Purposes

Generally, the primary purposes of pulmonary function testing are to identify pulmonary impairment and to quantify the severity of pulmonary impairment if present.² Pulmonary function testing has diagnostic and therapeutic roles and helps clinicians answer some general questions about patients with lung disease (Box 19-1).

The indications for pulmonary function testing are as follows:

There are also contraindications to pulmonary function testing.⁴ Patients with acute, unstable cardiopulmonary problems, such as hemoptysis, pneumothorax, myocardial infarction, and pulmonary embolism, and patients with acute chest or abdominal pain should not be tested. Testing could be harmful if needed treatment would be delayed. Patients who have nausea and who are vomiting should not be tested because there is a risk of aspiration. Testing for patients who have had recent cataract removal surgery should be delayed because changes in ocular pressure may be harmful to the eye. Pulmonary function testing requires patient effort and cooperation. Patients with dementia or confusion may not achieve optimal or repeatable results. Pulmonary function testing should not be performed if valid and reliable results cannot be predicted. 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

Pulmonary function testing provides the basis for classifying pulmonary diseases into two major categories, obstructive pulmonary disease and restrictive pulmonary disease. These two types of lung diseases sometimes occur together as a mixed impairment. Obstructive and restrictive types of lung diseases differ in several important ways. Figure 19-1 shows normal lungs with the pathophysiologic aspects of obstructive lung diseases and restrictive lung diseases, and the differences are summarized in Table 19-1. The primary problem in obstructive pulmonary disease is an increased airway resistance (R_aw). R_aw is the difference in pressure between the ends of the airways divided by the flow rate of gas moving through the airway, according to the following formula: R_aw = ΔP/.

There is an inverse relationship between R_aw and flow rates (). If the pressure difference is constant, a reduced flow rate indicates an increase in R_aw. Because the radius of the airways normally lessens slightly during expiration, flow rates are usually measured during expiration. By rearranging the symbols in Poiseuille’s law (see Chapter 6), R_aw is inversely related to the radius of the airways according to the following formula: R_aw = ΔP/ = η8l/r⁴.

When airway radius (r) decreases, R_aw increases, while the flow rate of gas through the airways () decreases. Airway radius can be reduced by excessive contraction of the bronchial and bronchiolar muscles (bronchospasm), excessive secretions in the airways, swelling of the airway mucosa, airway tumors, collapse of the bronchioles, and other causes. By measuring flow rates, pulmonary function tests measure indirectly the size of the airways, R_aw, and the presence of obstructive disease.

The primary problem in restrictive lung disease is reduced lung compliance, thoracic compliance, or both. Compliance is the volume of gas inspired per the amount of inspiratory effort; effort is measured as the amount of pressure created in the lung or in the pleural space when the inspiratory muscles contract. Compliance is calculated according to the following formula: C = ΔV/ΔP.

There is a direct relationship between compliance (C) and volume (V). If the pressure difference is constant, a reduced inspiratory volume indicates a reduction in compliance. Reduced lung compliance is usually the result of alveolar inflammation, pulmonary fibrosis, or neoplasms in the alveoli; a reduced thoracic compliance may be the result of thoracic wall abnormalities such as kyphoscoliosis. Neuromuscular diseases also can result in reduced lung volumes and restrictive-type pulmonary impairments, mainly by affecting the function of the inspiratory muscles. In these circumstances, lung compliance and thoracic compliance may be normal, but the patient is unable to generate enough subatmospheric pressure to take a full, deep breath.

Some obstructive diseases and some restrictive diseases also may affect the ability of the lung to diffuse gases. In some diseases, there is damage to the alveolar-capillary membrane, or less alveolar surface area is accessible for diffusion. Measuring the diffusing capacity of the lung for carbon monoxide (DLCO) can identify the destruction of alveolar tissue or the loss of functioning alveolar surface area.

For each measurement of pulmonary function, there is a normal value and a lower limit of normal (LLN). Measurements less than the LLN indicate the presence of an abnormality. The severity of pulmonary impairment is based on a comparison of each patient’s measurement with the predicted normal value for the patient. Several methods are used for comparison with the normal value. A common method of comparison is to compute a percentage of the predicted normal value according to the following equation:

< ?xml:namespace prefix = "mml" />% Predicted=Measured valuePredicted normal value×100

Determining if the patient’s value is within 1 or 2 standard deviations of the predicted normal value is an alternative method used in some laboratories. The predicted percentage or the number of standard deviations from the predicted normal value can be used to quantify severity of impairment. Typical degrees of severity are listed in Table 19-2.

