Pulmonary function testing
Pulmonary function test (PFT) is a complete evaluation of the respiratory system including patient history, physical examinations, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity of pulmonary impairment. Pulmonary function testing has diagnostic and therapeutic roles and helps clinicians answer some general questions about patients with lung disease. PFTs are normally performed by a respiratory therapist, physiotherapist, pulmonologist, and/or general practitioner.
|Pulmonary function testing|
|TLC||Total lung capacity: the volume in the lungs at maximal inflation, the sum of VC and RV.|
|TV||Tidal volume: that volume of air moved into or out of the lungs during quiet breathing (TV indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)|
|RV||Residual volume: the volume of air remaining in the lungs after a maximal exhalation|
|ERV||Expiratory reserve volume: the maximal volume of air that can be exhaled from the end-expiratory position|
|IRV||Inspiratory reserve volume: the maximal volume that can be inhaled from the end-inspiratory level|
|IC||Inspiratory capacity: the sum of IRV and TV|
|IVC||Inspiratory vital capacity: the maximum volume of air inhaled from the point of maximum expiration|
|VC||Vital capacity: the volume of air breathed out after the deepest inhalation.|
|VT||Tidal volume: that volume of air moved into or out of the lungs during quiet breathing (VT indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)|
|FRC||Functional residual capacity: the volume in the lungs at the end-expiratory position|
|RV/TLC%||Residual volume expressed as percent of TLC|
|VA||Alveolar gas volume|
|VL||Actual volume of the lung including the volume of the conducting airway.|
|FVC||Forced vital capacity: the determination of the vital capacity from a maximally forced expiratory effort|
|FEVt||Forced expiratory volume (time): a generic term indicating the volume of air exhaled under forced conditions in the first t seconds|
|FEV1||Volume that has been exhaled at the end of the first second of forced expiration|
|FEFx||Forced expiratory flow related to some portion of the FVC curve; modifiers refer to amount of FVC already exhaled|
|FEFmax||The maximum instantaneous flow achieved during a FVC maneuver|
|FIF||Forced inspiratory flow: (Specific measurement of the forced inspiratory curve is denoted by nomenclature analogous to that for the forced expiratory curve. For example, maximum inspiratory flow is denoted FIFmax. Unless otherwise specified, volume qualifiers indicate the volume inspired from RV at the point of measurement.)|
|PEF||Peak expiratory flow: The highest forced expiratory flow measured with a peak flow meter|
|MVV||Maximal voluntary ventilation: volume of air expired in a specified period during repetitive maximal effort|
Pulmonary function testing is a diagnostic and management tool used for a variety of reasons, such as:
- Chronic shortness of breath
- Chronic obstructive pulmonary disease
- Restrictive lung disease
- Preoperative testing
- Impairment or disability
- Early morning wheezing
Pediatric neuromuscular disorders
Neuromuscular disorders such as Duchenne muscular dystrophy are associated with gradual loss of muscle function over time. Involvement of respiratory muscles results in poor ability to cough and decreased ability to breathe well and leads to collapse of part or all of the lung leading to impaired gas exchange and an overall insufficiency in lung strength. Pulmonary function testing in patients with neuromuscular disorders helps to evaluate the respiratory status of patients at the time of diagnosis, monitor their progress and course, evaluate them for possible surgery, and gives an overall idea of the prognosis.
Spirometry includes tests of pulmonary mechanics – measurements of FVC, FEV1, FEF values, forced inspiratory flow rates (FIFs), and MVV. Measuring pulmonary mechanics assesses the ability of the lungs to move huge volumes of air quickly through the airways to identify airway obstruction.
The measurements taken by the spirometry device are used to generate a pneumotachograph that can help to assess lung conditions such as: asthma, pulmonary fibrosis, cystic fibrosis, and chronic obstructive pulmonary disease. Physicians may also use the test results to diagnose bronchial hyperresponsiveness to exercise, cold air, or pharmaceutical agents.
Complications of spirometry
Spirometry is a safe procedure; however, there is cause for concern regarding untoward reactions. The value of the test data should be weighed against potential hazards. Some complications have been reported, including pneumothorax, increased intracranial pressure, fainting, chest pain, paroxysmal coughing, nosocomial infections, oxygen desaturation, and bronchospasm.
There are four lung volumes and four lung capacities. A lung's capacity consists of two or more lung volumes. The lung volumes are tidal volume (VT), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV). The four lung capacities are total lung capacity (TLC), inspiratory capacity (IC), functional residual capacity (FRC) and vital capacity (VC).
Maximal respiratory pressures
Measurement of maximal inspiratory and expiratory pressures is indicated whenever there is an unexplained decrease in vital capacity or respiratory muscle weakness is suspected clinically. Maximal inspiratory pressure (MIP) is the maximal pressure that can be produced by the patient trying to inhale through a blocked mouthpiece. Maximal expiratory pressure (MEP) is the maximal pressure measured during forced expiration (with cheeks bulging) through a blocked mouthpiece after a full inhalation. Repeated measurements of MIP and MEP are useful in following the course of patients with neuromuscular disorders.
Oxygen desaturation during exercise
Arterial blood gases
Arterial blood gases (ABGs) are a helpful measurement in pulmonary function testing in selected patients. The primary role of measuring ABGs in individuals that are healthy and stable is to confirm hypoventilation when it is suspected on the basis of medical history, such as respiratory muscle weakness or advanced COPD.
ABGs also provide a more detailed assessment of the severity of hypoxemia in patients who have low normal oxyhemoglobin saturation.
The helium dilution technique for measuring lung volumes uses a closed, rebreathing circuit. This technique is based on the assumptions that a known volume and concentration of helium in air begin in the closed spirometer, that the patient has no helium in their lungs, and that an equilibration of helium can occur between the spirometer and the lungs.
The nitrogen washout technique uses a non-rebreathing open circuit. The technique is based on the assumptions that the nitrogen concentration in the lungs is 78% and in equilibrium with the atmosphere, that the patient inhales 100% oxygen and that the oxygen replaces all of the nitrogen in the lungs.
The plethysmography technique applies Boyle's law and uses measurements of volume and pressure changes to determine lung volume, assuming temperature is constant.
Interpretation of tests
Professional societies such as the American Thoracic Society/ European Respiratory Society have published guidelines regarding conduct and interpretation of pulmonary function testing to ensure standardization and uniformity in performance of tests. The interpretation of tests depends on comparing the patients values to published normals from previous studies. Deviation from guidelines can result in false-positive or false negative test results. Mohanka MR et al. recently demonstrated that only a small minority of pulmonary function laboratories followed published guidelines for spirometry, lung volumes and diffusing capacity in 2012.
Changes in lung volumes and capacities are generally consistent with the pattern of impairment. TLC, FRC, and RV increase with obstructive lung diseases and decrease with restrictive impairment.
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