Pulmonary function testing

Last updated: April 29, 2022

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).

Chalk Talk: Spirometry Chalk Talk: Pulmonary function testing - Body plethysmography

Spirometry

Spirometry measures breath volume and airflow rates and is routinely performed with the help of a spirometer.

Diagnostically relevant spirometric values
Parameter	Definition	Normal finding ^[1]^[2]^[3]
Peak expiratory flow (PEF)	The maximum airflow rate attained during forced expiration (in L/s)	≥ 80% of the predicted average value based on race, height, gender, and age
Vital capacity	The maximum volume of air that can be expired after maximal inspiration VC can be measured through: Slow respiratory maneuvers Inspiratory vital capacity (IVC): The maximum volume of air that can be inspired after maximal expiration. Expiratory vital capacity (EVC): The maximum volume of air that can be expired after maximal inspiration. Forced respiratory maneuvers Forced vital capacity (FVC): The maximum volume of air that can be forcefully expired after maximal inspiration. Among young individuals, IVC, EVC, and FVC have nearly the same value. However among patients with obstructive lung disease: IVC > EVC > FVC. ^[4]^[5]	Depends on race, height, age, and gender; approximately 4.5–5 L in healthy young adults
Forced expiratory volume in 1 second (FEV₁)	The maximum volume of air that can be forcefully expired (forced expiratory volume) within 1 second after maximal inspiration	≥ 80% of the predicted average value based on height, gender, and age (or > 75% of vital capacity)
FEV1/FVC (Tiffeneau-Pinelli index, relative FEV₁)	Ratio of FEV₁ to FVC expressed as a percentage	≥ 70%
Forced expiratory flow rate at 75%, 50%, and 25% of vital capacity (FEF_75%, FEF_50%, FEF_25%)	Average airflow rates observed during forced expiration when 75% (FEF_25%), 50% (FEF_50%), and 25% (FEF_75%) of the vital capacity remains in the lungs.	≥ 65% of the predicted average value based on race, height, gender, and age ^[6]

Physiological flow-volume loop Normal flow-volume loop Overview of flow-volume loops

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.

Measured parameters
- All parameters that are measured in spirometry
- Airway resistance (R_aw): 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
Indications ^[7]
- To obtain objective spirometric readings in uncooperative/unconscious patients
- Clinical suspicion of emphysema

Body plethysmography

Obstructive and restrictive lung diseases

Differential diagnoses of pulmonary function testing (PFTs)

Obstructive vs restrictive lung diseases ^[1]^[8]
Type		Obstructive lung disease	Restrictive lung disease
Description		Increased resistance to airflow caused by narrowing of airways	Impaired ability of the lungs to expand (as a result of reduced lung compliance)
Etiology		COPD (chronic bronchitis, emphysema) Bronchial asthma Bronchiectasis, cystic fibrosis Narrowing of extrathoracic airways: laryngeal tumors, vocal cord palsy	Intrinsic causes (parenchymal diseases) Interstitial lung disease Sarcoidosis Pneumoconioses ARDS Idiopathic pulmonary fibrosis Hypersensitivity pneumonitis Granulomatosis with polyangiitis Pulmonary Langerhans cell histiocytosis Radiation-induced lung injury Drug-induced interstitial lung disease (e.g., due to MTX, bleomycin, busulfan, amiodarone) Alveolar (e.g., pneumonia, pulmonary edema, hemorrhage) Extrinsic causes (i.e., extrapulmonary conditions that change the mechanics of respiration) Diseases of the pleura and pleural cavity (e.g., chronic pleural effusion, pleural adhesions, pneumothorax) Deformities of the thorax/mechanical limitation (e.g., kyphoscoliosis, ankylosing spondylitis, obesity, ascites, pregnancy) Respiratory muscle weakness (e.g., phrenic nerve palsy, myasthenia gravis, polio, GBS, ALS, myopathies): See “Respiratory muscle function.” ^[9]
Spirometric findings	FEV₁	↓	Normal or ↓
	FEV₁/FVC	↓ (↓ in FEV1 > ↓ in FVC)	Normal or ↑ (↓ in FEV1 is proportional to ↓ in FVC)
	VC	↓	↓
	Spirometer tracing	Air trapping: a "scalloping" of the expiratory limb in conditions such as emphysema or in patients who have undergone a pneumectomy.	Narrow flow-volume loop
Plethysmograph findings	RV	↑	Normal or ↓
	FRC	↑	↓
	TLC	Normal or ↑	↓
	Airway resistance	↑	Normal
	Lung compliance	Normal	Normal (extrinsic causes) or ↓ (intrinsic causes)
DLCO		↓ ^[10]	Normal (extrinsic causes) or ↓ (intrinsic causes)
A-a gradient		↑	Normal (extrinsic causes) or ↑ (intrinsic causes)
Imaging findings		Hyperlucency of lung tissue Horizontal ribs and widened intercostal spaces Increased anteroposterior diameter Diaphragm pushed down and flattened	Can be normal Depends on the underlying disease Interstitial lung disease: reticular opacities (sign of fibrosis) Pleural effusion: unilateral blunting of the costophrenic angle Pneumothorax: unilateral pleural line with reduced/absent lung markings