Infection Control

Pulmonary function testing is considered safe, but there is potential to transmit infective microorganisms to patients and technologists.⁸ Transmission can occur by direct or indirect contact. Standard precautions should be applied because of the potential exposure to saliva or mucus, which could possibly contain blood or other potentially hazardous microorganisms. Patients with oral lesions pose the greatest potential hazard, and patients with compromised immune systems are at the greatest risk. Practitioners should wear gloves when handling potentially contaminated mouthpieces, valves, tubing, and equipment surfaces. When performing procedures on patients with potentially infectious airborne diseases, practitioners should wear a personal respirator or a close-fitting surgical mask, especially if the testing induces coughing. Practitioners should always wash their hands between testing patients and after contact with testing equipment. Although it is unnecessary to clean the interior surfaces of the testing instruments routinely between patients,⁹ the mouthpiece, nose clips, tubing, and any parts of the instrument that come into direct contact with a patient should be disposed, sterilized, or disinfected between patients. Any equipment surface showing visible condensation from exhaled air should be discarded, disinfected, or sterilized before reuse. When testing instruments are disassembled for cleaning and disinfecting, manufacturer recommendations should be considered, and recalibration may be necessary before testing resumes. The routine use of low-resistance, in-line barrier filters is controversial.^10–12 Filters may be appropriate when internal surfaces of manifolds and valves proximal to mouthpieces are inaccessible or difficult to disassemble for cleaning and disinfecting. Filters provide visible evidence to reassure patients that their protection has been considered.

Equipment

Pulmonary function testing requires measurement of gas volume or flow, and various instruments and measurement principles are used to make these measurements. There are two general types of measuring instruments: instruments that measure gas volume and instruments that measure gas flow. Both types of instruments simultaneously measure time, and both compute various volumes and flow rates used in pulmonary function testing. The term spirometer is sometimes used as a generic term for all volume-measuring and flow-measuring devices.

Volume-measuring devices are specifically called spirometers and include water-sealed, bellows, and dry rolling seal types. These devices expand as they collect gas volumes. The magnitude of the expansion is the volume measured, and the speed of expansion represents the flow rate. In the absence of leaks and with low momentum forces, volume-measuring devices can be extremely accurate for measuring volumes, and with low inertia and friction forces, volume-measuring devices can be extremely accurate when computing flow rates.

Flow-measuring devices are commonly called pneumotachometers, although some practitioners reserve this term for only the device originally designed by Fleisch. These devices measure flow using a variety of unique principles. The Fleisch-type pneumotachometer measures the change in pressure as gas flows through a minimal, constant resistance according to the formula: = ΔP ÷ R. Different manufacturers have used several materials to provide the resistance, including screens, capillary tubes, and fiber sheets made of silk, nylon, or filter paper. With multiple uses, condensation from exhaled air can collect in these devices and alter the resistance and accuracy of the device; some devices are heated to body temperature to prevent condensation. Known as thermistors or mass flowmeters, another type of flow-measuring device measures the temperature change created by gas flowing through it. There are also tubinometers, which use rotation of a fan or blades similar to a windmill. The number of rotations indicates volume, and the speed of the rotations indicates flow. How gas flow affects the transmission of sound waves and the force of flow stretching a spring have also been used to measure flow. Detailed descriptions and examples of each type of device are beyond the scope of this chapter and are available elsewhere.¹³

Regardless of the type of device or the principle of measurement used, several important characteristics are common to all volume-measuring and flow-measuring devices. Having an understanding of these common characteristics provides RTs the ability to select and use these devices properly. Every measuring instrument has capacity, accuracy, error, resolution, precision, linearity, and output.14,15 The ideal instrument would have unlimited capacity to measure every pulmonary parameter, and it would have perfect accuracy and precision over its entire measurement range; there are no ideal instruments.

