednisolone daily for 2 weeks or 1000μg of ICS for 3 months) is undertaken. An increase in FEV₁ of >12% and >200mls from baseline is significant. An increase of >20% and >400mls is suggestive of asthma.

Always repeat a series of readings on another occasion before basing a diagnosis on spirometry.For a full assessment you need to consider values: FEV₁, FVC, PEFR; calculate the FEV₁/FVC ratio; and how these compare with the individual's predicted values (based on age, sex, race and height).

FEV₁

FEV₁ is strongly recommended as the measurement of choice in COPD as:

• It is reproducible and objective with well defined normal ranges.
• It can be measured quickly and easily at all stages of disease .
• Variation on different occasions on the same patient is low (<170mls).
• FEV₁ is a good predictor of future morbidity and mortality (better than FEV₁/VC).
• Serial measurements provide evidence of disease progression.
• Peak flow measurements do not distinguish between obstruction and restriction of airflow, and may seriously underestimate the degree of airway obstruction in COPD.
• In mild asthma, FEV₁ is likely to show up the lesser degrees of airflow obstruction occurring later in the expiratory effort.

However in some with advanced COPD, forced expiration may result in closing of airways and trapping of air, so relaxed vital capacity (SVC) may be a better measure of lung function.

FVC

• There is likely to be a reduction in FVC in patients with moderate to severe COPD, which is caused by the alveolar damage and coalescence, together with loss of elasticity of the lung tissue.
• Patients with chronic asthma may have a reduction in FVC.

FEV₁/FVC ratio

• A ratio of <70% implies obstructive disease.
• In the elderly, the FEV₁/FVC may fall to <70% in the absence of airways obstruction (so use tables to compare to predicted values for age etc), but in everyone, if the value is >70%, obstruction is effectively excluded.
• The hallmark of an obstructive defect is slowing of expiratory flow, so that a low proportion of the FVC is expired in the first second and the FEV₁/FVC ratio is reduced.
• If the patient has a restrictive ventilatory defect the FEV₁ and FVC are both reduced, but in proportion, so the FEV₁/FVC ratio remains normal (greater than 75%).
• Restrictive ventilatory defects can be due to various intrapulmonary diseases, eg pulmonary fibrosis, pulmonary oedema, collapse or consolidation of the lung but also importantly with extra-pulmonary conditions, eg large pleural effusion, rib cage deformity (scoliosis), after lung surgery, and with weakness of the respiratory muscles. Clearly the measurements need to be interpreted in the clinical context and if a restrictive abnormality is discovered a chest x-ray is usually essential for interpretation.

Restrictive vs obstructive patterns - summary

Flow volume loops

(see ⁶ and ⁴ for diagrammatic illustration of flow volume loops)
Flow volume loops show flow rate as the lung empties - the shape of the loop depends on the mechanical properties of the lung and different diagnoses provide different shaped loops:

• Normal - on exhalation there is a rapid rise to the maximal expiratory flow followed by a steady uniform decline until exhalation is complete.
• Asthma - typically the curve is a smooth concave shape as airway obstruction is relatively constant throughout expiration.
• COPD - typically the curve is angled or 'kinked' as COPD lungs collapse with forced expiration.
• Restrictive disease - the curve is typically a normal height but with a very steep gradient as the lung volume is diminished.

All patients with either suspected asthma or COPD should ideally have spirometry performed to aid initial diagnosis.

COPD ³

• A diagnosis of COPD should be considered in all patients over 35 years who smoke or have smoked and have chronic symptoms of breathlessness, cough and sputum.
• Symptoms may be accepted as a consequence of smoking and therefore medical help not sought or not apparent until significant airways obstruction is present (up to half of respiratory reserve may be lost prior to developing chronic symptoms).
• Early diagnosis with spirometry is vital - enabling the identification of susceptible patients, improve smoking cessation rates and prevent deterioration of patients identified with early COPD.

The grade of COPD is important in deciding on appropriate treatment for COPD patients:

• If patients have mild symptoms but their spirometry result confirms mild or moderate COPD then they might be more persuaded to stop smoking. Smoking cessation is the single most important intervention in controlling progression of COPD.
• All patients with COPD should have their FEV₁ monitored annually to assess progression of their disease as the level of FEV₁is related to complications such as development of respiratory failure or pulmonary hypertension. Serial measurements over a few years allow assessment of rate of decline of FEV ₁, an indicator of mortality risk in COPD.

Grading of COPD

Asthma

• Spirometry is not essential for the diagnosis of asthma.²
• Slowly progressive respiratory symptoms in a middle-aged or elderly smoker are most likely due to COPD but such patients may also have asthma (more likely if symptoms have onset prior to age of 35 years and have symptoms that vary in severity).
• Where diagnosis is in question, serial peak flows looking for diurnal variation of >20% may help.
• NICE guidance³ suggests that reversibility testing should not be routine if clinical features and spirometry are strongly suggestive of COPD. However, the diagnosis of asthma can be supported by the demonstration of 15% reversibility or more in either PEFR or FEV₁ post bronchodilator and/or following a two week course of high dose prednisolone.