Pulmonary function test: Wikis

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Flow-Volume loop showing successful FVC maneuver. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. (Note the FEV1, FEV1/2 and FEV3 values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment).
Example of a modern PC based spirometer printout.


Spirometry (meaning the measuring of breath) is the most common of the Pulmonary Function Tests (PFTs), measuring lung function, specifically the measurement of the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is an important tool used for generating pneumotachographs which are helpful in assessing conditions such as asthma, pulmonary fibrosis, cystic fibrosis, and COPD.

Contents

Spirometry testing

Device for spirometry. The patient places his or her lips around the blue mouthpiece. The teeth go between the nubs and the shield, and the lips go over the shield. A noseclip guarantees that breath will flow only through the mouth.
A modern USB PC-based spirometer.
Screen for spirometry readouts at right. The chamber can also be used for body plethysmography.


The spirometry test is performed using a device called a spirometer, which comes in several different varieties. Most spirometers display the following graphs, called spirograms:

  • a volume-time curve, showing volume (liters) along the Y-axis and time (seconds) along the X-axis
  • a flow-volume loop, which graphically depicts the rate of airflow on the Y-axis and the total volume inspired or expired on the X-axis

The most commonly used guidelines for spirometric testing and interpretation are set by the American Thoracic Society (ATS) and the European Respiratory Society (ERS).

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Procedure

The basic forced volume vital capacity (FVC) test varies slightly depending on the equipment used.

Generally, the patient is asked to take the deepest breath they can, and then exhale into the sensor as hard as possible, for as long as possible. It is sometimes directly followed by a rapid inhalation (inspiration), in particular when assessing possible upper airway obstruction. Sometimes, the test will be preceded by a period of quiet breathing in and out from the sensor (tidal volume), or the rapid breath in (forced inspiratory part) will come before the forced exhalation.

During the test, soft nose clips may be used to prevent air escaping through the nose. Filter mouthpieces may be used to prevent the spread of microorganisms, particularly for inspiratory maneuvers.

Limitations of test

The maneuver is highly dependent on patient cooperation and effort, and is normally repeated at least three times to ensure reproducibility. Since results are dependent on patient cooperation, FEV1* and FVC can only be underestimated, never overestimated.(*FEV1 can be overestimated in people with some diseases - a softer blow can reduce the spasm or collapse of lung tissue to elevate the measure

Due to the patient cooperation required, spirometry can only be used on children old enough to comprehend and follow the instructions given (typically about 4–5 years old), and only on patients who are able to understand and follow instructions - thus, this test is not suitable for patients who are unconscious, heavily sedated, or have limitations that would interfere with vigorous respiratory efforts. Other types of lung function tests are available for infants and unconscious persons.

Related tests

Spirometry can also be part of a bronchial challenge test, used to determine bronchial hyperresponsiveness to either rigorous exercise, inhalation of cold/dry air, or with a pharmaceutical agent such as methacholine or histamine.

Sometimes, to assess the reversibility of a particular condition, a bronchodilator is administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test (Post BD), and is an important part in diagnosing asthma versus COPD

Parameters

Measurement Approximate value
Male Female
Forced vital capacity (FVC) 4.8 L 3.7 L
Tidal volume (Vt) 500mL 390mL
Total lung capacity (TLC) 6.0 L 4.7 L

The most common parameters measured in spirometry are Vital capacity (VC), Forced vital capacity (FVC), Forced expiratory volume (FEV) at timed intervals of 0.5, 1.0 (FEV1), 2.0, and 3.0 seconds, Forced expiratory flow 25–75% (FEF 25–75) and Maximal voluntary ventilation (MVV).[1] Other tests may be performed in certain situations.

Results are usually given in both raw data (litres, litres per second) and percent predicted - the test result as a percent of the "predicted values" for the patients of similar characteristics (height, age, sex, and sometimes race and weight). The interpretation of the results can vary depending on the physician and the source of the predicted values. Generally speaking, results nearest to 100% predicted are the most normal, and results over 80% are often considered normal. However, review by a doctor is necessary for accurate diagnosis of any individual situation.

Functional residual capacity (FRC) cannot be measured via spirometry, but it can be measured with a plethysmograph or dilution tests (for example, helium dilution test).

Normal values for Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 Second (FEV1) and Forced Expiratory Flow 25–75% (FEF25–75%), according to a study in the United States 2007 of 3,600 subjects aged 4–80 years.[2] Y-axis is expressed in litres for FVC and FEV1, and in litres/second for FEF25–75%.