The capacity of an instrument refers to the range or limits of how much it can measure. Most instruments are designed with capacities to measure volumes and flow rates of all adults. The accuracy of a measuring instrument is how well it measures a known reference value. For volume measurements, standard reference values are provided by a graduated 3.0-L calibration syringe.¹⁶ No measuring instrument is perfect, and there usually is an arithmetic difference between reference values and measured values. This difference is called the error. Accuracy and error are opposing terms; the greater the accuracy, the smaller is the error. Accuracy and error are commonly expressed as percentages, with their sum always equaling 100%. To determine percent accuracy and percent error, several reference values are measured, and the mean of the measured values is computed and compared with the reference values according to the following equations:

% Accuracy=Mean measured valueReference value×100

or

% Error=Mean measured value−Reference valueReference value×100

Resolution is the smallest detectable measurement; instruments with high resolution can measure the smallest volumes, flows, and times. Precision is synonymous with reliability of measurements and the opposite of variability. When multiple known reference values are measured, the standard deviation of the mean measured reference value is the statistic that indicates the precision of an instrument. A small standard deviation indicates low variability and high precision. Linearity refers to the accuracy of the instrument over its entire range of measurement, or its capacity. Some devices may accurately measure large volumes or high flow rates but may be less accurate when measuring small volumes or low flow rates. To determine linearity, accuracy and precision are calculated at different points over the range (capacity) of the device.

Output includes the specific measurements made or computed by the instrument. Most volume-measuring and flow-measuring devices measure the FVC and forced expiratory volume in 1 second (FEV₁). Others calculate various forced expiratory flow (FEF) rates, and some measure tidal volume (V_T) and minute ventilation (). Diagnostic spirometers usually measure and calculate vital capacity (VC), FVC, FEV₁, peak expiratory flow (PEF) rate, and FEF rates. Some measure and calculate maximal voluntary ventilation (MVV). Some of these instruments may be a component of a laboratory system providing the volume-measuring or flow-measuring capability for other diagnostic tests of pulmonary function. For example, they may be used with gas analyzers to measure functional residual capacity (FRC) and total lung capacity (TLC) or the inspiratory VC during single-breath diffusing capacity (DLCOSB). Whether a spirometer or pneumotachometer is used in a diagnostic laboratory, a physician’s office, or at the bedside in an intensive care unit, it should meet or exceed the national performance standards for volume-measuring and flow-measuring devices.

In 1978, the American Thoracic Society (ATS) adopted the initial standards for diagnostic spirometers. These standards have been adopted by other medical organizations and government agencies. Updated most recently in 2005 in collaboration with the European Respiratory Society (ERS), the standards are now recognized internationally as the standards for the industry.¹⁷ Some instruments have been independently evaluated against the standards or compared with instruments that meet those standards. Regardless of the measuring principle used by the instrument or the purpose of the patient testing, RTs should use only devices that meet or exceed current ATS/ERS performance standards. According to the ATS/ERS standards, when measuring a slow VC, the spirometer should be able to measure for up to 30 seconds, and for the FVC, the time capacity should be at least 15 seconds. When measuring the VC, FVC, and forced expiratory volumes, a volume-measuring spirometer should have a capacity of at least 8 L and should measure volumes with less than a 3% error or within 50 ml of a reference value, whichever is greater. These standards, including the 8-L standard for capacity, also apply to children. A diagnostic spirometer that measures flow should be at least 95% accurate (or within 0.2 L/sec, whichever is greater) over the entire 0 to 14 L/sec range of gas flow. The standards are summarized in Table 19-3.

The spirometer standards also require spirometers to have a thermometer or to produce values corrected for body temperature, ambient pressure, and fully saturated with water vapor (BTPS). Standards also require that graphic outputs be of sufficient size and scale of display and recording to allow for visual inspection during testing, validation, and hand measurements. For visual display on a computer monitor, the resolution required is 0.050 L, and the scale of the volume axis must be 5 mm/L; the scale for the time axis must be at least 10 mm/sec. For validation and hand measurement functions from graph paper, the resolution must be 0.025 L, and the scale of the volume axis must be 10 mm/L; the scale for the time axis must be at least 20 mm/sec. Most manufacturers have designed their spirometers to meet or exceed the validation and hand measurement standards.

