Pulmonary function testing explained

Pulmonary function testing

Pulmonary function testing (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.[1] 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 pulmonary function technologist, respiratory therapist, respiratory physiologist, physiotherapist, pulmonologist, or general practitioner.

Indications

Pulmonary function testing is a diagnostic and management tool used for a variety of reasons, such as:

Neuromuscular disorders

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.[3]

Duchenne muscular dystrophy is 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.[4]

Tests

Spirometry

See main article: Spirometry. 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.[5]

Helium dilution

See main article: Helium dilution technique. The helium dilution technique for measuring lung volumes uses a closed, rebreathing circuit.[6] 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.

Nitrogen washout

See main article: Nitrogen washout. 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.[7]

Plethysmography

See main article: Plethysmograph and Lung volumes.

The plethysmography technique applies Boyle's law and uses measurements of volume and pressure changes to determine total lung volume, assuming temperature is constant.[8]

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

See main article: Respiratory pressure meter. 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.

Diffusing capacity

See main article: Diffusing capacity. Measurement of the single-breath diffusing capacity for carbon monoxide (DLCO) is a fast and safe tool in the evaluation of both restrictive and obstructive lung disease.

Bronchodilator responsiveness

When a patient has an obstructive defect, a bronchodilator test is given to evaluate if airway constriction is reversible with a short acting beta-agonist. This is defined as an increase of ≥12% and ≥200 mL in the FEV1 or FVC.[9]

Oxygen desaturation during exercise

The six-minute walk test is a good index of physical function and therapeutic response in patients with a chronic lung disease, such as COPD or idiopathic pulmonary fibrosis.[10] [11] [12]

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.

Risks

Pulmonary function testing is a safe procedure; however, there is cause for concern regarding untoward reactions and the value of the test data should be weighed against potential hazards. Some complications include dizziness, shortness of breath, coughing, pneumothorax, and inducing an asthma attack.[13] [14]

Contraindications

There are some indications against a pulmonary function test being done. These include a recent heart attack, stroke, head injury, an aneurysm, or confusion.[15]

Technique

Preparation

Subjects have measurements of height and weight taken before spirometry to determine what their predicted values should be. Additionally, a history of smoking, recent illness, and medications is taken.

Quality control

In order for the forced vital capacity to be considered accurate it has to be conducted three times where the peak is sharp in the flow-volume curve and the exhalation time is longer than 6 seconds.

Repeatability of the PFT is determined by comparing the values of forced vital capacity (FVC) and forced expiratory volume at 1 second (FEV1). The difference between the highest values of two FVCs need to be within 5% or 150 mL. When the FVC is less than 1.0 L, the difference between the highest two values must be within 100 mL. Lastly, the difference between the two highest values of FEV1 should also be within 150 mL. The highest FVC and FEV1 may be used from each different test. Until the results of three tests meet the criteria of reproducibility, the test can be repeated up to eight times. If it is still not possible to get accurate results, the best three tests are used.[16]

Clinical significance

Changes in lung volumes and capacities from normal are generally consistent with the pattern of lung impairment.

Spirometry is required for a diagnosis of COPD.[17]

Interpretation of tests

Classification of COPD based on spirometry[18] !Severity!FEV1 % predicted
Mild (GOLD 1)≥80
Moderate (GOLD 2)50–79
Severe (GOLD 3)30–49
Very severe (GOLD 4)<30

Professional societies such as the American Thoracic Society and the European Respiratory Society have published guidelines regarding the 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, even though only a small minority of pulmonary function laboratories followed published guidelines for spirometry, lung volumes and diffusing capacity in 2012.[19]

COPD

The Global Initiative for Chronic Obstructive Lung Disease provides guidelines for the diagnosis, severity, and management of COPD.[20] To determine obstruction in a patient's lungs, the post-bronchodilator FEV1/FVC needs to be <0.7. Then, the FEV1 percentage of predicted result is used to determine the degree of obstruction where the lower the percent the worse the obstruction.

Maximum respiratory pressures

Several calculations are needed for what a normal maximum inspiratory (MIP) and expiratory pressure (MEP) is. For males this found by:

MIP=120-(0.41 x age)

and

MEP=174-(0.83 x age)

To find the lower limit of what is acceptable in males the equations are:

MIPLLN=62-(0.15 x age)

and

MEPLLN=117-(0.83 x age)

For females, the equations are slightly different. For the normal values this is used:

MIP=108-(0.61 x age)

and

MEP=131-(0.86 x age)

For find the lower limit of what it should be without impairment this form of the equations is used:

MIPLLN=62-(0.50 x age)

and

MEPLLN=95-(0.57 x age)

where

MIP

= maximum inspiratory pressure in cmH20

MEP

= maximum expiratory pressure in cmH20

MIPLLN

= maximum inspiratory pressure lower limit of normal in cmH20

MEPLLN

= maximum expiratory pressure lower limit of normal in cmH20

age

= the patient's age in years[21]