Forced Vital Capacity (FVC)

Forced Vital Capacity (FVC) is the volume of air that can forcibly be blown out after full inspiration, measured in litres.

Forced Expiratory Volume in 1 Second (FEV1)

Forced Expiratory Volume in 1 Second (FEV1) is the maximum volume of air that can forcibly blow out in the first second during the FVC manoeuvre, measured in liters. Along with FVC it is considered one of the primary indicators of lung function.

FEV1/FVC ratio (FEV1%)

FEV1/FVC (FEV1%) is the ratio of FEV1 to FVC. In healthy adults this should be approximately 75–80%. In obstructive diseases (asthma, COPD, chronic bronchitis, emphysema) FEV1 is diminished because of increased airway resistance to expiratory flow and the FVC may be increased (for instance by air trapping in emphysema). This generates a reduced value (<80%, often ~45%). In restrictive diseases (such as pulmonary fibrosis) the FEV1 and FVC are both reduced proportionally and the value may be normal or even increased as a result of decreased lung compliance.

Forced Expiratory Flow 25–75% or 25–50%

Forced Expiratory Flow 25–75% or 25–50% (FEF 25–75% or 25–50%) is the average flow (or speed) of air coming out of the lung during the middle portion of the expiration (also sometimes referred to as the MMEF, for maximal mid-expiratory flow). In small airway diseases such as asthma this value will be reduced, perhaps <65% of expected value. This may be the first sign of small airway disease detectable.

Forced Inspiratory Flow 25–75% or 25–50%

Forced Inspiratory Flow 25–75% or 25–50% (FIF 25–75% or 25–50%) is similar to FEF 25–75% or 25–50% except the measurement is taken during inspiration.

Peak Expiratory Flow (PEF)

Normal values for Peak Expiratory Flow (PEF), shown on EU scale.[3]

Peak Expiratory Flow (PEF) is the maximal flow (or speed) achieved during the maximally forced expiration initiated at full inspiration, measured in litres per minute.

Tidal volume (TV)

Tidal volume (TV) is the During the normal, tidal breathing a specific volume of air is drawn into and then expired out of the lungs.

Total Lung Capacity (TLC)

Total Lung Capacity (TLC) is the maximum volume of air present in the lungs.

Diffusion capacity (DLCO)

Diffusing Capacity (DLCO) is the carbon monoxide uptake from a single inspiration in a standard time (usually 10 sec). This will pick up diffusion impairments, for instance in pulmonary fibrosis. This must be corrected for anemia (because rapid CO diffusion is dependent on hemoglobin in RBC's a low hemoglobin concentration, anemia, will reduce DLCO) and pulmonary hemorrhage (excess RBC's in the interstitium or alveoli can absorb CO and artificially increase the DLCO capacity).

Maximum Voluntary Ventilation (MVV)

Maximum Voluntary Ventilation (MMV) is a measure of the maximum amount of air that can be inhaled and exhaled in one minute, measured in liters/minute.

Others

Forced Expiratory Time (FET)
Forced Expiratory Time (FET) measures the length of the expiration in seconds.

Slow Vital capacity (SVC)
Slow Vital capacity (SVC) is the maximum volume of air that can be exhaled slowly after slow maximum inhalation.

Technologies used in spirometers

  • Volumetric Spirometers
  • Flow measuring Spirometers
    • Fleisch-pneumotach
    • Lilly (screen) pneumotach
    • Turbine (actually a rotating vane which spins because of the air flow generated by the subject. The revolutions of the vane are counted as they break a light beam)
    • Pitot tube
    • Hot-wire anemometer
    • Ultrasound

See also

References

  1. ^ surgeryencyclopedia.com > Spirometry tests Retrieved on Mars 14, 2010
  2. ^ Stanojevic S, Wade A, Stocks J, et al. (February 2008). "Reference ranges for spirometry across all ages: a new approach". Am. J. Respir. Crit. Care Med. 177 (3): 253–60. doi:10.1164/rccm.200708-1248OC. PMID 18006882. 
  3. ^ Nunn, A. J., and I. Gregg. 1989. New regression equations for predicting peak expiratory flow in adults. Br. Med. J. 298: 1068-1070. Adapted by Clement Clarke for use in EU scale - see Peakflow.com > Predictive Normal Values (Nomogram, EU scale)

Further reading

External links


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