For quality control, the standards include verifying volume accuracy with a 3.0-L calibration syringe at least daily, although best practice in many laboratories is to verify accuracy before each test subject. The 2005 standards recognize that 3.0-L calibration syringes may have up to 0.5% error, and error may be acceptable if in the ±3.5% range. Volume linearity should be verified quarterly using 1.0-L increments over the entire volume range; flow linearity should be checked weekly using at least three different flow ranges. Recorder speed should be checked with a stopwatch quarterly. When new versions of software are installed, testing known subjects and comparing results is recommended. For comprehensive quality assurance of pulmonary function testing, there are three equally important aspects to consider: verifying the accuracy and precision of the measuring instruments, the performance of the technologist, and the test results when measuring a standard.

Most modern pulmonary function laboratories use computers for data acquisition and reduction. Computer-assisted testing decreases the time necessary to complete the tests and enhances the effectiveness of pulmonary function testing by increasing accuracy, increasing patient acceptance, and monitoring patient performance. Although computer-assisted testing and interpretations of test results are often applied by a computer, pulmonary function testing always requires a trained and competent RT to administer the tests, and computer analysis should not replace human analysis.

Spirometry

Spirometry includes the tests of pulmonary mechanics—the measurements of FVC, FEV₁, several FEF values, forced inspiratory flow rates, and MVV. Measuring pulmonary mechanics is assessing the ability of the lungs to move large volumes of air quickly through the airways to identify airway obstruction. Some measurements are aimed at large intrathoracic airways, some are aimed at small airways, and some assess obstruction throughout the lungs. Measuring flow rates is a surrogate for measuring airways resistance according to the formula: Raw = ΔP ÷ . A decrease in flow rate signifies an increase in airways resistance and the presence of airway obstruction when patient effort creating the difference between mouth pressure and lung pressure is constant (see Clinical Practice Guideline 19-1).¹⁷

Although performing tests of pulmonary mechanics is considered safe, some adverse reactions have occurred, including pneumothorax,¹⁸ syncope, chest pain, paroxysmal coughing, and bronchospasm associated with exercise-induced asthma.¹⁹ The contraindications for pulmonary function testing are primarily for testing mechanics. The ATS/ERS 2005 standards for spirometry¹⁷ specify the validity and reliability criteria of the measurements and accuracy and precision limits of the measuring equipment. These standards have been incorporated into the clinical practice guidelines of the AARC, medical societies, and government agencies.^4,20–22

Forced Vital Capacity

FVC is the most commonly performed test of pulmonary mechanics, and many measurements are made while the patient is performing the FVC maneuver (Figure 19-2). Measuring FVC often occurs under baseline or untreated conditions. For baseline testing, patients should temporarily abstain from bronchodilator medications. Short-acting bronchodilators (e.g., β-agonist albuterol, anticholinergic agent ipratropium bromide) should not be used for 4 hours before baseline spirometry, whereas long-acting β-agonist bronchodilators and oral therapy with aminophylline should be stopped for 12 hours. When a patient’s baseline results show airway obstruction, performing FVC after treatment (e.g., albuterol bronchodilator aerosol or metered dose inhaler) can help determine if the treatment is effective. The FVC maneuver is also performed repeatedly during bronchial provocation testing.

FVC may be measured on a spirometer that measures volumes or flows, that presents a graph of volume and time or flow and volume, that is mechanical or electronic, and that has a calculator or computer. The forced expiratory VC sometimes is followed by a forced inspiratory VC to produce a complete image of forced breathing called a flow-volume loop.²³

FVC is an effort-dependent maneuver that requires careful patient instruction, understanding, coordination, and cooperation. Spirometry standards for FVC specify that patients must be instructed in the FVC maneuver, that the appropriate technique be demonstrated, and that enthusiastic coaching occur. When measuring FVC, the RT needs to coach the preceding inspiratory capacity (IC) as enthusiastically as the FVC. According to the standards, nose clips are encouraged, but not required, and patients may be tested in the sitting or standing position. Although standing usually produces a larger FVC compared with sitting, sitting is considered safer in case of lightheadedness. It is recommended that the position be consistent for repeat testing of the same patient. FVC should be converted to body temperature conditions and reported as liters under BTPS conditions.

 Rule of Thumb

If you don’t inhale it, you can’t exhale it. So, coach the preceding IC as enthusiastically as the FVC.

