An inaccurate spirometer is worse than useless. But having an accurate one is not a matter of belief or faith; it is a matter of proof. Furthermore, this must be documented proof. Without a documented daily accuracy check, your spirometry measurements are useless, no matter how accurate you believe your spirometer to be.
All spirometry standards state that daily checks are essential. I say it is also common sense. It only takes two minutes at the start of the day to assure yourself, and have documented proof, that your spirometer is accurate.
Also, as with any precision measuring instrument, a spirometer requires an ongoing programme of preventative maintenance that includes regular accuracy and linearity assessment, cleaning and performance checks. Just like your car, an annual service is essential to validate proper functioning, safety and routine replacement of certain components, but medical devices with a measuring function additionally require hygiene procedures and calibration certification traceable to international standards.
Precision syringe for checking accuracy of your spirometer
A 3-L precision syringe is the essential tool to check spirometer accuracy. All spirometry standards recommend a check each day that the device is used. This is different to the need to adjust the calibration, which will be very infrequent or never on today’s spirometers.
To conform to the required standards, spirometers must be accurate to within 3%. The precision syringe needs to be at the same temperature as the spirometer and, for this reason, it is usually stored near the spirometer.
Conducting a spirometry test
It is essential that the subject performing the test be clearly instructed in the procedure prior to the commencement of each test. To achieve good results, carefully explain the procedure to the patient. A very enthusiastic demonstration by the operator is required so that the subject makes a maximum effort when carrying out the forced expiratory test.
Over the past decade or so, spirometry guidelines have increasingly been moving towards recommending a sitting posture, but stressing the importance of recording the testing posture.
Comparing the spirometry measurements with previous measurements on a test subject is the most valuable information that can be gained from a spirometry test. This can be done very simply on a single sheet of paper by recording the FEV1 (forced expiratory volume in one second) value at age of test. This is a serial spirometry record.
Some computerised spirometers do this automatically, but often this simple paper chart is still the best thing to file in the worker’s medical record.
Make the test subject comfortable and fully explain the procedure to them. Use a new disposable noseclip and mouthpiece with each subject.
Spirometry steps:
Measure their standing height (without shoes)
Explain and demonstrate the test procedure
Observe and enthusiastically coach the subject on each effort
Repeat until test repeatability is obtained.
Use the Serial Spirometry Record to note: date of test; measured values (name, ID, date of birth, gender, height, posture).
The vital capacity (VC) test
The slow VC test, to discover air trapping and thus not underestimate obstruction, should always precede the forced vital capacity (FVC) test as bronchospasm (a sudden constriction of the muscles in the walls of the bronchioles) may be induced in susceptible subjects during the forced test.
The operator demonstrates using a mouthpiece, blowing out slowly from full inspiration.
The subject practices with their mouthpiece. They must lightly bite the mouthpiece (don’t use it like a trumpet).
The VC spirometry test is performed, with enthusiastic coaching from the operator. They must:
Breathe in fully
Lightly bite the mouthpiece
Slowly exhale until their lungs are completely empty
Squeeze out the last air in the final few seconds
This is usually repeated two or three times until a consistent VC is achieved.
The FVC technique
The simple FVC manoeuvre must be performed with maximum effort immediately following maximum inhalation. It should have a rapid start and the subject must continue to blow out for as long as possible.
The operator demonstrates using a mouthpiece, blowing out rapidly from full inspiration.
The subject practices with their mouthpiece.
The FVC spirometry test is performed, with enthusiastic coaching from the operator. They must:
Breathe in fully
Lightly bite the mouthpiece
Then blast air out as fast and as long as they can until their lungs are completely empty.
Continue exhalation for at least six seconds, longer if necessary.
This must be repeated at least twice more until a good FVC session is achieved. Make sure you keep testing until this is achieved or the subject becomes fatigued – a maximum of eight blows is normally the limit.
A good FVC session has the following characteristics:
Blows with good start of test
Free of cough or other artefact (such as a second breath)
Tests which last for a duration of more than six seconds and have a plateau on the volume/time curve at the end of the blow
The two best blows are within 5% for both FEV1 and FVC volumes.
Some tips
You must demonstrate the full procedure to the subject.
Ensure a good seal on the mouthpiece by biting.
Encourage a vigorous effort from the start of the manoeuvre.
Continue until absolutely no more air can be exhaled.
Make sure they do not lean forward during test.
When using FEV, in Office Spirometry, the same technique applies.
Diurnal and occupational variations
The test should be carried out noting the time of the test and the date. Diurnal variations, both daily and seasonal, are greater in abnormal than healthy subjects although they do occur to some lesser extent in the latter. It is, therefore, advisable that repeat examinations should be carried out at the same time of the day.
Also, especially in the case of people engaged in industry where exposure to an allergen is possible, the examinations should be on the same day of the working week.
How to calculate the results from a spirogram
The calculation of results from a spirogram is today hardly required, since most spirometers have electronics and software to do it for you. But you should at least understand where the two main indices come from, namely the FEV1 and FVC, or the FVC surrogate, FEV6. In the example below these are the same value, as is the VC. This would be expected with normal lung function.
Spirometry fundamentals for OH professionals
What spirometry measures
Spirometry is all about measuring the lung mechanics – how the volume in the lung empties. This is a very important part of ventilating the lungs for the purpose of getting oxygen into the blood stream and carrying away carbon dioxide and other waste gasses and vapours excreted by the lungs.
