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Measurement of Ventilatory Function |
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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.
Figure 1
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(Click to enlarge)
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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.
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Figure 2
Content Updated 28 March, 2008
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Last Updated ( Tuesday, 21 April 2009 )
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