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

  	ACTUAL	PRED	%PRED		ACTUAL	PRED	%PRED
  TLC	4.93	5.27		FVC	3.67	3.86
  FRC	2.41	2.43		%FEV1	84%	78%
  RV	1.29	1.35		FEF200–1200	5.66	5.74
  VC	3.64	3.86		FEF25%–75%	3.53	3.49

  • 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:

  	ACTUAL	PRED	% PRED		ACTUAL	PRED	% PRED
  TLC	4.34	7.73		FVC	2.86	4.74
  FRC	1.73	4.36		%FEV1	96%	83%
  RV	1.45	2.63		FEF200-1200	6.89	6.71
  VC	2.89	4.74		FEF25%-75%	2.78	2.88

  • Indicates restrictive disorder

• Case 3:

  	ACTUAL	PRED			ACTUAL	PRED
  TLC	4.75	4.90		FVC	2.96	3.63
  FRC	2.31	2.21		%FEV1	82%	80%
  RV	1.28	1.20		FEF200-1200	4.33	5.45
  VC	3.48	3.63		FEF25%-75%	1.95	3.37

  • 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