To ensure validity, each patient must perform a minimum of three acceptable FVC maneuvers. To ensure reliability, the largest FVC and second largest FVC from the acceptable trials should not vary by more than 0.150 L. To perform an FVC trial, the patient should inhale rapidly and completely to TLC from the resting FRC level. The forced exhalation of an acceptable FVC trial should begin abruptly and without hesitation. A satisfactory start of expiration is defined as an extrapolated volume at the zero time point less than 5% of FVC or 0.150 L, whichever is greater (Figure 19-3). The volume exhaled before the zero time point is called the extrapolated volume. To be valid, no more than 5% of the VC or 0.150 L is allowed to be exhaled before the zero time point. An acceptable FVC trial also is smooth, continuous, and complete. A cough, an inspiration, a Valsalva maneuver, a leak, or an obstructed mouthpiece while an FVC maneuver is being performed disqualifies the trial. FVC must be completely exhaled or an exhalation time of at least 6 seconds must occur for adults and children older than 10 years (longer times are commonly needed for patients with airway obstruction). A 3-second exhalation is acceptable for children younger than 10 years old. An end expiratory plateau must be obvious in the volume-time curve; the objective standard is less than 0.025 L exhaled during the final second of exhalation. Consistent with its definition, the largest acceptable FVC (BTPS) measured from the set of three acceptable trials is the patient’s FVC.

Forced Expiratory Flow Between 200 ml and 1200 ml of Forced Vital Capacity and Forced Expiratory Flow Between 25% and 75% of Forced Vital Capacity

FEF_200-1200 and FEF_25%-75% represent average flow rates that occur during specific intervals of FVC. Both measurements can be made on a volume-time spirogram as the slope of a line connecting the two points in their subscripts. For FEF_200-1200, the 200-ml point and the 1200-ml point are identified. A straight line is drawn connecting these points, and the line is extended to intersect two vertical time lines 1 second apart on the graph (Figure 19-4). The volume of air measured between the two time lines is FEF_200-1200 in liters per second. The volume measured must be corrected to BTPS.

FEF_25%-75% is a measure of the flow during the middle portion of FVC, or the time necessary to exhale the middle 50%. For FEF_25%-75%, the VC of the best curve is multiplied by 25% and 75%, and the points are identified on the tracing. A straight line is drawn connecting these points, and the line is extended to intersect two vertical time lines 1 second apart on the graph. The volume of air measured between the two time lines is FEF_25%-75% in liters per second. The volume measured must be corrected to BTPS (Figure 19-5).

Maximal Voluntary Ventilation

Another measurement of pulmonary mechanics is MVV. MVV is another effort-dependent test for which the patient is asked to breathe as deeply and as rapidly as possible for at least 12 seconds. MVV is a test that reflects patient cooperation and effort, the ability of the diaphragm and thoracic muscles to expand the thorax and lungs, and airway patency. Because of the potential for acute hyperventilation and fainting or coughing, the patient should be seated. Measuring systems that incorporate rebreathing may minimize hyperventilation. After a demonstration of the expected breathing pattern is performed, the patient should be instructed to breathe as rapidly and as deeply as possible for at least 12 seconds. The patient’s breathing is measured on a spirogram (Figure 19-6) or electronically for the specific number of seconds (t) and the volume (V) breathed when the MVV is converted to liters per minute. As with all volumes measured on a spirometer, the recorded values should be in BTPS conditions. The validity of MVV depends on the duration of the maneuver, which should be at least 12 seconds; the breathing frequency, which should be at least 90/min; and the average volume, which should be at least 50% of FVC. Patients should perform at least two MVV trials when the first trial does not exceed 80% of the subject’s FEV₁ × 40, which may indicate less than maximal effort, or 80% of the predicted normal value, which may indicate disease. Reliability is shown when there is less than 20% variability between the two largest trials. The largest MVV (BTPS) should be reported.

Mini Clini

Decreased Forced Vital Capacity and Forced Expiratory Volume in 1 Second: Is It Obstruction or Restriction?

 Problem

Both obstructive and restrictive diseases may exhibit decreased FVC and FEV₁. How can the two kinds of patterns be differentiated?

Solution

FVC and FEV₁ are reduced in both obstructive and restrictive diseases for different reasons. With restrictive disease, lung expansion is reduced. If a person can inhale only a small volume, he or she can exhale only a small volume. All lung volumes are smaller than normal, including TLC, FVC, and FEV₁.

Buy Membership for Pulmolory and Respiratory Category to continue reading. Learn more here