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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/FVC are outside the normal range.
The inter-relationships of the various
measurements are also important diagnostically (see Table
and Figure 4). For example,
1. |
A reduction of FEV1
in relation to the forced vital capacity will result
in a low FEV1/FVC and is typical of
obstructive ventilatory defects (e.g. asthma and
emphysema). The lower limit of normal for FEV1/FVC
is around 70-75% but the exact limit is dependent on
age. The exact values by age, sex and height are
given in the tables in Appendix B. In obstructive
lung disease the FVC may be less than the slow VC
because of earlier airway closure during the forced
manoeuvre. This may lead to an overestimation of the
FEV1/FVC. Thus, the FEV1/VC
may be a more sensitive index of airflow
obstruction. |
2. |
The FEV1/FVC
ratio remains normal or high (typically > 80%)
with a reduction in both FEV1 and FVC
in restrictive ventilatory defects (e.g.
interstitial lung disease, respiratory muscle
weakness, and thoracic cage deformities such as
kypho-scoliosis). |
3. |
A reduced FVC together
with a low FEV1/FVC ratio is a
feature of a mixed ventilatory defect
in which a combination of both obstruction
and restriction appear to be present, or
alternatively may occur in airflow
obstruction as a consequence of airway
closure resulting in gas trapping, rather
than as a result of small lungs. It is
necessary to measure the patient's total
lung capacity to distinguish between these
two possibilities. |
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 (Click to enlarge) |
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Figure 4
Schematic diagram illustrating idealised shapes
of flow-volume curves and spirograms for
obstructing, restrictive and mixed ventilatory
defects. |
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).
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(Click to enlarge) |
Figure 5
Maximum expiratory and inspiratory flow volume curves with examples of
how respiratory disease can alter its shape
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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
nebuliser. Beta2
agonists (e.g. salbutamol, terbutaline, etc.)
are generally considered the benchmark
bronchodilator.
To express the degree of
improvement,
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FEV1
(post-bronchodilator) - FEV1
(baseline) |
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%
Improvement |
=100 X
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FEV1
(baseline) |
There is presently no universal agreement on the
definition of significant bronchodilator reversibility.
According to the ATS/ERS 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 calibre 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 patients with asthma), the
pattern of the graph can be helpful in
identifying particular aspects of the patient's
disease. Typical patterns are
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the fall in PEF during the week with
improvement on weekends and holidays which occurs in
occupational asthma; and
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the ‘morning dipper’ pattern of some
patients with asthma 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 for people with poor perception of their
own airway calibre. 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.
PEF self-monitoring can be
useful in asthma management, particularly in
those with poor perception of their own airway
calibre.
It is worth trying to recognise
clinical situations and choosing the appropriate
test for each. For example,
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If upper airway obstruction is
suspected, flow-volume curve with particular emphasis on
inspiration is the best test.
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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.
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In patients suspected of having asthma
but in whom baseline spirometry is normal, it may be
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 patient can be helpful.
Spirometry is most useful for:
Detection of disease
and its severity
Identification of
asthma triggers
Progress/natural
history monitoring
Treatment response
assessment
Preoperative assessment
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