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Pulmonary Function Testing (PFT) made simple

last modified on: Mon, 11/05/2018 - 11:14

 

Pulmonary Function Testing (PFT) made simple under construction

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Pulmonary Function Testing

Pulmonary function testing is a group of tests that provide objective data on a patient's lung function.  These tests must be interpreted within the context of the patient's history and physical examination, though their patterns can suggest different categories of respiratory disease.

  • PFTs can be used in a variety of settings, and they are generally ordered to:
    • Look for evidence of respiratory disease when patients present with respiratory symptoms (e.g. dyspnea, cough, cyanosis, wheezing, etc.);
    • Assess for any progression of lung disease;
    • Monitor the efficacy of a given treatment;
    • Evaluate patients pre-operatively; and
    • Monitor for potentially toxic side effects of certain drugs (e.g. amiodarone)
  • The components of PFTs include:
    1. Lung volumes
    2. Spirometry and flow volume loops
    3. Diffusing capacity
  • Difficulties in interpreting PFT's can arise from variation in effort expended as well as muscle weakness compromising an otherwise intense effort
    • PIMAX  = maximum inspiratory mouth pressure (also termed NIF = negative inspiratory force)
      • Is an index of inspiratory muscle strength (Black 1969)
      • Is a routine procedure in many pulmonary function laboratories
      • Hautmann et al in 2000 published the mean PIMAX (kPA)
        • for men (N=248) to be 9.95 kPa
        • for women (N=256) to be 7.43 kPa
        • "strongly dependent on the skill of the technician" (may account for 5%- 12% variation) and "the motivation of the patient"
        • strongest correlation in results were for sex and age, but also height/weight/BMI and different lung function parameters.

LUNG VOLUMES

  • Tidal volume (TV) is the volume of air inspired or expired with each normal breath at rest
  • Inspiratory reserve volume (IRV) is the maximum volume of air that can be inspired over and above the tidal volume
  • Expiratory reserve volume (ERV) is the volume of air that can be expired after the expiration of the tidal volume
  • Residual volume (RV) is the volume of air that remains in the lungs after maximal exhalation
  • Functional residual capacity (FRC) is the volume of air in the lungs following expiration of the tidal volume (ERV + RV)
  • Vital capacity (VC), also known as forced vital capacity (FVC) is the total volume that can be forcefully expired following a maximal inspiratory effort (IRV + TV + ERV)
  • Total lung capacity (TLC) is the volume of air in the lungs at maximal inspiration (IRV + TV + ERV + RV)
     

Graphical demonstration of various lung volumes.  Image obtained from http://commons.wikimedia.org/wiki/File:Lungvolumes.svg.

  • Some of the lung volumes, such as TV, IRV, and ERV, can be measured via simple spirometry.  Others, including RV, FRC, and TLC, cannot be so easily measured and must be obtained via other techniques, such as body plethysmography, helium dilution, and/or nitrogen washout.
    • Body plethysmography
      • Body plethysmography is a technique based on Boyle's law (P1V1 = P2V2) that can be used to determine a patient's FRC, which can then be used to determine RV and TLC
      • With this technique, the patient sits in a closed chamber with a fixed volume and inhales through a closed mouthpiece, causing increased pressure in the box as his or her lungs expand
    • Helium dilution
      • In this test, the patient breathes in a known amount and concentration of helium (an inert gas that has poor solubility in blood and lung tissues), and the spirometer measures the new concentration of helium after an equilibrium between the spirometer and the patient's lungs is reached.  The FRC can then be calculated according to the relationship C1V1 = C2V2, where C = concentration and V = volume.
    • Nitrogen washout
      • In this test, the patient breathes in pure oxygen, the gas he or she exhales is collected, and the nitrogren concentration in the exhaled gas is measured.  The volume of nitrogen-containing gas that was present at the beginning of the test (i.e. the FRC) can be calculated from the initial concentration of nitrogen (atmospheric) and the amount of nitrogen washed out from the lungs.
    • Of note, both the helium dilution and nitrogen washout methods are sensitive to leaks in the system, and they are not as good as body plethysmography at measuring areas of poor air movement (e.g. lung bullae)

SPIROMETRY and FLOW VOLUME LOOPS

  • Spirometry records the flow of air in and out of a patient's lungs plotted against the volume of air inhaled and exhaled during various respiratory maneuvers
  • The values obtained from a given patient are compared with normal values established from reference patients that are matched in size, age, gender, and ethnicity
  • Data obtained from spirometry
    • The most commonly used measures include the forced vital capacity (FVC), the forced expiratory volume in one second (FEV1), and the ratio of the two (FEV1/FVC), which should be about 80% in normal patients
      • An FEV1/FVC <80% suggests obstructive lung disease, while restrictive lung disease typically has normal or increased FEV1/FVC
    • Other useful data from spirometry include measures of flow, such as peak inspiratory flow (PIF) and peak expiratory flow (PEF) ***

Image of a normal flow volume loop. The small loop on the right corresponds to breathing at rest, while the larger loop corresponds to maximal inspiratory and expiratory efforts. Image obtained from http://commons.wikimedia.org/wiki/File:Normal_Spirometry.png.