Flow-volume loop in obstructive and restrictive lung diseases Flow-volume loop in restrictive lung disease Overview of flow-volume loops Osmosis: Restrictive lung disease - causes, symptoms, diagnosis, treatment, pathology

Bronchial challenge tests

Bronchial challenge tests help distinguish bronchial asthma from other causes of obstructive lung disease.

Bronchial challenge tests
	Methacholine challenge test (bronchoprovocation test) ^[11]	Bronchodilator reversibility test (post-bronchodilator test) ^[12]
Description	Lung function tests are performed before and after the administration of methacholine.	FEV₁ and airway resistance are measured before and 10 minutes after the inhalation of a fast-acting bronchodilator (e.g., albuterol, ipratropium bromide).
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).	An increase in FEV₁ by 200 mL and 12% of the initial value indicates reversible airway obstruction (bronchial asthma). ^[13]
Indication	To determine if airway hyperresponsiveness is present	Allows reversible airway obstruction to be differentiated from irreversible obstruction

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
- Carbon monoxide transfer coefficient (K_CO): the amount of CO per unit time per unit partial pressure that is transferred from the alveolus to the pulmonary capillary
- Diffusing capacity of the lung for carbon monoxide (DL_CO) : the product of K_CO and total alveolar volume (V_A)
Interpretation: See table below.
- Position and/or certain maneuvers also affect results.
- Exercise will increase DL_CO through recruitment of additional lung zones and increased perfusion with reduced capillary transit time. ^[14]

Spirometric findings
	Restrictive lung disease (normal or ↑ FEV1/FVC)	Obstructive lung disease (FEV1/FVC < 70%)	Normal
↓ DL_CO	Late interstitial lung disease Post-pneumonectomy (↑ K_CO) Pulmonary edema (e.g., as a result of severe congestive heart failure) ^[15] Obesity ^[16]	Emphysema	Pulmonary vascular diseases (pulmonary hypertension, pulmonary embolism, hepatopulmonary syndrome) (↓ K_CO) Early interstitial lung disease Pre-existing carboxyhemoglobinemia (e.g., due to smoking) Anemia
Normal DL_CO	Respiratory muscle weakness Pleural disorders Thoracic cage deformities Obesity	Alpha-1-antitrypsin deficiency ^[17] Bronchiectasis, cystic fibrosis ^[18] Chronic bronchitis Bronchial asthma	Healthy findings
↑ DL_CO	Obesity	Bronchial asthma	Polycythemia Mild heart failure and left-to-right cardiac shunts

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

Tests ^[19]^[20]
- 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)
Indication: diagnose and monitor patients with respiratory muscle weakness ^[19]
- Depression of the respiratory center
  - Severe metabolic encephalopathy
  - Opiate poisoning
  - Brainstem stroke
  - Traumatic brain injury
- 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)

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
	Definition	Normal range
Total lung capacity (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