Notes and References

  1. Burrows B . Pulmonary terms and symbols: A report of the ACCP-ATS joint committee on pulmonary nomenclature. . Chest . May 1975 . 67 . 5 . 583–593 . 10.1378/chest.67.5.583 . 1126197 .
  2. Web site: Pulmonary Function Tests . June 15, 2022 . American Thoracic Society.
  3. Sharma GD . Pulmonary function testing in neuromuscular disorders . Pediatrics . 123 . Suppl 4 . S219–S221 . May 2009 . 19420147 . 10.1542/peds.2008-2952D . free .
  4. Finder JD, Birnkrant D, Carl J, Farber HJ, Gozal D, Iannaccone ST, Kovesi T, Kravitz RM, Panitch H, Schramm C, Schroth M, Sharma G, Sievers L, Silvestri JM, Sterni L . 6 . Respiratory care of the patient with Duchenne muscular dystrophy: ATS consensus statement . American Journal of Respiratory and Critical Care Medicine . 170 . 4 . 456–465 . August 2004 . 15302625 . 10.1164/rccm.200307-885ST .
  5. Web site: Pulmonary Function Test in New York . June 2010 . Gafanovich M .
  6. Hathirat S, Mitchell M, Renzetti AD . Measurement of the total lung capacity by helium dilution in a constant volume system . The American Review of Respiratory Disease . 102 . 5 . 760–70 . November 1970 . 5475674 . 10.1164/arrd.1970.102.5.760 . 31 January 2024 .
  7. Boren HG, Kory RC, Syner JC . The Veterans Administration-Army cooperative study of pulmonary function: II. The lung volume and its subdivisions in normal men. . The American Journal of Medicine . July 1966 . 41 . 1 . 96–114 . 10.1016/0002-9343(66)90008-8 .
  8. DuBois AB, Botelho SY, Bedell GN, Marshall R, Comroe JH . A rapid plethysmographic method for measuring thoracic gas volume: a comparison with a nitrogen washout method for measuring functional residual capacity in normal subjects . The Journal of Clinical Investigation . 35 . 3 . 322–6 . March 1956 . 13295396 . 438814 . 10.1172/JCI103281 .
  9. Sim YS, Lee JH, Lee WY, Suh DI, Oh YM, Yoon JS, Lee JH, Cho JH, Kwon CS, Chang JH . 6 . Spirometry and Bronchodilator Test . Tuberculosis and Respiratory Diseases . 80 . 2 . 105–112 . April 2017 . 28416951 . 5392482 . 10.4046/trd.2017.80.2.105 .
  10. Enright PL . The six-minute walk test . Respiratory Care . 48 . 8 . 783–785 . August 2003 . 12890299 .
  11. Swigris JJ, Wamboldt FS, Behr J, du Bois RM, King TE, Raghu G, Brown KK . The 6 minute walk in idiopathic pulmonary fibrosis: longitudinal changes and minimum important difference . Thorax . 65 . 2 . 173–177 . February 2010 . 19996335 . 3144486 . 10.1136/thx.2009.113498 .
  12. ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories . ATS statement: guidelines for the six-minute walk test . American Journal of Respiratory and Critical Care Medicine . 166 . 1 . 111–117 . July 2002 . 12091180 . 10.1164/ajrccm.166.1.at1102 .
  13. Web site: 2019-11-19 . Pulmonary Function Tests . 2022-06-15 . www.hopkinsmedicine.org . en.
  14. Web site: Pulmonary function tests: MedlinePlus Medical Encyclopedia . 2022-06-15 . medlineplus.gov . en.
  15. Web site: Lung Function Tests . 2022-06-15 . www.lung.org . en.
  16. Sim YS, Lee JH, Lee WY, Suh DI, Oh YM, Yoon JS, Lee JH, Cho JH, Kwon CS, Chang JH . 6 . Spirometry and Bronchodilator Test . Tuberculosis and Respiratory Diseases . 80 . 2 . 105–112 . April 2017 . 28416951 . 5392482 . 10.4046/trd.2017.80.2.105 .
  17. Web site: 2022 GOLD Reports . 2022-06-15 . Global Initiative for Chronic Obstructive Lung Disease - GOLD . en-US.
  18. Book: Global Initiative for Chronic Obstructive Lung Disease . Pocket Guide to COPD Diagnosis, Management, and Prevention . 11.
  19. Mohanka MR, McCarthy K, Xu M, Stoller JK . A survey of practices of pulmonary function interpretation in laboratories in Northeast Ohio . Chest . 141 . 4 . 1040–1046 . April 2012 . 21940775 . 10.1378/chest.11-1141 .
  20. Web site: About Us . 2022-06-16 . Global Initiative for Chronic Obstructive Lung Disease - GOLD . en-US.
  21. Evans JA, Whitelaw WA . October 2009 . The assessment of maximal respiratory mouth pressures in adults . Respiratory Care . 54 . 10 . 1348–1359 . 19796415.