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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 manoeuvre is usually characterised by poor
reproducibility.
The measurements which are usually made are as follows:
| 1. |
VC (vital capacity) is the
maximum volume of air which can be exhaled or
inspired during either a maximally forced (FVC) or a slow (VC)
manoeuvre. VC is normally equal to FVC unless
airflow obstruction is present, in which case VC is
usually higher than FVC. |
| 2. |
FEV1
(forced expired volume in one second) is the
volume expired in the first second of maximal
expiration after a maximal inspiration and is a
useful measure of how quickly full lungs can be
emptied. |
| 3. |
FEV1/VC
or (FEV1/FVC) is the FEV1
expressed as a percentage of the VC or FVC
(whichever volume is larger) and gives a clinically
useful index of airflow limitation. |
| 4. |
FEF25-75%
is the average expired flow over the middle half of
the FVC manoeuvre and is regarded as a more
sensitive measure of small airways narrowing than
FEV1.
Unfortunately FEF25-75%
has a wide range of normality, is less reproducible
than FEV1,
and is difficult to interpret if the VC (or FVC) is
reduced or increased. |
| 5. |
PEF (peak expiratory flow)
is the maximal expiratory flow rate achieved and
this occurs very early in the forced expiratory
manoeuvre. |
| 6. |
FEF50%
and FEF75%
(forced expiratory flow at 50% or 75% FVC) is the
maximal expiratory flow measured at the point where
50% of the FVC has been expired (FEF50%)
and after 75% has been expired (FEF75%).
Both indices have a wide range of normality but are
usually reproducible in a given subject provided the
FVC is reproducible. |
| 7. |
FVC6 is the
forced expiratory volume during the first 6 seconds
and is a surrogate of the FVC. The FVC6
(and FEV1/FVC6) is gaining
popularity because stopping the expiratory manoeuvre
after 6 seconds is less demanding and easier to
perform for patients with airflow obstruction and
the elderly yet is similar to conventional FVC and
FEV1/FVC for diagnosing and grading
airflow obstruction. |
All indices of ventilatory function should
be reported at body temperature and pressure saturated with
water vapour (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.
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(Click to enlarge) |
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Figure
1
Normal spirogram showing the
measurements of forced vital capacity (FVC),
forced expired volume in one second (FEV1)
and forced expiratory flow over the middle half
of the FVC (FEF25-75%).
The left panel is a typical recording from a
water-sealed (or rolling seal) spirometer with
inspired volume upward; the right panel is a
spirogram from a dry wedge-bellows spirometer
with expired volume upward. |
(Click to enlarge) |
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Figure 2
Normal maximal expiratory and
inspiratory flow-volume loop. |
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