Summary
Pulmonary function tests (PFTs) measure different lung volumes and other functional metrics of pulmonary function. They can be used to diagnose ventilatory disorders and differentiate between obstructive and restrictive lung diseases. The most common PFT is spirometry, which involves a cooperative patient breathing actively through his or her mouth into an external device. This simple and cost-effective test measures both dynamic and static lung volumes (with the exception of residual volume and total lung capacity), as well as airflow rates. Full-body plethysmography is an additional PFT that is able to estimate both residual volume and total lung capacity and is performed with the patient in a closed space. Lastly, single-breath diffusing capacity helps determine if the alveolar membrane is thickened (e.g., pulmonary fibrosis) or destroyed (e.g., emphysema), or if the pulmonary vasculature is affected (e.g., pulmonary hypertension).
Spirometry
Spirometry measures breath volume and airflow rates and is routinely performed with the help of a spirometer.
Diagnostically relevant spirometric values | ||
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Parameter | Definition | Normal finding [1][2][3] |
Peak expiratory flow (PEF) |
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Vital capacity |
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Forced expiratory volume in 1 second (FEV1) |
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FEV1/FVC (Tiffeneau-Pinelli index, relative FEV1) |
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Forced expiratory flow rate at 75%, 50%, and 25% of vital capacity (FEF75%, FEF50%, FEF25%) |
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Ergospirometry
Ergospirometry is used to objectively measure cardiopulmonary performance when a patient is subjected to stress. Oxygen utilization (VO2 max), the rate of carbon dioxide production, and minute ventilation are measured in addition to lung volumes.
Body plethysmography
Body plethysmography is performed to detect functional limitations during respiration.
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Measured parameters
- All parameters that are measured in spirometry
- Airway resistance (Raw): the resistance to airflow from the mouth to the alveoli during inspiration and expiration
- Residual volume (RV)
- Total lung capacity (TLC)
- Thoracic gas volume (TGV or VGT): the intrathoracic air volume after normal expiration
- With the aid of an esophageal probe: lung compliance
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Indications [7]
- To obtain objective spirometric readings in uncooperative/unconscious patients
- Clinical suspicion of emphysema
Obstructive and restrictive lung diseases
Obstructive vs restrictive lung diseases [1][8] | |||
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Type | Obstructive lung disease | Restrictive lung disease | |
Description |
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Etiology |
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Spirometric findings | FEV1 |
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FEV1/FVC | |||
VC |
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Spirometer tracing |
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Plethysmograph findings | RV |
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FRC |
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TLC |
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Airway resistance |
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Lung compliance |
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DLCO |
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Imaging findings |
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Bronchial challenge tests
Bronchial challenge tests help distinguish bronchial asthma from other causes of obstructive lung disease.
Bronchial challenge tests | ||
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Methacholine challenge test (bronchoprovocation test) [11] | Bronchodilator reversibility test (post-bronchodilator test) [12] | |
Description |
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Interpretation |
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Indication |
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Because the methacholine challenge test can trigger a life-threatening asthma attack, medications that reverse bronchospasm (e.g., epinephrine, atropine) should be kept at hand during the test!
Single-breath diffusing capacity
Single-breath diffusing capacity measures the ability of the alveoli to exchange gases with pulmonary capillaries.
- Measured parameters
- Interpretation: See table below.
Spirometric findings | |||
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Restrictive lung disease (normal or ↑ FEV1/FVC) | Obstructive lung disease (FEV1/FVC < 70%) | Normal | |
↓ DLCO |
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Normal DLCO |
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↑ DLCO |
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Indications
- To differentiate between intrapulmonary (e.g., interstitial lung disease) and extrapulmonary (e.g., pleural effusion, respiratory muscle weakness) causes of restrictive lung disease
- To monitor disease progression among patients with intraparenchymal lung diseases
- Hypoxemia that remains unexplained even after spirometry (e.g., pulmonary embolism)
Respiratory muscle function
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Tests [19][20]
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Test of inspiratory muscle function (e.g., diaphragm)
- Maximal inspiratory pressure (MIP)
- Sniff nasal inspiratory pressure (SNIP)
- Test of expiratory muscle function: maximal expiratory pressure (MEP)
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Test of inspiratory muscle function (e.g., diaphragm)
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Indication: diagnose and monitor patients with respiratory muscle weakness [19]
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Depression of the respiratory center
- Severe metabolic encephalopathy
- Opiate poisoning
- Brainstem stroke
- Traumatic brain injury
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Phrenic nerve palsy due to:
- Anterior horn cell disorders (e.g., amyotrophic lateral sclerosis)
- Peripheral neuropathies (e.g., Guillain-Barré syndrome)
- Myasthenia gravis
- Myopathies; (e.g., thyrotoxic myopathy, muscular dystrophy)
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Depression of the respiratory center
Patients with respiratory muscle weakness show spirometric findings of restrictive lung disease.
Clinical features of respiratory muscle weakness do not manifest until diaphragmatic strength is reduced to a quarter of its normal strength (unilateral diaphragmatic paralysis decreases ventilatory capacity by only 20%).
Lung volumes
Lung volumes depend on age, height, and gender. The values that are listed below are for a healthy young adult.
Normal lung volumes | ||
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Definition | Normal range | |
Total lung capacity (TLC) |
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Vital capacity (VC) |
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Residual volume (RV) |
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Tidal volume (TV) |
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Inspiratory reserve volume (IRV) |
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Inspiratory capacity (IC) |
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Expiratory reserve volume (ERV) |
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Expiratory capacity (EC) |
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Functional residual capacity (FRC) |
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