Interpretive
Strategies
Figure 2 shows a simple algorithm to
guide the interpretation of spirometry results. In the first instance,
interpretation should be based on the FEV1/FVC ratio, FEV1,
and FVC to determine if the results demonstrate normal, obstructive, restrictive
or mixed patterns. Categorizing the severity of an obstructive defect
should be based on the percent predicted FEV1
rather than the FEV1/FVC ratio.
Figure 2 Guideline for
Spirometry Interpretation
* National Asthma Council (NAC) 2002 Asthma Management
Handbook.
(Note: Edition 6, 2006 now available
www.nationalasthma.org.au/amh2006/)
^ The COPDX Plan: Australian and New Zealand
Guidelines for the management of
Chronic Obstructive Pulmonary Disease 2003.
Medical Journal of Australia, Vol 178. Supplement.
Pages 1 - 40, 17 March 2003.
www.lungnet.org.au/ |
There are three classifications for abnormal spirometry (see
Figure 3):
- Obstructive Ventilatory Defect: characterised by
reduced expiratory flows e.g. reduced FEV1/FVC
ratio, FEV1, FEF25-75%
or if the expiratory flow volume curve is scooped-out (see examples Figure
2). Common examples include asthma and COPD.
- Restrictive Ventilatory Defect: characterised by loss
of lung volume in the absence of airflow obstruction – i.e. as suggested by
a low SVC or FVC but normal or high FEV1/FVC
ratio. Examples include interstitial lung disease, respiratory muscle
weakness, and thoracic cage deformities.
- Mixed Obstructive and Restrictive Ventilatory Defect:
characterised by both airflow obstruction and loss of lung volume i.e. a low
FEV1/FVC ratio and low SVC or FVC. An
example is cystic fibrosis.
Figure 3
Generalised classification of ventilatory defects
 |
Additionally, certain respiratory conditions alter the shape
of the flow volume loop and it is important to learn how to recognise these.
Examples are given in Figure 4.
Figure 5 shows
examples of normal and abnormal volume-time curves.
Figure
4 The normal flow-volume curve
Shown together with examples of how respiratory disease can
alter the shape of the flow-volume relationship.
a) Flow volume loop from a healthy
subject;
b) Obstructive airway disease (e.g. asthma) before
(shaded curve) and after (dashed line) the administration of a bronchodilator;
c) Severe obstructive disease (e.g. emphysema)
before (shaded curve) and after (dashed line) the administration of a
bronchodilator;
d) Restrictive lung disease (e.g. pulmonary fibrosis) –
the predicted FVC is marked;
e) Fixed major airway obstruction (e.g. laryngeal
obstruction).
 |
Figure 5 The normal expiratory
volume-time curve (spirogram)
a) normal curve is shown and as a dotted line in (b) and
(c).
b) is an example of airflow obstruction with significant improvement
after the administration of a bronchodilator (dashed line).
c) shows a restrictive ventilatory defect .
 |
Asthma and COPD
In these diseases FEV1/FVC,
and percent predicted FEV1 are critical to
detect and grade the severity of airflow obstruction, respectively, and are used
in the interpretation algorithm (Figure 2). Although both asthma and COPD
are characterised by airflow obstruction, the mechanisms of each disease are
different. In COPD due to emphysema, airway obstruction is predominantly due to
airway collapse whereas in asthma it is mainly due to bronchoconstriction,
inflammation of the airway wall and mucous plugging. In general, spirometry
improves significantly after effective treatment in asthma but not at all, or
only marginally, in patients with COPD although their symptoms may improve.
Spirometry screening of smokers and ex-smokers has been
shown to enhance early detection of COPD when treatment and intervention can
have a positive effect on disease progression. Furthermore, the demonstration of
airflow limitation to the patient has been shown to motivate smokers to quit.
Reversibility of Airflow Obstruction
If there is evidence of airflow obstruction, spirometry is
usually performed before and after the administration of a short-acting
bronchodilator to assess whether the airflow obstruction can be reversed:
- Perform pre-bronchodilator spirometry (see above).
- Administer the bronchodilator (eg 4 puffs of
salbutamol via a spacer).
- Wait 10 minutes.
- Perform post-bronchodilator spirometry (as above).
If the clinical reason for performing the reversibility
test was to check the patients’ usual response to bronchodilator, it may be more
appropriate to use the patients’ usual bronchodilator device and dose. During
this test it is helpful to observe the patient’s normal inhaler technique and
correct any errors.
The American Thoracic Society recommends the following
criteria for a significant improvement in spirometry: at least a 12% improvement
in measured FEV1 (or FVC) and an absolute improvement of at least 200ml in
either of these two measures.
It is important to note that in some patients the degree
of reversibility can vary between clinic visits and will be reduced if the
patient has taken a bronchodilator within prior to testing. It is important to
ask the patient when they last used their bronchodilator (short and long acting)
and to take this into account when assessing the degree of reversibility.
The absence of significant reversibility does not
necessarily exclude the diagnosis of asthma.
Note that the FEV1/FVC
ratio is not a reliable index of reversibility as the FVC can increase more than
FEV1 causing the FEV1/FVC ratio to decrease in the presence of a useful degree
of bronchodilatation. Do not use FEF25-75%
for assessing reversibility.
Reversibility may also be assessed by measuring spirometry
before and several weeks after a trial of inhaled glucocorticosteroids or oral
Prednisone.
