Spirometry - Introduction

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 filled.

A spirogram is thus a volume-time curve and Figure 1 shows a typical curve. Alternatively, measures of flow can be made either absolutely (e.g. peak expiratory flow) or as a function of volume, thus generating a flow-volume curve (Figure 2), the shape of which is reproducible for any individual but varies considerably between different lung diseases. A poorly performed maneuver is usually characterized by poor reproducibility.

The measurements which are usually made are as follows:

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.

Figure 2

(Click to enlarge)

Normal maximal expiratory and inspiratory flow-volume curve.

The Technique - How To Do It and Common Pitfalls and Problems

How to Do It

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.

To achieve good results, carefully explain the procedure to the patient, ensuring that he/she is sitting erect with feet firmly on the floor (the most comfortable position, though standing gives a similar result in adults, but in children the vital capacity is often greater in the standing position). Apply a nose clip to the patient's nose (this is recommended but not essential) and urge the patient to: · breathe in fully (must be absolutely full);

• seal his/her lips around the mouthpiece;

• blast air out 'as fast and as far as you can until the lungs are completely empty;

• breathe in again as forcibly and fully as possible (if inspiratory curve is required).

If only peak expiratory flow is being measured then the patient need only exhale for a couple of seconds. Essentials are:

• to breathe in fully (must be absolutely full);

• a good seal on the mouthpiece and

• very vigorous effort right from the start of the maneuver and continuing until absolutely no more air can be exhaled;

• no leaning forward during the test.

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.

Attention to fine detail in the performance of the breathing maneuver is critical to obtaining reliable results.

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

FVC

• Minimum of 3 acceptable blows

• A rapid start is essential: this is defined as a back-extrapolated volume of <5% of FVC or 0.15 L, whichever is greater (See Figure 4)

• At least 6 second expiration

• End of test - no change in volume for at least 1 second after exhalation time of 6 seconds; or FET >15 seconds; or stopped for clinical reasons

• Spirometer temperature between 17 and 40 degrees Celsius; measure spirometer temperature to one degree Celsius

• Use of nose clip is encouraged

• Sitting or standing

• Reproducibility: the highest and second highest FVC should agree to within 0.2L

• Largest VC or FVC is recorded

FEV₁

• As for FVC

• Take largest FEV1 even if not from the same curve as the best FVC

• "Zero time" determined by back-extrapolation - extrapolated volume should be <5% of FVC or 0.15 liters, whichever is greatest (Figure 4)

• Smooth, rapid take off with no: hesitation, cough, leak, tongue obstruction, glottic closure, valsalva or early termination

• Reproducibility: the highest and second highest FEV1 should agree to within 0.2L

FEF_25-75% and Expiratory Flows

• From the single spirogram with the largest sum of FEV1 + FVC

PEF (Using a peak flow meter)

• Minimum of 3 acceptable blows

• Standing position is preferred

• Nose clip not necessary

• No cough

• Blow duration 1 to 2 seconds

Figures 3 (a) and 3 (b) show some problematic examples compared with well-performed maneuvers.

Patient-Related Problems

The most common patient-related problems when performing the FVC maneuver are:

1. Sub maximal effort

2. Leaks between the lips and mouthpiece

3. Incomplete inspiration or expiration (prior to or during the forced maneuver)

4. Hesitation at the start of the expiration

5. Cough (particularly within the first second of expiration)

6. Glottic closure

7. Obstruction of the mouthpiece by the tongue

8. Vocalization during the forced maneuver

9. Poor posture.

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.

The vigorous effort required for spirometry is often facilitated by demonstrating the test yourself.

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.

Classifying Abnormal Ventilatory Function

The inter-relationships of the various measurements are also important diagnostically (see Table and Figure 4). For example,

Classification Of Ventilatory Abnormalities by Spirometry
	OBSTRUCTIVE	RESTRICTIVE	MIXED
FEV1		or Normal
FVC	or Normal
FEV1/FVC		Normal or

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).

A "tail" on the expiratory curve as residual volume is approached is suggestive of obstruction in the small peripheral airways. Examination of the shape of the flow-volume curve can help to distinguish different disease states, but note that the inspiratory curve is effort-dependent.

For example, a plateau of inspiratory flow may result from a floppy extra-thoracic airway, whereas both inspiratory and expiratory flow are truncated for fixed lesions.

Expiratory flows alone are reduced for intra-thoracic obstruction (Figure 5).

