Spirometry

Spirometry (meaning the measuring of breath) is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) 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 volume-time curves and also flow-volume loops, which are charts that plot the volume and flow of air coming in and out of the lungs from one inhalation and one exhalation.

Spirometry (meaning the measuring of breath) is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) 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 volume-time curves and also flow-volume loops, 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== y.]] ===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 (litres) 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 ===Procedure=== The basic forced volume vital capacity (FVC) 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 preceding exhale. In some cases each round of test will be preceded 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 are 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 (Post BD), 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 (angina, recent myocardial infarction, etc.) 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 (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), also known as Maximum breathing capacity. 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. 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 (FRC) cannot be measured via spirometry, but it can be measured with a plethysmograph or dilution tests (for example, helium dilution test). ===Forced vital capacity (FVC)=== Forced vital capacity (FVC) 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=== FEV1/FVC is the ratio of FEV1 to FVC. In healthy adults this should be approximately 70–80% (declining with age). In obstructive diseases (asthma, COPD, chronic bronchitis, emphysema) 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 (for instance, both FEV1 and FVC are reduced, but the former is more affected because of the increased airway resistance). This generates a reduced value ( st )=== When estimating static lung compliance, volume measurements by the spirometer needs to be complemented by pressure transducers in order to simultaneously measure the transpulmonary pressure. When having drawn a curve with the relations between changes in volume to changes in transpulmonary pressure, C st is the slope of the curve during any given volume, or, mathematically, ΔV/ΔP. Static lung compliance is perhaps the most sensitive parameter for the detection of abnormal pulmonary mechanics. It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition. In those with acute respiratory failure on mechanical ventilation, "the static compliance of the total respiratory system is conventionally obtained by dividing the tidal volume by the difference between the 'plateau' pressure measured at the airway opening (PaO) during an occlusion at end-inspiration and positive end-expiratory pressure (PEEP) set by the ventilator". ===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. Maximal pressure (P max and P i ) P max is the asymptotically maximal pressure that can be developed by the respiratory muscles at any lung volume and P i is the maximum inspiratory pressure that can be developed at specific lung volumes. This measurement also requires pressure transducers in addition. It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition. A derived parameter is the coefficient of retraction (CR) which is P max /TLC . Mean transit time (MTT) Mean transit time is the area under the flow-volume curve divided by the forced vital capacity. Maximal inspiratory pressure (MIP) MIP, also known as negative inspiratory force (NIF), is the maximum pressure that can be generated against an occluded airway beginning at functional residual capacity (FRC). It is a marker of respiratory muscle function and strength. Represented by centimeters of water pressure (cmH2O) and measured with a manometer. Maximum inspiratory pressure is an important and noninvasive index of diaphragm strength and an independent tool for diagnosing many illnesses. Typical maximum inspiratory pressures in adult males can be estimated from the equation, M IP = 142 - (1.03 x Age) cmH 2 O, where age is in years. ==Technologies used in spirometers== Volumetric Spirometers Water bell Bellows wedge Flow measuring Spirometers Fleisch-pneumotach Lilly (screen) pneumotach Turbine/Stator Rotor (normally incorrectly referred to as a 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 ==Further reading== ==External links== American Thoracic Society (ATS) European Respiratory Society (ERS)

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