14 Pulmonary Function Testing

Question 1

Clinical pulmonary function tests (PFTs) vs. individual tests

Answer 1

• Clinical pulmonary function tests (PFTs)
  
  • Provide a practical assessment of the components of the respiratory system
  • Rarely provide a specific diagnosis in the absence of complementary clinical data
  • No single pulmonary function test is able to adequately assess all the physiologic processes performed by the respiratory system
• Individual tests
  
  • May be influenced by more than one process

Question 2

Common indications for pulmonary function testing

Answer 2

• Categorization of the type and severity of physiologic perturbation
  
  • Restrictive vs. obstructive categorization
  • Asthma vs. emphysema
• The objective assessment of pulmonary symptoms
  
  • Documentation of abnormality
  • Disability assessment
• Documentation of progression of disease
  
  • COPD
  • Neuromuscular disease, such as ALS
• Documentation of the patient’s response to therapy
  
  • Asthma control
  • Lung volume reduction surgery
• Preoperative assessment
  
  • Lung cancer resection
  • Timing of lung transplantation
• Screening for sub-clinical disease
  
  • Emphysema in a tobacco smoker
  • Occupational risk

Question 3

Corresponding PFTs of physiologic processes

Answer 3

• Ventilation
  
  • Spirometry (FVC, FEV1 etc.)
  • Lung volume (RV, FRC, TLC)
    
    • Plethysmography
    • Helium dilution
  • Inspiratory and expiratory pressure (MIP, MEP)
  • Lung compliance (rarely measured clinically)
  • Maximal voluntary ventilation (MVV)
  • Exercise ventilation (VE-max, VD/VT)
• Diffusion
  
  • Diffusing capacity
  • Exercise oxygen saturation
• Circulation
  
  • Diffusing capacity (DLCO)
  • Cardiopulmonary exercise testing (VO2-max)

Question 4

Spirometry

• Spirometry
• Measures…
• Does not measure…

Answer 4

• Spirometry
  
  • The most commonly performed and standardized measurement of pulmonary function
  • One of the tests required to evaluate the volumes and capacities of the lung
• Measures…
  
  • The volume and flow rate of air that leaves the lungs
• Does not measure…
  
  • Residual volume (RV)
  • Functional residual capacity (FRC)
  • Total lung capacity (TLC)
  • These require the use of lung volume measurement techniques capable of assessing the air remaining in the lung after an expiration has been completed

Question 5

Spirometry

• Vital capacity (VC)
• Forced vital capacity (FVC)
• Slow vital capacity (SVC)
• FVC vs. SVC

Answer 5

• Vital capacity (VC)
  
  • The most important of these volumes
  • Represents the total volume of gas that can be exhaled starting from a full inspiration
  • One of the most powerful prognostic indicators of survival because it is decreased in both cardiac failure and in pulmonary diseases
• Forced vital capacity (FVC)
  
  • If this maneuver is performed with maximal effort
  • Technologist screams “take a deep breath all the way in, now BLOW as hard and as fast as you can…keep on blowing all the way out…”
• Slow vital capacity (SVC)
  
  • If this maneuver is performed slowly and steadily
• FVC vs. SVC
  
  • Normally the FVC and SVC are equal
  • In patients with airway obstruction, the FVC is commonly lower than the SVC because the extra effort results in airway compression and early airway closure

Question 6

Spirometry

• Forced maneuver measures…
• Forced expiratory volume in 1 second (FEV1)
• Forced expiratory flow rate which occurs between 25% and 75% of the forced vital capacity (FEF 25-75)
• Reasons why the FEV1 is decreased
• FEV1/FVC ratio

Answer 6

• Forced maneuver measures…
  
  • Volume
  • Rate of expiratory flow can be obtained from the forced maneuver
• Forced expiratory volume in 1 second (FEV1)
  
  • The most commonly employed measure of the rate of expiratory flow
  • The volume of air exhaled after one second of a forced vital capacity maneuver
• Forced expiratory flow rate which occurs between 25% and 75% of the forced vital capacity (FEF 25-75)
  
  • A second, more subtle indicator of airflow
• Reasons why the FEV1 is decreased
  
  • There is simply less volume in the lungs to start with (e.g. mouse lungs)
  • There is an obstruction to the air flowing out of the lungs
• FEV1/FVC ratio
  
  • Values less than 0.7 indicate the presence of airways obstruction

Question 7

Spirometry

• Quality control
  
  • An acceptable spirogram requires that…
  • Flow-volume loop
• Predicted values
  
  • Spirometric values
  • Individuals of Asian or African descent
• Detection of disease
  
  • Variability
  • Normal limits

Answer 7

• Quality control
  
  • An acceptable spirogram requires that…
    
    • Each effort last at least 4 seconds
    • Each effort contain no coughs or pauses
    • 3 acceptable maneuvers be within 5% of each other
  • Flow-volume loop
    
    • May be a better representation of the data to assist in determining consistency of effort
• Predicted values
  
