Spirometry

Spirometry is the most common of the pulmonary function tests. It measures lung function, specifically the amount and/or speed of air that can be inhaled and exhaled. Spirometry is helpful in assessing breathing patterns that identify conditions such as asthma, pulmonary fibrosis, cystic fibrosis, and COPD. It is also helpful as part of a system of health surveillance, in which breathing patterns are measured over time.
Spirometry generates pneumotachographs, which are charts that plot the volume and flow of air coming in and out of the lungs from one inhalation and one exhalation.

Testing

Spirometer

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 along the Y-axis and time 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
Procedure

The basic forced volume vital capacity test varies slightly depending on the equipment used. It can be in the form of either closed or open circuit. Regardless of differences in testing procedure providers are recommended to follow the ATS/ERS Standardisation of Spirometry. The standard procedure ensures an accurate and objectively collected set of data, based on a common reference, to reduce incompatibility of the results when shared across differing medical groups.
The patient is asked to put on soft nose clips to prevent air escape and a breathing sensor in their mouth forming an air tight seal. Guided by a technician, the patient is given step by step instructions to take an abrupt maximum effort inhale, followed by a maximum effort exhale lasting for a target of at least 6 seconds. When assessing possible upper airway obstruction, the technician will direct the patient to make an additional rapid inhalation to complete the round. The timing of the second inhale can vary between persons depending on the length of the proceeding exhale. In some cases each round of test will be proceeded by a period of normal, gentle breathing for additional data.

Limitations

Clinically useful results are highly dependent on patient cooperation and effort and must be repeated for a minimum of three times to ensure reproducibility with a general limit of ten attempts. Given variable rates of effort, the results can only be underestimated given an effort output greater than 100% is not possible.
Due to the need for patient cooperation and an ability to understand and follow instructions, spirometry can typically only be done in cooperative children when they at least 5 years old or adults without physical or mental impairment preventing effective diagnostic results. In addition, General anesthesia and various forms of sedation are not compatible with the testing process.
Another limitation is that persons with intermittent or mild asthma can present normal spirometry values between acute exacerbation, reducing spirometry's effectiveness as a diagnostic tool in these circumstances.

Supplemental diagnostics

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.
To assess the reversibility of a particular condition, a bronchodilator can be administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test, and is an important part in diagnosing asthma versus COPD.
Other complementary lung functions tests include plethysmography and nitrogen washout.

Indications

Spirometry is indicated for the following reasons:

to diagnose or manage asthma
to detect respiratory disease in patients presenting with symptoms of breathlessness, and to distinguish respiratory from cardiac disease as the cause
to measure bronchial responsiveness in patients suspected of having asthma
to diagnose and differentiate between obstructive lung disease and restrictive lung disease
to follow the natural history of disease in respiratory conditions
to assess of impairment from occupational asthma
to identify those at risk from pulmonary barotrauma while scuba diving
to conduct pre-operative risk assessment before anaesthesia or cardiothoracic surgery
to measure response to treatment of conditions which spirometry detects
to diagnose the vocal cord dysfunction.
Contraindications

Forced expiratory maneuvers may aggravate some medical conditions. Spirometry should not be performed when the individual presents with:

Hemoptysis of unknown origin
Pneumothorax
Unstable cardiovascular status
Thoracic, abdominal, or cerebral aneurysms
Cataracts or recent eye surgery
Recent thoracic or abdominal surgery
Nausea, vomiting, or acute illness
Recent or current viral infection
Undiagnosed hypertension
Parameters

The most common parameters measured in spirometry are vital capacity, forced vital capacity, forced expiratory volume at timed intervals of 0.5, 1.0, 2.0, and 3.0 seconds, forced expiratory flow 25–75% and maximal voluntary ventilation, also known as Maximum breathing capacity. Other tests may be performed in certain situations.
Results are usually given in both raw data and percent predicted—the test result as a percent of the "predicted values" for the patients of similar characteristics. 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. Multiple publications of predicted values have been published and may be calculated based on age, sex, weight and ethnicity. However, review by a doctor is necessary for accurate diagnosis of any individual situation.
A bronchodilator is also given in certain circumstances and a pre/post graph comparison is done to assess the effectiveness of the bronchodilator. See the example printout.
Functional residual capacity cannot be measured via spirometry, but it can be measured with a plethysmograph or dilution tests.
Image:LungVolume.jpg|frameless|600px|Output of a 'spirometer'

Forced vital capacity (FVC)

Forced vital capacity is the volume of air that can forcibly be blown out after full inspiration, measured in liters. FVC is the most basic maneuver in spirometry tests.

Forced expiratory volume in 1 second (FEV1)

FEV1 is the volume of air that can forcibly be blown out in first 1-second, after full inspiration. Average values for FEV1 in healthy people depend mainly on sex and age, according to the diagram.
Values of between 80% and 120% of the average value are considered normal. Predicted normal values for FEV1 can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.

FEV1/FVC ratio

is the ratio of FEV1 to FVC. In healthy adults this should be approximately 70–80%. In obstructive diseases FEV1 is diminished because of increased airway resistance to expiratory flow; the FVC may be decreased as well, due to the premature closure of airway in expiration, just not in the same proportion as FEV1. This generates a reduced value. In restrictive diseases the FEV1 and FVC are both reduced proportionally and the value may be normal or even increased as a result of decreased lung compliance.
A derived value of FEV1 is FEV1% predicted, which is defined as FEV1 of the patient divided by the average FEV1 in the population for any person of the same age, height, gender, and race.

Forced expiratory flow (FEF)

Forced expiratory flow is the flow of air coming out of the lung during the middle portion of a forced expiration.
It can be given at discrete times, generally defined by what fraction of the forced vital capacity has been exhaled.
The usual discrete intervals are 25%, 50% and 75%, or 25% and 50% of FVC that has been exhaled.
It can also be given as a mean of the flow during an interval, also generally delimited by when specific fractions remain of FVC, usually 25–75%. Average ranges in the healthy population depend mainly on sex and age, with FEF25–75% shown in diagram at left. Values ranging from 50 to 60% and up to 130% of the average are considered normal. Predicted normal values for FEF can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.
MMEF or MEF stands for maximal expiratory flow and is the peak of expiratory flow as taken from the flow-volume curve and measured in liters per second. It should theoretically be identical to peak expiratory flow, which is, however, generally measured by a peak flow meter and given in liters per minute.
Recent research suggests that FEF25-75% or FEF25-50% may be a more sensitive parameter than FEV1 in the detection of obstructive small airway disease. However, in the absence of concomitant changes in the standard markers, discrepancies in mid-range expiratory flow may not be specific enough to be useful, and current practice guidelines recommend continuing to use FEV1, VC, and FEV1/VC as indicators of obstructive disease.
More rarely, forced expiratory flow may be given at intervals defined by how much remains of total lung capacity. In such cases, it is usually designated as e.g. FEF70%TLC, FEF60%TLC and FEF50%TLC.

Forced inspiratory flow 25–75% or 25–50%

Forced inspiratory flow 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)

is the maximal flow achieved during the maximally forced expiration initiated at full inspiration, measured in liters per minute or in liters per second.