Pulmonary Function Test

Original Editor - Uchechukwu Chukwuemeka

Top Contributors - Lucinda hampton, Kim Jackson and Admin  

Description

Pulmonary function tests (PFTs) are routinely used in patients with chronic respiratory disorders to monitor lung indices. They are performed to assess respiratory function and to determine the extent of dysfunction. Such tests include measurements of lung volumes, ventilatory function, and the mechanics of breathing, diffusion, and gas exchange. PFTs are useful in following the course of a patient with an established respiratory disease and assessing the response to therapy. They are useful as screening tests in potentially hazardous industries, such as coal mining and those that involve exposure to asbestos and other noxious fumes, dusts, or gases. They are useful for screening patients scheduled for thoracic and upper abdominal surgery, and symptomatic patients with a history suggesting high risk.

Indication

• Investigation of a patient with sign/symptoms that shows respiratory problems. e.g. wheezing, coughing, crackles and abnormal chest xray e.t.c.
• Monitoring of patients with respiratory condition for disease progression and response to treatment.
• Evaluation of degree of cardio-respiratory disability
• Preoperative evaluation for patient that will undergo thoracic and abdominal surgery
• Montoring of patients at risk of pulmonary complications
• Routine assessment for individual with high exposure to pulmonary toxic agent e.g abestos, dust and fumes

Contraindication

• Myocardial Infarction in the last month
• Unstable Angina
• Recent thoracic and abdominal surgeries
• Recent Opthalmic surgery
• Abdominal, thoracic and cerebral Aureysms
• Active Hemoptysis
• Pneumothorax

Different Pulmonary Function Tests

1. Spirometry 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).
2. Techniques for measuring Residual volume (RV) and Functional residual capacity (FRC): Nitrogen Washout Technique ( 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); Helium Dilution Technique ( 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); 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)
3. Diffusing Capacity of Carbon monoxide (DLCo) Oxygen. 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. 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.
4. Peak Expiratory flow Meter Reading to generate a flow volume loop.
5. Arterial Blood Gas Analysis
6. Bronchodilator Test. One of the defining characteristics of asthma is an increase in the responsiveness of the airways to a number of stimuli. If lung function is normal but the patient experiences intermittent episodes of cough, dyspnea, or wheezing, the demonstration of bronchial hyper-responsiveness may be useful in establishing a diagnosis of asthma. Two general types of bronchoprovocation studies may be performed: direct and indirect. Direct testing employs pharmacologic agents that directly stimulate airway smooth muscle receptors to cause bronchoconstriction. The most commonly utilized drugs in direct testing are methacholine and histamine. Indirect testing incorporates agents that stimulate release of pro-inflammatory mediators, which ultimately induce bronchoconstriction. Indirect tests employ hypertonic saline or mannitol or are based on exercise protocols used to elicit exercise-induced asthma or eucapnic voluntary hyperpnea.
7. Cardiorespiratory Exercise Test
8. Oximetry
9. Respiratory Muscle Function Test. Maximal inspiratory and expiratory pressures (MIP/MEPs) are measured by having the patient perform maximal inspiratory and expiratory efforts against a closed valve and measuring the static pressures that are generated. The maximal voluntary ventilation is often coupled with spirometry by having the patient breathe in as deeply and as quickly as possible for ten to fifteen seconds and then extrapolating that volume to one minute.

Patterns of Respiratory Diseases

• 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)

• 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

Interpretation

• Step 1: FEV1/FVC ratio

FEV1/FVC <70% (or less than the Lower limit of normal) : Obstructive Pattern (COPD, Asthma)

≥70%: Normal or Restrictive disease

• Step 2:

Severity of obstruction (GOLD Criteria for COPD)

FEV1 80-100% predicted: Mild (Stage 1)

FEV1 50- 80% predicted: Moderate (Stage 2)

FEV1 30- <50% predicted: Severe (Stage 3)

FEV1 <30% predicted: Very severe (Stage 4)

FEV1 <50% with right side heart failure: Very severe (Stage 4)

Severity of obstruction (ATS General Criteria)

FEV1 over 70% predicted: MILD

FEV1 60-70%predicted: MODERATE

FEV1 50-60% predicted: MODERATELY SEVERE

FEV1 35-50% predicted: SEVERE

FEV1 <35% predicted: VERY SEVERE

• Step 3

≥12% increase AND an absolute improvement of at least 200ml after inhaling a beta agonist.

FEV1, OR FVC is considered a significant response.

• Step 4: Total Lung Capacity (TLC) (Normal range: 80-120% of predicted)

TLC >120 = Hyperinflation

TLC <80% = Restrictive disease (ATS criteria for severity)

70-80% predicted: MILD

60-70% predicted: MODERATE

50-60% predicted: MODERATELY SEVERE

<50% predicted: SEVERE

• Step 5: RV/TLC ratio (Normal range: <35% or < predicted)

RV/TLC >35% or > predicted indicates Air trapping

• Step 6: DLCO (Normal range: 80-120% of predicted)

In obstructive disease:

Decreased in Emphysema

Normal in chronic bronchitis

Normal or increased in Asthma

In restrictive disease:

Decreased in parenchymal disease

Normal in non-parenchymal (e.g. chest wall) restriction