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Pulmonary Function Testing Notes

Pulmonary Function Testing

Learning Objectives

  • List the three categories of pulmonary PFTs.
  • State the primary purposes of pulmonary function testing.
  • Compare and contrast the four general principles that should be considered for PFTs.
  • Describe different physiologic patterns seen on PFTs.
  • Discuss general principles of infection control in the PFT laboratory.
  • Evaluate various devices used for measurement of pulmonary function and discuss differences and similarities among them.
  • Discuss the grading system for PFT quality.
  • Describe how reference values are obtained.
  • Evaluate various tests based on their characteristics such as sensitivity and specificity, validity, and reliability.
  • Define individual tests of pulmonary function and explain how they are obtained.
  • Describe bronchodilator responsiveness and discuss its importance.
  • Describe the purpose and technique for the bronchoprovocation test.
  • Describe the purpose and techniques used to measure diffusing capacity.
  • Analyze pulmonary function reports.

Introduction

  • The most important function of the lungs is gas exchange.
  • The ability of the lungs to perform gas exchange depends on the following four general physiologic functions:
    • The diaphragm and thoracic muscles expand the thorax and lungs.
    • The airway size (radius) is suitable to allow gas to flow into the lungs and reach the alveoli.
    • O2 and CO2 diffuse through the alveolar-capillary membrane.
    • The cardiovascular system circulates blood through the lungs and ventilated alveoli.

Pulmonary Function Testing (PFT)

  • Provide valuable information about important individual processes that support gas exchange.
  • Three categories of PFT:
    • Measuring dynamic flow rates of gases through the airways.
    • Lung volumes and capacities.
    • The ability of the lungs to diffuse gases.

Purposes of PFT

  • To identify and quantify changes in pulmonary function
  • To evaluate need and quantify therapeutic effectiveness
  • To perform epidemiologic surveillance for pulmonary disease
  • To assess patients for risk of postoperative complications
  • To determine pulmonary disability

Contraindication to PFT

  • Patients with acute, unstable cardiopulmonary problems.
  • Patients who have nausea and who are vomiting.
  • Testing for patients who have had recent cataract removal surgery should be delayed.
  • Patients with dementia or confusion may not achieve optimal or repeatable results.
  • In patients who are acutely ill or who have recently smoked a cigarette, the test validity of measuring the forced vital capacity (FVC) may be hindered.

Pathophysiologic Patterns

  • Two major categories of pulmonary disease exist:
    • Obstructive
    • Restrictive
  • Primary abnormality in obstructive disease:
    • Increased airways resistance
  • Primary problem in restrictive disease:
    • Reduced lung compliance
    • Reduced lung volumes
  • Some pulmonary diseases cause both obstructive and restrictive disease.

Infection Control

  • Generally regarded as a very low-risk procedure
  • Potential exists to transmit infective microorganisms to patients and technologists
  • Direct or indirect contact
  • Standard precautions should be applied
  • Potential exposure to saliva, mucus, or blood
  • When testing instruments are disassembled for cleaning and disinfecting:
    • Manufacturer recommendations should be considered and recalibration may be necessary

Equipment

  • Two general types of measuring devices exist:
    • Measure volume
    • Measure flow
  • Volume-measuring devices—spirometers
  • Flow-measuring devices—pneumotachometers, Turbine Meters
  • Every measuring device has capacity, accuracy, error, resolution, precision, linearity, and output.

Equipment Terms:

  • Capacity: The range or limits of how much it can measure.
  • Accuracy: How well it measures a known reference value.
  • Error: An arithmetic difference between reference values and measured values.
    • Accuracy and error are opposing terms.
    • The greater the accuracy, the smaller is the error.
  • Resolution: The smallest detectable measurement.
  • Precision: Synonymous with reliability (repeatability) of measurements and the opposite of variability
  • Linearity: The accuracy of the instrument over its entire range of measurement.
  • Output: Specific measurements made or computed by the instrument.

Grading of Quality

  • Most modern pulmonary function laboratories use computers for data acquisition and reproduction
  • PFTs always requires a trained and competent RT to administer the tests, and computer analysis should not replace human analysis

Reference Values and Interpretation of Results

  • Based for population studies
  • The "normal" values for the PFT measurements are based on:
    • Height, age, gender, and race/ethnicity
  • ATS recommends routine reporting of the following reference values:
    • Predicted value, upper and lower limit of normal (ULN, LLN), and z-score
  • % Predicted =
    ewline {Measured value
    ewline Predicted normal value} ×100

Principles of Measurement and Significance

  • For tests of pulmonary function, three general principles should be considered:
    • Test sensitivity and specificity
      • Address the test’s ability to detect disease, or absence of it
    • Validity
      • Relates to its meaningfulness, or the ability to measure what it is intended to measure
    • Reliability
      • Consistency

Individual Tests and Measurements

  • There are three basic tests of pulmonary function:
    • Spirometry
    • Lung volumes
    • Diffusing capacity

Spirometry

  • Tests of pulmonary mechanics
    • Forced vital capacity (FVC)
    • Forced expiratory volume in 1 second (FEV1)
    • Other forced expiratory flow measurements
    • Maximum voluntary ventilation
  • These measurements assess the ability of the lungs to move large volumes of air quickly through airways.

