Desktop Helper No.14 - Quick guide to spirometry - online version

Before the test

When performing spirometry, consider potential contraindications (Table 1). This test is highly dependent on the person’s collaboration and the testing circumstances, therefore, the procedure should be explained beforehand and a decision made by the prescribing physician if the person should stop taking any respiratory medications prior to the test (see Table 2 for minimum timings). It may not be necessary to withhold medication if the purpose of the test is to determine whether the person’s lung function can be improved with therapy in addition to their regular treatment. Instruct the person not to smoke, vape or use a water pipe and abstain from any strenuous physical exercise for at least one hour prior to the test, or to consume intoxicants up to 8 hours before the test. Ask them to loosen any tight clothing. Spirometry must be conducted in a comfortable and well-ventilated room (ideally, specific for spirometry), with the person sitting on a chair without arms, wheels or height adjustment. There must be scales, a stadiometer and a basic weather station (if not already integrated with the test equipment). The spirometer should have a maximum error range of ±2.5% when tested with a 3L calibration syringe.

Table 1: Contraindications for spirometry

Any situation that puts the person's health at serious risk when making a significant effort such as:

• Significant haemoptysis
• Active or recent pneumothorax. Having a pneumothorax in the past does not contraindicate spirometry
• Unstable CV disease (e.g. angina, recent MI, PTE)
• Brain, thoracic or abdominal aneurysms
• Recent retinal detachment or recent eye surgery (e.g. cataracts)
• Recent chest or abdominal surgery

Situations in which minimal acceptable quality manoeuvres cannot be obtained such as:

• Inability to understand directions or unwillingness to follow the directions
• Not understanding the manoeuvre well (e.g. children under 6 years old, mental deterioration, some elderly people)
• Poor physical state (e.g. cachexia)
• Presence of a tracheotomy. If it is considered necessary to perform spirometry on a person with a tracheostomy, they should be referred to a specialist clinic
• Oral and/or facial problems that prevent correct sealing of the mouth around the mouthpiece (e.g. facial paralysis)
• Uncomfortable nausea when inserting the mouthpiece

CV, cardiovascular; MI, myocardial infarction; PTE, pulmonary thromboembolism.

Although modern spirometers can yield multiple-variable results, three basic parameters are enough to interpret them: the FVC measured in litres, the forced expiratory volume in the first second (FEV₁), a flow measurement expressed in litres per second, and their ratio (FEV₁/FVC) given as a ratio or percentage.

• The FVC is the total air volume that can be forcefully exhaled after taking the deepest breath possible. Make sure the person has emptied all the available air from their lungs to finish the manoeuvre.
• The FEV₁ is the volume exhaled in the first second of a forced expiratory manoeuvre.
• The relation between the forced exhaled volume in the first second and the forced vital capacity (FEV₁/FVC), also known as the forced expiratory ratio (FER or FEV₁%), indicates the percentage of the vital capacity exhaled during the first second of the manoeuvre. It is the key parameter for measuring airways obstruction.

Diagnosis of obstructive airway disease

Under normal conditions, in the first second of the forced expiration, over 70% of the FVC is exhaled. If the FEV₁/FVC ratio is below 70%, it means that there is an airway obstruction.

To reduce underdiagnosis of obstruction in young patients and overdiagnosis in the elderly, which happens when using the fixed limit of 70%, we propose to use the Lower Limit of Normal (LLN) of the FEV₁%, as the cut-off value to determine the presence of an obstruction, which corresponds to the fifth percentile, or to -1.64 on the z-score of the reference value. However the LLN is not included in the software of most current spirometers, and is only indicated without the need for further conversions in the theoretical values of the Global Lung Function Initiative (GLI).(7) For now, 70% remains the cut-off point for obstruction  recommended by international organisations, such as the Global Initiative for Chronic Obstructive Lung Disease (GOLD),(8,9) when assessing people with COPD.

Result interpretation

Types of spirometric curves

The first step in the correct interpretation of a spirometry result is to check the acceptability of the curves generated during the manoeuvre using two different graphs: the volume-time graph (V-T) and the flow-volume graph (F-V). Review of the curves on each of these graphs will indicate whether the manoeuvre has been performed correctly and can be considered as acceptable, or if there have been significant errors that require a repetition of the test.

• Volume-time graph: Volume is measured in litres on the y axis (vertical), while time is measured in seconds on the x axis (horizontal). A normal V-T curve has a steep rise, given that a large proportion of the air is expelled in the first second. Following this, the slope becomes gradually smoother until flattening out, when it reaches maximum volume (FVC). The volume of expelled air in the first second is the FEV₁ (Figure 1).
• Flow-volume graph: In this type of graph, flow is measured in litres per second on the y axis (vertical), and the volume is measured in litres on the x axis (horizontal). The normal F-V curve has a very steep rise, near the horizontal axis (flow) until reaching a peak (peak expiratory flow [PEF]). From there, it decreases in a somewhat straight line, with a less pronounced slope, until it finishes by asymptotically reaching the x axis (volume), showing the FVC (Figure 2).

Manoeuvre acceptability

By observing the curves, it is possible to identify a correct manoeuvre and thus meaningful results. The curve must indicate test, and that the peak of effort is very close to the start of the manoeuvre; that the shape of the curve is appropriate, without irregularities pointing at an incorrect or abnormal flow; and that it finishes properly – gradually rather than abruptly – verifying that virtually all of the previously inspired volume was exhaled (Figure 3).

To determine if there has been a good start in each curve, the Back Extrapolated Volume (BEV) (calculated by the spirometer) is used, which must be < 100 ml or 5% of the FVC, whichever is greater.

Manoeuvre repeatability

As well as yielding acceptable curves, spirometry must be repeatable in order to be interpreted; we must be sure it will yield similar results every time it is repeated. So, after having obtained at least three acceptable curves, the difference between the two best curves must be less than 150 ml or 5% (for both FVC and FEV₁), whichever is greater. In children under 6 years it must be less than 100 ml or 5%, whichever is greater. Most spirometers provide this information automatically (Figure 4).

Frequent errors

By observing the curves, it is possible to identify errors by the person and/or operator that may disrupt the test and require a repetition (Figure 5).

Spirometry does not provide a diagnosis in and of itself - it merely supports or contradicts a suspected diagnosis, based on related clinical data and other complementary tests. However, spirometry does provide valuable clues, and by analysing its results, it is possible to identify four spirometric patterns: normal, obstructive, restrictive and mixed. These distinct patterns, together with the person's clinical data, make it possible to make a diagnosis.

Post-bronchodilator responsiveness test

The Bronchodilator (BD) Responsiveness Test is used to study the dilation that can occur in the person's bronchi after being administered a bronchodilator (usually salbutamol/ albuterol), and whether that bronchodilation reaches full reversibility (normalisation of a previously obstructive pattern). FVC pre- and post-BD are compared. If there is >10% of predicted value improvement in either the FEV₁ or FVC the test is considered to have a positive BD response. The previous criteria for a positive BD test of >200 ml has been recently removed from ERS-ATS guidelines to simplify the interpretation of the test.

Next steps

In cases where spirometry reveals a mixed pattern (obstruction + restriction), the person should be referred for a lung volume test (e.g., plethysmography), to evaluate whether the reduced FVC is due to restrictive disease, or is just a functional restriction of the volume due to air trapping, with an increase in the residual volume, which happens to the vast majority of people with severe cases of COPD.