DIFFUSING CAPACITY

  • Diffusing capacity is a measure of the ability of the lungs to transfer gas into the blood.  Diffusion of gas to blood in the lungs is the most efficient when there is a high surface area for transfer, and when the blood is able to accept the gas being transferred

    • Situations in which the diffusing capacity may be abnormally low include:
      1. Conditions that decrease the available surface area for gas exchange (e.g. emphysema, pulmonary embolism)
      2. Conditions that impair the blood's ability to accept gas (e.g. anemia)
      3. Conditions that alter membrane permeability or increase its thickness (e.g. pulmonary fibrosis)
    • The diffusing capacity is helpful primarily in distinguishing between types of obstructive lung disease.  For example, the diffusing capacity will generally be normal or increased in asthma, while it will be decreased in emphysema.
  • In this test, after maximally exhaling, the patient then maximally inhales a gas mixture that contains trace amounts of carbon monoxide (CO, a gas that has a 210 times higher affinity for hemoglobin than oxygen).  The patient holds this breath for 10 seconds, allowing for gas exchange to occur.  As the patient exhales, the amount of CO in the exhaled gas is measured, allowing for calculation of the diffusing capacity, or DLCO via the equation DLCO = VCO /PaCO, where VCO is the volume of CO and PaCO is the alveolar concentration of CO.
    • Adjustments are made according to the patient's age, sex, height, altitude, level of serum hemoglobin, as these factors may also impact the diffusing capacity

PATTERNS OF RESPIRATORY DISEASE

  • Obstructive pattern
    • Decreased FEV1, normal or decreased FVC, and decreased FEV1/FVC
    • Classically, these are the patients with asthma, chronic bronchitis, or emphysema
      • PFTs can help further distinguish between the above three:
        • Bronchodilator responsiveness - an increase in the FEV1 by 12% following bronchodilator use suggests asthma
        • Bronchial provocation - inducing asthmatic obstruction of reactive lower airways by administering methacholine, histamine, or adenosine monophosphate
        • DLCO will be decreased in patients with emphysema, and can be normal or increased in patients with asthma
    • Lower airway obstruction vs. upper airway obstruction
      • Lower airway obstruction typically displays impaired expiratory capacity (see image below), while upper airway obstruction has impaired inspiratory capacity, which can be evident on the flow volume loop (seen as flattening of the inspiratory arm).

Flow volume loop in a patient with COPD demonstrating impaired inhalation.  Image obtained from http://commons.wikimedia.org/wiki/File:COPD.png.

  • Restrictive pattern
    • Decreased TLC, FEV1, and FVC with a normal FEV1/FVC, and a low DLCO
    • Typically these are patients with interstitial lung disease, severe skeletal abnormalities, or diaphragmatic paralysis
    • The flow volume loop is generally normal in appearance, but has low lung volumes

SUGGESTED READING

  1. Miaskiewicz JJ. Chapter 103. Pulmonary Function Tests. In: Lawry GV, McKean SC, Matloff J, Ross JJ, Dressler DD, Brotman DJ, Ginsberg JS, eds. Principles and Practice of Hospital Medicine. New York: McGraw-Hill; 2012. 
  2. Al-Ashkar F, Mehra R, Mazzone PJ. "Interpreting pulmonary function tests: recognize the pattern, and the diagnosis will follow." Cleve Clin J Med. 2003 Oct;70(10):866-81.
  3. Chu MW, Han JK. "Introduction to Pulmonary Function." Otolaryngol Clin N Am. 2008 Apr;41(2):387-96.
  4. Hautmann H, Hefele S, Schotten K, Huber RM: Maximal inspiratory mouth pressures (PIMAX) in Healthy Subjectes – what is the lower limit of normal. Respir Med. 2000 Jul; 94(7):689-93
  5. Black LF, Hyatt RE. Maximal respiratory pressures: normal values and relationship to age and sex. Am Rev Respir Dis 1969;99:696-702
  6. More References for PIF

  7. Miller RD, Hyatt RE. Evaluation of obstructing lesions of the trachea and larynx by flow-volume loops. Am Rev Respir Dis. 1973 Sep;108(3):475-81.
  8. Rotman HH, Liss HP, Weg JG. Diagnosis of upper airway obstruction by pulmonary function testing. Chest. 1975 Dec;68(6):796-9.
  9. Harrison BD. Upper airway obstruction–a report on sixteen patients. Q J Med. 1976 Oct;45(180):625-45.
  10. Nouraei SA, Winterborn C, Nouraei SM, Giussani DA, Murphy K, Howard DJ, Sandhu GS. Quantifying the physiology of laryngotracheal stenosis: changes in pulmonary dynamics in response to graded extrathoracic resistive loading. Laryngoscope. 2007 Apr;117(4):581-8.
  11. Owens GR, Murphy DM. Spirometric diagnosis of upper airway obstruction. Arch Intern Med. 1983 Jul;143(7):1331-4.
  12. Engström H, Grimby G, Söderholm B. Dynamic spirometry in patients with tracheal stenosis. Acta Med Scand. 1964;146:329-34.
  13. Yernault JC, Englert M, Sergysels R, DeCoster A. Upper airway stenosis: a physiologic study. Am Rev Respir Dis. 1973 Oct;108(4):996-1000.
  14. Modrykamien AM, Gudavalli R, McCarthy K, Liu X, Stoller JK. Detection of upper airway obstruction with spirometry results and the flow-volume loop: a comparison of quantitative and visual inspection criteria. Respir Care. 2009 Apr;54(4):474-9.
  15. Farmer W, Littner MR, Gee BL. “Assessment of Therapy of Upper Airway Obstruction.” Arch Intern Med. 1977 Mar;137(3):309-12.
  16. Ejnell H, Bake B, Mansson I. “Spirometric indices in the assessment of laryngeal obstruction.” Eur J Respir Dis (1984) 65, 600-10.