Spirometry is a simple way to get an objective measure of a complex process. In the lungs, expanded alveoli (which are responsible for gas exchange with the blood) empty into small airways, and small airways empty into large airways. Measuring the volume of the forced expiratory air is a way to determine the limits of the lung mechanics. This is measured by a timed volume displacement measuring device or a flow measuring device which then integrates the flow with respect to time to estimate volumes.
The thoracic cavity
Inspiration (inhalation) causes upward and outward movement of the ribs and a flattening of the diaphragm using the intercostal muscles (which run between the ribs, and help form and move the chest wall) and the muscle of the diaphragm. This causes the expansion of the thoracic (chest) cavity and lowers the pressure of the pleural space surrounding the lung (the potential space between the two layers of the pleura which surround the lungs). Thus air enters the lungs and they expand.
Quiet exhaling is a passive act – the inspiratory muscles relax and the intra-pleural pressure becomes less negative. The elastic recoil of the lungs that had resisted inhaling causes the pressure in the periphery of the lung to increase and the flow of air is reversed. A forced exhalation is assisted by the use of muscles, particularly contraction of the abdominal muscles, which push the diaphragm upward to reduce the size of the rib cage.
The work of respiration is done during inhaling. The intercostal muscles perform the majority of the work, helped by the diaphragm. Exhaling is a normally passive act, but of course not during forced expiratory spirometry.
Static and Dynamic Lung Volume Measurements
There are two main kinds of tests performed on simple spirometers:
Static lung volume measurements, obtained in spirometry using the slow VC (vital capacity) measurement method
Dynamic lung volume measurement, or forced spirometry.
The VC (sometimes called SVC, or slow VC) is the air that can be exhaled from fully inflated lungs. The residual volume (RV) is the air that remains (cannot be exhaled due to closure of the small conducting air passages). Thus, the total lung capacity (TLC) is the sum of the VC (also FVC or forced vital capacity in normal lungs) and the RV.
In obstructive disease, the RV and TLC are usually increased. In restrictive disease, they are decreased. Tidal volume (TV) is the amount of air exhaled in resting breathing, which is an important measure when compared to the inspiratory capacity (IC). In many types of lung disease the IC is almost the same as the TV. This means that the sufferer has no more capacity to breathe in – the classic case of an acute asthma attack. The asthmatic feels they can’t breathe in enough. While this is true, it is in fact caused by them not breathing out enough, because of the severe narrowing of the airways.
The remaining lung volumes shown above cannot be measured on a simple spirometer. It requires laboratory lung function analysers with trace gasses, and therefore are of no relevance in routine occupational health spirometry.
Expressing expiratory airflow
The expiratory volume/time curve gives the simplest and most informative picture of the expiratory spirogram in a forced expiratory test. This is because the FEV1, FVC, and expiratory time can be directly visualised from the curve.
Exactly the same information is available from the flow volume curve. But the FEV1 and expiratory time cannot be directly visualised. Some experts believe that there are some quantitative advantages to being able to view the contours of the expiratory flow/volume (f/v) curve (ie, the progressive concavity associated with the loss of elastic recoil). But there are many disadvantages to only viewing the f/v curve, not least the loss of quality control.
Inspiratory airflow
Some spirometers also allow the reading and measurement of inspiratory airflow. Inspiratory curves can detect the presence of extrathoracic airways obstruction such as caused by retrosternal goiter, vocal cord paralysis, tracheal stenosis or tracheal tumours, because the inspiratory airflow is truncated or flattened at the bottom of the curve. However, the conditions characterised by upper airway obstruction are relatively rare and not of concern to occupational health practitioners. Also, such conditions can be observed without the need of a spirometer.
Spirometry curve patterns: Airways obstruction and restriction
Lung diseases are grouped into two types of disease: obstructive disease and restrictive disease. Obstruction of the airways means that they are narrower than they should be so that air cannot exit the lungs as fast as a normal lung. Restriction of the airways means they are limited in the amount of air they can exhale – in other words, the exhaled volume is smaller than a normal lung.
Of course, some people can have both types of pattern simultaneously, called mixed pattern. But by far the most prevalent pattern is obstruction. This is particularly true in occupational health where there may be problems with poor air quality, sick building syndrome, exposure to respiratory sensitisers, dusts and fumes.
As well as the degree of severity, these patterns occur in different parts of the lung, which characterise the shape of the spirometry curve. Put simply, the big airways empty first, followed by the smaller airways and the alveoli, which empty more slowly.
Interpretation
Spirometric interpretation needs to be simple. This is why the two basic parameters of FVC and FEV1 are normally the only ones used. The normal ratio between FEV1 and FVC is 0.70 to 0.75. FEV1 is an index of air flow, which is the volume expired in a second. Since the FEV1 is an index of flow, a reduced FEV1 indicates a flow or obstructive abnormality. Since the FVC is an index of volume, a reduced FVC indicates a volume or restrictive ventilatory abnormality.
Thus, an obstructive defect is present when the FEV1/FVC ratio is <0.70. In an adult, a restrictive defect is usually present when the FEV1/FVC is >0.80.
In the case of mixed obstructive and restrictive spirometry abnormalities, measurements of lung volumes are also useful. In particular the finding of a VC much larger than the FVC can reveal airways obstruction that can be overlooked using only a FVC curve.
Bernard Garbe is managing director of Vitalograph
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References
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