Spirometry - Introduction
|A great deal can be learned about the
mechanical properties of the lungs from measurements of forced maximal
expiration and inspiration. Since Hutchinson first developed the spirometer
in 1846, measurements of the so-called dynamic lung volumes and of maximal
flow rates have been used in the detection and quantification of diseases
affecting the respiratory system. Over the years it has become obvious that
the spirometer and peak flow meter used to measure ventilatory function are
as deserving of a place in the family practitioner's surgery as the
sphygmomanometer. After all, who would dream of managing hypertension
without measurement of blood pressure?
It is important to appreciate that the clinical value of spirometric measurements is critically dependent on the correct operation and accuracy of the spirometer, performance of the correct breathing maneuver and use of relevant predicted normal values.
Measurement of Ventilatory Function
Conventionally, a spirometer is a device
used to measure timed expired and inspired volumes, and from these we can
calculate how effectively and how quickly the lungs can be emptied and
All indices of ventilatory function should be reported at body temperature and pressure saturated with water vapor (BTPS). If this is not done the results will be underestimated, because when the patient blows into a ‘cold’ spirometer, the volume recorded by the spirometer is less than that displaced by the lungs.
The Technique - How To Do It and Common Pitfalls and Problems
To ensure an acceptable result, the FVC
maneuver must be performed with maximum effort immediately following a
maximum inspiration; it should have a rapid start and the spirogram should
be a smooth continuous curve.
If only peak expiratory flow is being measured then the patient need only exhale for a couple of seconds. Essentials are:
Remember, particularly in patients with airflow obstruction, that it may take many seconds to fully exhale. It is also important to recognize those patients whose efforts are reduced by chest pain or abdominal problems, or by fear of incontinence, or even just by lack of confidence. There is no substitute for careful explanation and demonstration - demonstrating the maneuver to the patient will overcome 90% of problems encountered and is critical in achieving satisfactory results. Observation and encouragement of the patient's performance are also crucial. Be sure to examine the spirogram or flow volume curve for acceptability and reproducibility.
At least three technically acceptable maneuvers should be obtained, ideally with less than 0.2 L variability for FEV1 (and FVC) between the highest and second highest result. Quote the largest value. The American Thoracic Society (ATS) provides the following guidelines for maneuver performance. 1
FEF25-75% and Expiratory Flows
PEF (Using a peak flow meter)
Figures 3 (a) and 3 (b) show some problematic examples compared with well-performed maneuvers.
The most common patient-related problems when performing the FVC maneuver are:
Once again, demonstration of the procedure will prevent many of these problems, remembering that all effort-dependent measurements will be variable in patients who are uncooperative or trying to produce low values.
Glottic closure should be suspected if flow ceases abruptly during the test rather than being a continuous smooth curve. Recordings in which cough, particularly if this occurs within the first second, or hesitation at the start has occurred should be rejected. Vocalization during the test will reduce flows and must be discouraged - performing the maneuver with the neck extended often helps.
Predicted Normal Values
To interpret ventilatory function tests in any individual, compare the results with reference values obtained from a well-defined population of normal subjects matched for gender, age, height and ethnic origin and using similar test protocols; and carefully calibrated and validated instruments.2
Normal predicted values for ventilatory function generally vary as follows:
Interpretation of Ventilatory Function Tests
Measurements of ventilatory function may be very useful in a diagnostic sense but they are also useful in following the natural history of disease over a period of time, assessing preoperative risk and in quantifying the effects of treatment. The presence of ventilatory abnormality can be inferred if any of FEV1, VC, PEF or FEV1/VC% are outside the normal range.
The inter-relationships of the various measurements are also important diagnostically (see Table and Figure 4). For example,
The shape of the expiratory flow-volume
curve varies between obstructive ventilatory defects where maximal flow
rates are diminished and the expiratory curve is scooped out or concave to
the x-axis, and restrictive diseases where flows may be increased in
relation to lung volume (convex).
To measure the degree of reversibility (typically increased in asthma) of airflow obstruction, perform spirometry before and 10 to 15 minutes after administering a bronchodilator by metered dose inhaler or jet nebulizer. beta2 agonists (e.g. salbutamol, terbutaline, etc.) are generally considered the benchmark bronchodilator.
To express the degree of improvement,
There is presently no universal agreement on the
definition of significant bronchodilator reversibility. According to the ATS
the criteria for a significant response in adults is:
Normal subjects generally exhibit a smaller degree of reversibility (up to 8% in most studies). The absence of reversibility does not exclude asthma because an asthmatic person’s response can vary from time to time and at times airway caliber in asthmatic subjects is clearly normal and incapable of dramatic improvement.
When peak expiratory flow is measured repeatedly over a period and plotted against time (e.g. by asthmatic patients), the pattern of the graph can be very important in identifying particular aspects of the patient's disease. Typical patterns are
Further practical information about measuring peak flow is given in the National Asthma Council’s Asthma Management Handbook. Remember that many patients have poor perception of their own airflow obstruction and their PEF is a better index of the state of their airways than how they feel.
It is worth trying to recognize clinical situations and choosing the appropriate test for each. For example,
To identify asthma triggers or treatment responses over long periods of time, regular PEF monitoring by the asthmatic patient is best.
Infection Control Measures
In patients with a known infectious
disease, many laboratories prefer to measure ventilatory function using a
pneumotachograph or other electronic sensor, as these can be more easily
cleaned and sterilized than conventional bellows or water-sealed
Measurements of ventilatory function
should be part of the routine assessment of patients with respiratory
The use of a fixed percent of predicted (eg 80%) to define the lower limit of normal is widespread despite being shown to be statistically invalid. A more appropriate approach is based on the use of the residual standard deviation (RSD) from regression analyses (e.g. FEV1 versus Age) but this is only possible if the survey population data are normally distributed for subjects of all ages and heights. The addition or subtraction of 1.64 times the RSD from the mean predicted value results in an upper or lower limit of normality with a confidence level such that 95% of the subjects in the survey lie above the lower limit.
If the population data is not normally distributed then the 95th percentile may be used. This represents the point at which 95% of the normal population falls. Lower limits of normal are also given at the 95th percentile.
The mean predicted normal values of Caucasian males and females between 10 and 80 years of age are given in the following tables3. The 95% lower limit of normal are age-sex specific and are listed as percent predicted in the table below.
Tables by age in years; height in centimetres. (Click to view)