  • Spirometric values
    
    • Compared to “normal” values obtained from large series of individuals
    • Vary with height, gender, age and race
  • Individuals of Asian or African descent
    
    • On average normally have values for FVC and FEV1 which are 10% lower than people of European descent
• Detection of disease
  
  • Measurements of lung volume and flow in normal individuals may vary by as much as 20%
  • Therefore, any value which is 80-120% of the predicted value is considered within normal limits
    
    • I.e. FVC ≥ 80% of predicted is considered normal and values < 80% of predicted are considered abnormal

Question 8

Spirometry:
Obstructive lung disease

• Obstructive lung diseases
• Clinical examples of obstructive lung diseases
• Indicator of minimal airway obstruction

Answer 8

• Obstructive lung diseases
  
  • Diseases characterized by an increased resistance to airflow
  • Characterized by a reduction in the FEV1/FVC ratio to less than 0.70
    
    • Hence FEV1 is generally decreased
• Clinical examples of obstructive lung diseases
  
  • Chronic bronchitis, emphysema, and asthma
• Indicator of minimal airway obstruction
  
  • FEF25-75
  • Only when other lung volumes are normal
  • Because this value has a greater variability than other spirometric indices, only values less than 65% of predicted are considered abnormal
  • The previous use of FEF25-75 as an indicator of “small airway disease” has been shown to be a misnomer but still persists in many general medicine texts

Question 9

Spirometry:
Restrictive lung disease

• Restrictive diseases
• Processes that cause restrictive physiology
• In some situations obstructive lung diseases can result in…

Answer 9

• Restrictive diseases
  
  • Diseases that cause a reduction in lung volume are called restrictive diseases
  • Characterized by a reduction in FVC (and usually FEV1) but a normal to increased FEV1/FVC ratio
• Processes that cause restrictive physiology
  
  • Diseases which decrease lung compliance (pulmonary fibrosis, pulmonary edema)
  • Diseases which decrease chest wall compliance (e.g. kyphoscoliosis)
  • Pleural diseases
  • Diseases of the respiratory muscles.
• In some situations obstructive lung diseases can result in…
  
  • A decreased FVC in the presence of actually overall increased lung volumes (TLC)
  • This is due to trapped air at the end of expiration, which must be measured using the indirect methods discussed below

Question 10

Spirometry:
Bronchodilator response

Answer 10

• Spirometry is often performed sequentially before and after a bronchodilator medication (usually a beta-agonist) in order to determine reversibility of airway obstruction (typical of asthma)
• Generally an increase in FEV1 or FVC of at least 12% and at least 200ml is considered a positive response to a bronchodilator

Question 11

Spirometry:
Bronchial challenge

• Measurements of pulmonary function in the laboratory in episodic diseases
• Challenges given to patients in the laboratory in order to induce spirometric abnormalities
• Positive challenge test

Answer 11

• Measurements of pulmonary function in the laboratory in episodic diseases
  
  • May not represent the function at the time of symptoms
• Challenges given to patients in the laboratory in order to induce spirometric abnormalities
  
  • Chemicals or particulates that are encountered in the work place
  • Cold air
  • Exercise
  • Methacholine (a histamine derivative) is the exposure of choice to induce bronchospasm in occult asthmatics (non-asthmatics do not respond)
• Positive challenge test
  
  • A drop in FEV1, FVC, or Peak expiratory flow (PEF) of greater than 20%

Question 12

Spirometry:
Flow-volume loops

• How the spirometric tracing can be represented
• The greatest advantages of the flow-volume loop over simple volume time representation

Answer 12

• How the spirometric tracing can be represented
  
  • A volume vs. time tracing (figs 5B and 6B)
  • A flow vs. volume loop (figs 5A and 6A)
  • These are not separate tests but two separate graphical representations of the same data, which offer a different perspective
• The greatest advantages of the flow-volume loop over simple volume time representation
  
  • The assessment of patient effort on repetitive testing
  • The presence of specific patterns for upper airway obstruction

Question 13

Spirometry:
Flow-volume loops:
Extrathoracic variable upper airway obstruction

• During inspiration
• During expiration

Answer 13

• During inspiration
  
  • Intraluminal airway pressure is negative relative to surrounding atmospheric pressure
  • Intraluminal obstruction will increase
• During expiration
  
  • Reverse is true
  • Resulting flow-volume loop demonstrates a plateau during inspiration

Question 14

Spirometry:
Flow-volume loops:
Intrathoracic variable upper airway obstruction

• During inspiration
• During expiration

Answer 14

• During inspiration
  
  • Intraluminal airway pressure is greater than pressure surrounding the airway (pleural pressure)
  • Leads to…
    
    • Expansion of the intraluminal diameter
    • A decrease in a variable intrathoracic obstruction
• During expiration
  
  • The reverse is true
  • The resulting flow-volume loop demonstrates a plateau during expiration

