What does spirometry measure?
Spirometry provides an objective measurement of lung function.
It will measure:
a) Expiratory Vital Capacity (EVC): The maximum volume of gas which can be expired from the lungs during a relaxed expiration from a position of full inspiration.
b) Inspiratory Vital Capacity (IVC): The maximum volume of gas which can be inspired into the lungs during a relaxed inspiration from a position of full expiration. The expiratory phase is the one more commonly used to measure obstruction and restriction within the lungs. This can be done in two ways:
Static test - performed without regard to time e.g. Vital Capacity (VC)
Dynamic test - performed at forcible and maximum effort against time. e.g. FEV, (Forced expiratory volume in the first second from a maximum inspiration)
What do the different parameters within spirometry testing mean?
A. Static tests:
Vital Capacity (VC):
The change in volume of gas in the lungs from complete inspiration to complete expiration.
Forced Vital Capacity (FVC):
The maximum volume of air in litres that can be forcibly and rapidly exhaled following a maximum inspiration. FVC is the basic manoeuvre in spirometry tests.
B. Dynamic tests.
Forced Expiratory Volume in first second (FEV1):
Is the volume of air expelled in the first second of a forced expiration starting from full inspiration. FEV1%: This is the FEV1 expressed as a percentage of the total volume. It is sometimes called the FEV1 Ratio or theFEV1/ VC% when it is shown as a percentage of the VC volume, or the FEV1/FVC% when shown as a percentage of the FVC.
This parameter has nothing to do with predicted values. (In normal lung function this should generally be over 75%, ie. the subject should get at least three quarters of their total air out in the first second).
Forced Expiratory Flow Rate (FEFR 25% - 75%):
This is the average forced expiratory flow rate at the middle part of the FVC manoeuvre. Expressed in Litres per second it gives an indication of what is happening in the lower airways. It is a more sensitive parameter and not as reproducible as the others. It is a useful serial measurement because it will be affected before FEV, so
can act as an early warning sign of disease.
How do you interpret spirometry readings?
Spirometry results may be used to classify a patient's lung function into one of four different disease patterns or classifications of ventilatory function; Normal, Obstructive, Restrictive and Combined.
Normal Ventilatory Function:
A person with Normal Spirometry will have lung volumes and flow rates within the normal range for people their age, sex and height.
Obstructive Ventilatory Function:
An obstructive disorder refers to any disease that affects the lumen of the airways. This could be due to excessive mucus production, inflammation or bronchoconstriction. Asthma and Chronic Bronchitis are examples of obstructive disorders. In general terms the obstructive pattern presents itself as reduced flow rates and normal lung volumes (but with a reduced FEV1) on the FVC manoeuvre.
Restrictive Ventilatory Function:
A restrictive disorder is one that may affect the lung tissue itself or the capacity of the lungs to expand and hold predicted volumes of air. This could be due to fibrosis and scarring, or a physical deformity that is restricting expansion. Someone who has had part of their lung removed would show a restrictive pattern and another example of a restrictive disease is pneumoconiosis. The restrictive pattern usually presents itself as reduced volumes and normal flow rates on the FVC manoeuvres.
Combined Ventilatory Function:
A combined disorder is a ventilatory disorder exhibiting the features of both an obstructive and restrictive deficit. Examples of this pattern include Cystic Fibrosis, which causes excess mucus production and damage to the lung tissue.
What standards/procedures are recommended when taking spirometry readings?
It is recommended that the best of 3 FVC manoeuvres are taken, the best 2 being within 5% of each other.
Can FEV1 values be used to classify Chronic Obstructive Pulmonary Disease (COPD)?
Yes. Generally the following FEV1 values (expressed as a percentage of predicted) may classify the severity of the COPD:
60% - 79% predicted: MILD COPD
40% - 59% predicted: MODERATE COPD
Less than 40% predicted: SEVERE COPD
What's the difference between spirometry measurement and peak flow reading?
Spirometry devices record the whole of the FVC manoeuvre against time, allowing the dynamics of the resulting time/volume curve to be examined.
Peak Expiratory Flow records the greatest flow that can be sustained for 10 milliseconds on forced expiration starting from full inflation of the lungs.
Which measurement is more accurate/appropriate in patients with COPD; spirometry or peak flow?
Many doctors and nurses have become used to measuring serial peak flow measurements as part of asthma care. PEF is a more simple measure than FEV1 and repeat measurements can be easily performed at home by the patient using a hand held peak flow meter. As such, they might appear of immediate value in COPD. However, there are important physiological differences between COPD and asthma which limit the value of PEF in COPD. In asthma there is a reasonably good correlation between PEF and FEV1 which allows the use of PEF as a surrogate for FEV1. In COPD this relationship breaks down. Because the amount of airway collapsibility varies between COPD patients, the relationship between PEF and FEV1 will also vary. PEF can be misleadingly optimistic and therefore it may be severely limited as a diagnostic tool.
In addition, PEF is not a sensitive measure for detecting the small treatment changes typical of COPD.
Where does peak flow fit in and what is its role?
Peak flow meters measure the rate at which a patient can exhale. Serial peak flow measurements are therefore useful in the management of asthma where there's good correlation between PEF and FEV1 and where the importance of distinguishing diurnal variability as a reliable defining characteristic in asthma is well established.
PEF monitoring can be a useful tool when titrating treatment to prevent an asthma attack.
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