Forced Vital Capacity

  • Most common test of pulmonary mechanics
  • Many measurements are made while patient is performing FVC maneuver
  • FVC is an effort-dependent maneuver requiring careful patient instruction and cooperation
  • To ensure validity, each patient must perform at least three acceptable FVC maneuvers

Other Measures of Pulmonary Mechanics

  • FEV1—volume of gas exhaled in first 1- second of FVC maneuver
  • FEV1/FVC—calculated by dividing largest FEV1 by largest FVC
  • FEF200-1200—average flow rate early in FVC maneuver
  • FEF25-75—measure of flow during middle 50% of FVC
  • PEFR—highest point on flow-volume graph

Maximal Voluntary Ventilation (MVV)

  • Effort-dependent test; patient asked to breathe deep and fast for 12 seconds
  • Results reflect:
    • Patient effort
    • Function of respiratory muscles
    • Ability of chest wall to expand
    • Patency of airways

Significance of Results

  • Normal FEV1 = 5.6 L for average 20-year-old man
  • FEV1 is reduced with both obstructive and restrictive lung disease.
  • FEV1/FVC
    • Reduced with obstructive disease
    • Normal with restrictive disease
  • Other measures of expiratory flow are also reduced when obstructive disease is present
  • Normal MVV for males is 160 to 180 L/min and slightly lower in females
  • A measured value less than 75% of predicted is significant
  • MVV is reduced in patients with moderate to severe obstructive lung disease
  • MVV may be normal or slightly reduced in patients with restrictive disease
  • Undernourished patients may have reduced MVV

Reversibility of Airway Obstruction

  • If obstruction is present, reversibility must be evaluated
  • Done by performing spirometry before and after therapy
  • Bronchodilator is administered by small- volume nebulizer or MDI
  • Reversibility indicates effective therapy
  • Reversibility is defined as 12% or greater improvement in FEV1 or FVC and at least 200 -ml increase in FEV1

Bronchoprovocation

  • Indicated when the patient’s history suggests episodic symptoms of hyperreactive airways and airway obstruction
  • Uses an agent to stimulate a hyperreactive airway response and to create airway obstruction
  • Usually begins with saline and then repeat the FVC maneuver
  • A positive response to saline is defined as a decrease in FEV1 of 10% or greater
  • Methacholine provocation protocol systematically exposes the patient to increasing doses of methacholine
  • Usually starting with a low dose of 0.03 mg/ml, patients inhale the methacholine aerosol and then repeat the FVC maneuver
  • A positive response to methacholine is defined as a decrease in FEV1 of 20% or greater

Lung Volumes and Capacities

  • Lung Volumes
    • Tidal volume
    • Inspiratory reserve volume
    • Expiratory reserve volume
    • Residual volume
  • Lung Capacities
    • Total lung capacity
    • Inspiratory capacity
    • Functional residual capacity
    • Vital capacity
  • Values measured during spirometry
    • VT
    • IC
    • ERV
    • VC
    • Most commonly measured
  • Values not measured during spirometry
    • RV
    • FRC
    • TLC

Techniques for Measuring RV

  • Helium dilution
    • Based on fact that known amount of helium will be diluted by size of patient’s RV
  • Nitrogen washout
    • Based on fact that 78% of RV is nitrogen
  • Plethysmography
    • Applies Boyle’s law to measure RV

Significance of Results

  • TLC, FRC, and RV increase with obstructive disease and decrease with restrictive impairment
  • Normal tidal volume is 500 to 700 ml; VT measurement alone not helpful
  • Normal IRV is about 3.1L
  • Normal ERV is about 1.2 L
  • Normal VC is about 4.8 L in adult; results vary with age, gender, height, and ethnicity
  • Normal TLC is about 6 L
  • Normal RV is about 1.2 L
  • Normal FRC is about 2.4 L
  • RV and FRC are usually enlarged in acute and chronic obstructive lung diseases
  • Because of hyperinflation and air trapping TLC also may be increased in COPD
  • TLC is always reduced in restrictive lung diseases because of a loss of lung volume; RV and FRC are often reduced proportionately