Physiological lung volumes

References

Walker BR, Colledge NR, Raston SR, Penman ID. Davidson's Principles and Practice of Medicine. Elsevier ; 2013
Polkey MI, Green M, Moxham J. Measurement of respiratory muscle strength. Thorax. 1995; 50 (11): p.1131-1135.
Criée CP, Sorichter S, Smith HJ, et al. Body plethysmography – Its principles and clinical use. Respir Med. 2011; 105 (7): p.959-971. doi: 10.1016/j.rmed.2011.02.006 . | Open in Read by QxMD
Barreiro TJ, Perillo I. An Approach to Interpreting Spirometry. Am Fam Physician. 2004; 69 (5): p.1107-1115.
NIOSH Spirometry Training Guide 6: Compairing observed to predicted normal Values.
Culver BH. How should the lower limit of the normal range be defined?. Respir Care. 2012; 57 (1): p.136-145. doi: 10.4187/respcare.01427 . | Open in Read by QxMD
Constán E, Medina J, Silvestre A, Alvarez I, Olivas R. Difference Between The Slow Vital Capacity And Forced Vital Capacity: Predictor Of Hyperinflation In Patients With Airflow Obstruction. nternet Journal of Pulmonary Medicine. 2004; 4 (2).
Lung Function Indices. http://spirxpert.ers-education.org/en/spirometry/lung-function-indices/spirographic-indices-an-overview/. Updated: February 19, 2017. Accessed: February 19, 2017.
Ciprandi G, Capasso M, Tosca M, et al. A forced expiratory flow at 25-75% value <65% of predicted should be considered abnormal: a real-world, cross-sectional study. Allergy Asthma Proc. 2012; 33 (1): p.e5-8. doi: 10.2500/aap.2012.33.3524 . | Open in Read by QxMD
Groves L, Brade S, Wright SP. Pushing it to the limit: enhanced diffusing membrane capacity facilitates greater pulmonary diffusing capacity in athletes during exercise.. J Physiol. 2016; 594 (24): p.7171-7172. doi: 10.1113/JP273529 . | Open in Read by QxMD
Puri S, Baker BL, Dutka DP, Oakley CM, Hughes JMB, Cleland JGF. Reduced Alveolar–Capillary Membrane Diffusing Capacity in Chronic Heart Failure. Circulation. 1995; 91 (2769-2774). doi: 10.1161/01.CIR.91.11.2769 . | Open in Read by QxMD
Porhomayon J, Papadakos P, Singh A, Nader ND. Alteration in respiratory physiology in obesity for anesthesia-critical care physician.. HSR proceedings in intensive care & cardiovascular anesthesia. 2011; 3 (2): p.109-18.
Wilson JS, Galvin JR. Normal diffusing capacity in patients with PiZ alpha(1)-antitrypsin deficiency, severe airflow obstruction, and significant radiographic emphysema. Chest. 2000; 118 (3): p.867-871.
Espiritu JD, Ruppel G, Shrestha Y, Kleinhenz ME. The diffusing capacity in adult cystic fibrosis. Respir Med. 2003; 97 (6): p.606-611. doi: 10.1053/rmed.2003.1487 . | Open in Read by QxMD
Lung Diseases: Restrictive and Obstructive. http://www.ugr.es/~jhuertas/EvaluacionFisiologica/Espirometria/restobst.htm. Updated: February 19, 2017. Accessed: February 19, 2017.
Polkey MI, Green M, Moxham J. Measurement of respiratory muscle strength.. Thorax. 1995; 50 (11): p.1131-1135. doi: 10.1136/thx.50.11.1131 . | Open in Read by QxMD
Bailey KL. The importance of the assessment of pulmonary function in COPD.. Med Clin North Am. 2012; 96 (4): p.745-52. doi: 10.1016/j.mcna.2012.04.011 . | Open in Read by QxMD
Crapo RO, Casaburi R, Coates AL, et al. Guidelines for methacholine and exercise challenge testing-1999: This official statement of the American Thoracic Society was adopted by the ATS Board of Directors, July 1999. Am J Respir Crit Care Med. 2000; 161 (1): p.309-329. doi: 10.1164/ajrccm.161.1.ats11-99 . | Open in Read by QxMD
Respiratory Health Bronchodilator Procedures Manual. https://www.cdc.gov/nchs/data/nhanes/nhanes_07_08/Bronchodilator.pdf. Updated: January 1, 2008. Accessed: June 22, 2018.
Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests.. The European respiratory journal. 2005; 26 (5): p.948-68. doi: 10.1183/09031936.05.00035205 . | Open in Read by QxMD
Herold G. Internal Medicine. Herold G ; 2014

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