Spirometry

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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, FEV_1/2 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.

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).

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, FEV_1* 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.

Note that 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).

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.

Technologies used in spirometers

• Volumetric Spirometers
  • Water bell
  • Bellows wedge
• 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

• Peak flow meter
• Fowler method

References

1. ^ Miller, MR; Crapo R, Hankinson J et al. (July 2005). "General considerations for lung function testing". European Respiratory Journal 26 (1): 153–161. PMID 15994402. http://erj.ersjournals.com/cgi/content/full/26/1/153. 

External links

• Detailed information on spirometric testing, interpretation and physiology at spirxpert.com
• History of spirometers and lung function testing at medizin.li
• General information on spirometry at spirometrie.info
• American Thoracic Society (ATS)
• European Respiratory Society (ERS)
• General Practice Airways Group