Diffusing Capacity

  • Most PF labs use carbon monoxide to measure the diffusion capacity of the lungs
  • Results reported in ml/min/mm Hg
  • Results may be low in both obstructive and restrictive lung disease
  • Emphysema and pulmonary fibrosis are two common causes of reduced DLCO

Single-Breath Technique

  • Most common measurement technique for diffusing capacity of the lung for CO
  • It is quick and reproducible
  • The reliability of the DLCO is based on repeatability of the test
  • At least 4 minutes should be allowed between tests to allow an adequate elimination of CO from the lungs
  • Patients with obstructive airway disease, a longer period (e.g., 10 minutes) may be necessary

Interpreting the DLCO

  • Factors that Decrease DLCO
    • Anemia
    • Carboxyhemoglobin
    • Pulmonary Embolism
    • Diffuse pulmonary fibrosis
    • Pulmonary emphysema
  • Factors that Increase DLCO
    • Polycythemia
    • Exercise
    • Congestive heart failure

PFT Report Interpretation

  • FEV1/FVC ratio is good place to start; reduced (less than 70%) with obstructive lung disease
  • If TLC less than LLN of predicted normal and FEV1/FVC is normal—restrictive disease is present
  • If DLCO is below LLN—diffusion defect is present
    • Reduced surface area = emphysema
    • Thickened AC membrane = pulmonary fibrosis

PFT Results Cases

  • Case 1: 32-year-old 53-kg woman admitted for elective surgery

    ACTUALPRED%PREDACTUALPRED%PRED
    TLC4.935.27FVC3.673.86
    FRC2.412.43%FEV184%78%
    RV1.291.35FEF200–12005.665.74
    VC3.643.86FEF25%–75%3.533.49
    • NORMAL PULMONARY FUNCTION
    • When considering a pulmonary function report, the %FEV1/VC ratio is a good place to start, because it provides an initial focus as normal, restrictive, or obstructive impairment.
    • When the %FEV1/FVC is less than the limit of normal (LLN), there is airway obstruction.
    • When the %FEV1/FVC is greater than the LLN, there is no airway obstruction.
    • The LLN %FEV1/FVC can be determined directly for various population using regression equations in Table 19-9 or simply estimated at 70%.
    • If the %FEV1/FVC ratio is greater than the LLN or 70% and if the TLC is less than the LLN, often defined as less than 80% predicted normal, the patient has a restrictive impairment, according to this algorithm.
    • The severity of the restriction is based on the percent predicted or on the number of standard deviations below the LLN TLC according to Table 19-2.
    • If the %FEV1/FVC ratio is less than 70%, the patient likely has an obstructive impairment; the severity of the obstruction is based on the percent predicted normal FEV1 according to Table 19-2.
    • If the percent predicted normal DLCO is less than 80%, the patient has a diffusion impairment.
    • Some laboratories also report the DLCO/VA ratio, which indexes the DLCO for lung volume measured during the single breath test.
    • If the DLCO/VA ratio is also less the 80% of the indexed value, the cause of the diffusion impairment is considered within the lung, and if the DLCO/VA ratio is greater than 80% of the indexed value, the cause of the diffusion impairment is considered due to small lung volume.

  • Case 2:

    ACTUALPRED% PREDACTUALPRED% PRED
    TLC4.347.73FVC2.864.74
    FRC1.734.36%FEV196%83%
    RV1.452.63FEF200-12006.896.71
    VC2.894.74FEF25%-75%2.782.88
    • Indicates restrictive disorder

  • Case 3:

    ACTUALPREDACTUALPRED
    TLC4.754.90FVC2.963.63
    FRC2.312.21%FEV182%80%
    RV1.281.20FEF200-12004.335.45
    VC3.483.63FEF25%-75%1.953.37
    • Results indicate small airway obstruction.

Interpreting ABG

  • ABG # 1
    • pH 7.18
    • PCO2 50
    • PaO2 80
    • HCO3-18
  • First ABG Combined metabolic and respiratory acidosis with normal oxygenation or no hypoxemia
  • ABG 2
    • pH 7.48
    • PCO2 60
    • PaO2 39
    • HCO3-28
  • 2 ABG Partially compensated metabolic alkalosis with severe hypoxemia
  • With calculations, co2 should be around 49.5- 53.5, CO2 is higher, so this is a combined respiratory acidosis and metabolic alkalosis

How to correct ABG

  • ABG 1 Treat the metabolic acidosis issue while increasing minute ventilation to reduce the CO2
  • ABG 2 Treat the metabolic alkalosis, maintain patient’s ventilation to try and normalize the CO2