- Clinical science
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).
This convenient pulmonary function test measures breath volume and airflow rates and is routinely performed with the help of a spirometer.
|Diagnostically relevant spirometric values|
|Peak expiratory flow (PEF)|| || |
| || |
|Forced expiratory flow rate at 75%, 50%, and 25% of vital capacity (FEF75%, FEF50%, FEF25%)|| |
|Type||Obstructive lung disease||Restrictive lung disease|
|Spirometric findings||FEV1||↓||Normal or ↓|
|FEV1/FVC||↓||Normal or ↑|
|Residual volume||↑||Normal or ↓|
|Total lung capacity||Normal or ↑||↓|
|Resistance to air flow||↑||Normal|
|Lung compliance||Normal||Normal (extrinsic causes) or ↓ (intrinsic causes)|
|Spirometer tracing|| |
In restrictive lung disease, alterations in blood gas analysis are seen much later in the course of the disease!
Methacholine challenge test (bronchoprovocation test)
- Indication: : This test is used to determine if airway hyperresponsiveness is present.
- Description: : Lung function tests are performed before and after the administration of methacholine.
- Interpretation: : A doubling of airway resistance with a reduction of FEV1 of at least 20% points to a diagnosis of airway hyperresponsiveness (e.g., bronchial asthma).
Bronchodilator reversibility test (post-bronchodilator test)
- Indication: : This test allows reversible airway obstruction to be differentiated from irreversible obstruction.
- Description: To perform this test, FEV1 and airway resistance are measured before and 10 minutes after the inhalation of a fast-acting bronchodilator (e.g., albuterol, ipratropium bromide).
- Interpretation: : An increase in FEV1 by 200 mL or 12% of the initial value indicates reversible airway obstruction (bronchial asthma).
- Description: : Single-breath diffusing capacity measures the ability of the alveoli to exchange gases with pulmonary capillaries.
- To differentiate between intrapulmonary (e.g., interstitial lung disease) and extrapulmonary causes (e.g., pleural effusion, respiratory muscle weakness) 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)
- Measured parameters
|Single breath diffusing capacity||Spirometric findings|
|Restrictive lung disease (normal or ↑ FEV1/FVC)||Obstructive lung disease (FEV1/FVC < 70%)||Normal|
|Normal DLCO|| |
- Position and/or certain maneuvers also affect results.
The following non-invasive tests are used to diagnose and monitor patients with respiratory muscle weakness:
- 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)
Causes of respiratory muscle weakness
- Depression of the respiratory center
- Phrenic nerve palsy due to:
- Myasthenia gravis
- Myopathies; (e.g., thyrotoxic myopathy, muscular dystrophy)
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 volume||Definition||Normal range|
|Total lung capacity (TC,TLC)||Volume of air in the lungs after maximal inhalation||6–6.5 L|
|Vital capacity (VC)||Difference in lung volume between maximal exhalation and maximal inhalation||4.5–5 L|
|Residual volume (RV)||Volume of air that remains in the lungs after maximal exhalation||1–1.5 L|
|Tidal volume (TV)||Volume of air that is inhaled and exhaled in a normal breath at rest||∼ 500 mL or 7 mL/kg|
|Inspiratory reserve volume (IRV)||Maximum volume of air that can still be forcibly inhaled following the inhalation of a normal TV||3–3.5 L|
|Inspiratory capacity (IC)||Maximum volume of air that can be inhaled after the exhalation of a normal TV||3.5–4 L|
|Expiratory reserve volume (ERV)||Maximum volume of air that can still be forcibly exhaled after the exhalation of a normal TV||1.5 L|
|Expiratory capacity (EC):||Maximum volume of air that can be exhaled after the inspiration of a normal TV||2 L|
|Functional residual capacity (FRC)||Volume of air that remains in the lungs after the exhalation of a normal TV||2.5–3 L|