Figure 5
(Click to enlarge)	Maximum expiratory and inspiratory flow volume curves with examples of how respiratory disease can alter its shape:   a) normal subject;   b) obstructive airway disease     (e.g. asthma); c) severe obstructive disease     (e.g. emphysema); d) restrictive lung disease     (e.g. pulmonary fibrosis); and e) fixed major airway obstruction     (e.g. carcinoma of the trachea).

Measuring Reversibility of Airflow Obstruction

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,

• calculate the absolute change in FEV₁ (i.e. post-bronchodilator FEV₁ minus baseline FEV₁) and

• calculate the percentage improvement from the baseline FEV₁.
   

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:

>12% improvement in FEV₁ (or FVC) and an absolute improvement of >0.2 L

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.

Peak Flow Monitoring

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

• the fall in PEF during the week with improvement on weekends and holidays which occurs in occupational asthma; and

• the ‘morning dipper’ pattern of some asthmatic patients due to a fall in PEF in the early morning hours. Isolated falls in PEF in relation to specific allergens or trigger factors can help to identify and quantify these for the doctor and patient. A downward trend in PEF and an increase in its variability can identify worsening asthma and can be used by the doctor or patient to modify therapy. PEF monitoring is particularly useful in the substantial number of asthmatic people with poor perception of their own airway caliber. Response to asthma treatment is usually accompanied by an increase in PEF and a decrease in its variability.

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.

PEF self-monitoring is very useful in asthma management, particularly in those with poor perception of their own airway caliber.

Choosing an Appropriate Test

It is worth trying to recognize clinical situations and choosing the appropriate test for each. For example,

• If upper airway obstruction is suspected, flow-volume curve with particular emphasis on inspiration is the best test.

• For the diagnosis of asthma, spirometry before and after the administration of a bronchodilator, looking for an obstructive pattern with significant improvement, would apply. It is usually necessary to repeat spirometric assessment of airway function at follow-up visits in asthma and other lung conditions where change can occur over short periods of time.

• In patients suspected of having asthma but in whom baseline spirometry is normal, it is appropriate to try bronchial challenge testing with measurement of spirometry before and after provocation by exercise or by inhalation of histamine, methacholine or hypertonic saline.

To identify asthma triggers or treatment responses over long periods of time, regular PEF monitoring by the asthmatic patient is best.

Spirometry

• Detection of disease and its severity

• Identification of asthma triggers

• Progress/natural history monitoring

• Treatment response assessment

• Preoperative assessment

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 spirometers.

Although the transmission of respiratory pathogens (e.g. Mycobacterium avium, M. tuberculosis and aspergillus species) via spirometers has not been fully established, the potential risks are difficult to disprove. During spirometry patients can generate flows up to 14 L/sec (840 L/min) which can easily mobilize saliva and create dense macro- and micro-aerosols by entrainment of the fluid lining the mucous membranes. These can then be deposited in the equipment. Unless such deposition is prevented or the equipment is rigorously cleaned and decontaminated, the chance of cross-infection exists.

Mouthpieces must be disposed of or cleaned and disinfected between patients because the greatest danger of cross-infection is via direct contact with bodily fluids.

Since it is usually impractical to effectively decontaminate the interior surfaces of a spirometer between patients, most lung function laboratories clean and disinfect their equipment periodically (weekly or monthly) or use a disposable, low-resistance micro-aerosol filter inserted between the subject and spirometer to prevent contamination. Filters also have the advantage of protecting sensors and the internal surfaces of the spirometer from damage and reduce the corroding effects of cleaning agents and disinfectants. The extent to which the use of filters can effectively obviate the need for cleaning and disinfection is unclear. The cost of filters may be offset by reduced cleaning and disinfection costs. Other laboratories use disposable mouthpieces containing a one-way valve to prevent inspiration from equipment, but this is only possible when performing solely expiratory spirometry. However, their effectiveness at reducing the risk of cross-infection does not appear to have been studied.

If disassembling the spirometer for cleaning, it is essential to

• thoroughly dry the components before reassembling;

• check the spirometer for correct operation;

• adjust the calibration, if necessary.

Summary

Measurements of ventilatory function should be part of the routine assessment of patients with respiratory disease.

Patient self-monitoring with a peak flow meter or similar device is now an accepted part of the day-to-day management of asthma.

Spirometry measurements performed in the general practitioner's surgery or respiratory specialist's office can detect respiratory abnormalities and help to differentiate the various disease processes which result in ventilatory impairment.

They also have an important role in following the natural history of respiratory disease and its treatment.