Question 15

Lung volume measurement:
Helium dilution

• Technique
• Amount of helium
• Change in total gas volume

Answer 15

• Closed-circuit technique
  
  • A spirometer is filled with a mixture of helium and oxygen
  • The amount of helium in the spirometer (helium concentration x volume of gas) is known at the beginning of the test
  • The patient is then allowed to breathe from the spirometer starting from end expiration
  • This point, also termed the functional residual capacity, is chosen as the starting point since it tends to be a very reproducible value
• Amount of helium
  
  • Because the breathing circuit is closed (no leaks), the amount of helium remains constant during the test
• Change in total gas volume
  
  • By measuring the change in spirometer helium concentration during the period when the patient is breathing the gas mixture, the change in total gas volume (which reflects the addition of the patient’s lung volume) can be determined

Question 16

Lung volume measurement:
Body plethysmography

• Body plethysmograph
• The test employs Boyle’s law
• Technique
• Measurements

Answer 16

• Body plethysmograph
  
  • Instrument used to calculate the residual volume
• The test employs Boyle’s law
  
  • The relationship between the pressure and volume of a gas at a given temperature remains constant (P1xV1 = P2xV2)
• Technique
  
  • The patient sits in a large, glass-enclosed box and breathes through a mouthpiece
  • During the test, an electronic shutter temporarily occludes the mouthpiece and the patient continues to “pant” against the closed shutter
  • During an inspiratory pant against the closed airway a negative pressure swing occurs at the alveolus, which can be measured at the mouth
  • The negative alveolar pressure swing without airflow results in gas expansion within the chest
• Measurements
  
  • Because the relationship of gas/pressure within the lungs and gas/pressure within the box must remain constant, the increase in chest volume is associated with a corresponding change in box pressure
  • Measurement of the change in pressure at the mouth and change in box pressure allows one to solve for lung volume at the time of airway closure

Question 17

Lung volume measurement:
Body plethysmography

• An individual with a small amount of air left in the lungs at end expiration
• An individual with a large amount of trapped air
• Plethysmography vs. helium dilution
• Patients with bullous emphysema
• Once FRC has been measured using plethysmography,…

Answer 17

• An individual with a small amount of air left in the lungs at end expiration
  
  • Small FRC and RV
  • Will have a high mouth pressure change for a given change in thoracic volume (reflected in box pressure)
• An individual with a large amount of trapped air
  
  • Large FRC and RV
  • Will have a small mouth pressure change for a similar change in thoracic volume or box pressure
• Plethysmography vs. helium dilution
  
  • Plethysmography measures the total volume within the chest (including non-communicating bullae)
  • Helium dilution measures only the gas which communicates with and equilibrates at the mouth
• Patients with bullous emphysema
  
  • Often have RV and FRC measurements underestimated using the Helium technique
• Once FRC has been measured using plethysmography,…
  
  • The remaining volumes and capacities can be measured or calculated

Question 18

Diffusing capacity

• The measurement of single breath carbon monoxide diffusing capacity
• Measurement gas used
• The limiting factor for diffusion
• The most common technique

Answer 18

• The measurement of single breath carbon monoxide diffusing capacity
  
  • Used to assess the efficiency of gas exchange at the pulmonary-capillary membrane
  • Influenced by the volume of blood in the capillaries and the capillary surface area
  • Represents clinically the amount of functioning capillary bed in contact with ventilated alveoli
• Measurement gas used
  
  • Very small quantities of CO
  • Used because of its high affinity for hemoglobin and high partial pressure gradient, which leads to rapid and continuous diffusion of the gas across the alveolar-capillary membrane
• The limiting factor for diffusion
  
  • The functional status of the alveolar-capillary membrane
• The most common technique
  
  • Single breath measurement

Question 19

Diffusing capacity:
Single breath measurement

• Pros of this method
• The test requires…
• Technique

Answer 19

• Pros of this method
  
  • Rapid, reproducible, less affected by an uneven distribution of ventilation, and is well standardized
• The test requires…
  
  • A full inspiration to TLC and a breath holding time of approximately 10 seconds
• Technique
  
  • The subject inspires a gas mixture, which contains low concentrations of CO, helium, and oxygen
  • Measurement of the initial (inspired) and final (exhaled) concentration of CO adjusted for gas dilution (using inert Helium) determines diffusing capacity
  • A value less than 80% predicted is considered abnormal

Question 20

Diffusing capacity

• Decreased diffusing capacity
  
  • Associated with…
  • Disorders
• Increased diffusing capacity
  
  • Disorders

Answer 20

• Decreased diffusing capacity
  
  • Associated with…
    
    • Loss of contact area between alveolar gas and pulmonary capillary blood
  • Disorders
    
    • Emphysema
    • Interstitial lung diseases
    • Pulmonary vascular diseases
• Increased diffusing capacity
  
  • Disorders
    
    • Polycythemia
    • Mild congestive heart failure
    • Asthma
    • Pulmonary hemorrhage