20. In combination with spirometry and lung volume values, apply Dlco measurements to differentiate among blood/pulmonary vascular disorders and potential causes of restrictive and obstructive ventilatory impairments.
26. Outline the basic steps in the 6-Minute Walk Test, specify the cut-off points associated with abnormal functional capacity, and stipulate the percentage change needed to indicate improvement due to medical or surgical interventions.
Pulmonary function testing commonly includes spirometry, static lung volume measurements, and diffusing capacity studies. Pulmonary function testing may also involve measurement of the mechanical properties of the lungs and thorax, such as airway resistance and compliance. Some pulmonary function laboratories also may assess airway hyperresponsiveness using bronchoprovocation testing or exhaled nitric oxide measurements and perform exercise testing.
Because spirometry is commonly performed by respiratory therapists (RTs) at the bedside, this chapter emphasizes the measurement and interpretation of parameters obtained using this assessment method. However, because full comprehension of the role of pulmonary function testing in diagnosis requires some knowledge of the more advanced tests, they will be discussed in brief. Also covered are basic equipment needs, quality assurance procedures, and infection control considerations. This chapter ends with a selection of case studies designed to assist the learner in interpreting pulmonary function data.
Fundamental to understanding pulmonary function testing is knowledge of the lung’s volumes and capacities. Figure 9-1 portrays the four lung volumes and four lung capacities both as a graphic plot of inspired and expired volumes over time (called a spirogram) and as a block diagram. Each measure includes its standard abbreviation, with the spirogram also providing average values for the lung volumes and capacities for a healthy young 70-kg adult male. Reference ranges for these measures vary primarily with the height of the individual but are also affected by gender, age, and race.
As indicated in Figure 9-1, the four lung volumes (inspiratory reserve volume [IRV], tidal volume [VT], expiratory reserve volume [ERV], and residual volume [RV]) are separate from each other and do not overlap. On the other hand, a lung capacity consists of two or more lung volumes. For example, the total lung capacity (TLC) represents the sum of all four lung volumes. As such, lung capacities do overlap with each other. Note that the residual volume (RV) and the capacities that include it (functional residual capacity and total lung capacity) are not recorded on the spirogram plot in Figure 9-1. This is because the residual volume remains in the lungs at all times.
Table 9-1 provides functional definitions for each of these volume and capacities.
|Tidal volume||VT||Volume of air inhaled or exhaled during each normal breath|
|Inspiratory reserve volume||IRV||Maximal volume of air that can be inhaled over and above the inspired tidal volume|
|Expiratory reserve volume||ERV||Maximal volume of air that can be exhaled after exhaling a normal tidal breath|
|Residual volume||RV||Volume of air remaining in the lungs after a maximal exhalation|
|Total lung capacity||TLC||Maximal volume of air in the lungs at the end of a maximal inhalation (sum of RV + VT + ERV + RV)|
|Functional residual capacity||FRC||Volume of air present in the lung at end-expiration during tidal breathing (sum of RV + ERV)|
|Inspiratory capacity||IC||Maximal volume of air that can be inhaled from the resting end-expiratory level (sum of IRV + VT)|
|Vital capacity||VC||Maximal volume of air that can be exhaled after a maximal inhalation (sum of IC + VT + ERV)|
Minute volume is commonly measured at the bedside using a simple mechanical or electronic respirometer. For a typical adult breathing at a frequency of 12 breaths/minute with an average tidal volume of 500 mL, the minute volume would be computed as:
Normal minute volumes range between 5 and 10 L/minute, depending primarily on the size of the subject. The minute volume increases with fever, pain, hypoxia, acidosis, and increased metabolic rate. It can rise to 60 L/minute or more during strenuous exercise.
Also commonly measured at the bedside is the vital capacity. Two different procedures are used, resulting in different measures. If the patient gently but fully exhales from a maximal inspiration, the resulting measure is called a slow vital capacity (SVC). If the patient forcefully empties the lungs from a maximal inspiration, the measure is called the forced vital capacity (FVC).
Typically, the SVC is measured using a respirometer. The SVC measurement can help in assessing perioperative risk because it reflects the patient’s ability to take a deep breath, cough, and clear the airways of excess secretions. In general, SVC values lower than 20 mL/kg of predicted body weight indicate an increased risk for postoperative respiratory complications. The SVC also has been used as one of many measures to evaluate a patient’s need for mechanical ventilation or readiness to wean. The most commonly cited threshold is 15 mL/kg, with lower values indicating inadequate ventilatory reserve and the need for mechanical ventilatory support.
Measurement of the FVC requires more sophisticated instrumentation capable of nearly instantaneous measurement of airflow. The measurement of FVC and its components is the basis for clinical spirometry.
Spirometry is the most commonly performed pulmonary function test. Spirometry testing can be conducted in the pulmonary function laboratory, at a patient’s bedside, or in an outpatient clinic or doctor’s office. Spirometry primarily involves measurement of parameters during patient performance of an FVC maneuver. Spirometry may also include measurement of a patient’s maximal voluntary ventilation (MVV).
Relative contraindications for performing spirometry include unstable cardiovascular status, hemoptysis, pneumothorax, and any acute condition that might hinder test performance, such as nausea or vomiting.
|Forced vital capacity||FVC||Total volume of air that can be exhaled during a maximal forced expiration effort|
|Forced expiratory volume in 1 second||FEV1||Volume of air exhaled in the first second after a maximal forced inhalation|
|Ratio of FEV1 to FVC||FEV1/FVC||Proportion or percentage of the FVC expired during the first second of the maneuver|
|Forced expiratory volume in 3 seconds||FEV3||Volume of air exhaled in 3 seconds after a forced maximal inhalation|
|Forced expiratory volume in 6 seconds||FEV6||Volume of air exhaled in 6 seconds after a forced maximal inhalation|
|Peak expiratory flow||PEF||Maximal expiratory flow, typically achieved within 120 msec of the start of the forced exhalation|
|Maximal midexpiratory flow||FEF25-75%||Average flow occurring between 25% and 75% of the FVC|
Figure 9-2 illustrates these parameters on a typical volume-versus-time spirogram. Note that the forced expiratory volume in 1, 3, and 6 seconds (FEV1, FEV3, and FEV6) are all volumes measured at specific times during the forced exhalation. Note also that a normal individual is able to exhale more than 75% of the FVC in 1 second and generally 95% or more in 3 seconds. However, the minimal time needed to assure a valid FVC measurement is 6 seconds, with some patients requiring more than 10 seconds to fully empty their lungs.
Unlike the volume measures, the peak expiratory flow (PEF) and forced expiratory flow (FEF25-75%) are represented as sloped lines on the graph. The slope of any line on a graph of volume versus time is flow. The line representing PEF corresponds to the steepest slope of the FVC curve and thus the highest generated flow. The FEF25-75% line is drawn by connecting the two points corresponding to the 25th and 75th percentiles of the FVC. In this example, the FVC is 4 L, so the 25th percentile is reached at 1 L and the 75th percentile occurs at 3 L. Also in this example, as always should be the case, the slope of the FEF25-75% line is less than that plotted for the PEF, signifying what is a progressive decrease in flow throughout the maneuver after the initial PEF blast.
In general, the volume measures obtained by spirometry, including the FEV1/FVC ratio, are more reproducible than the flow measures. In particular, PEF is very effort dependent. For this reason, it is not given much weight in the overall assessment of pulmonary function.
On the other hand, the PEF is often used by patients with asthma to monitor for evidence of bronchospasm. The test is performed regularly using an inexpensive handheld peak flowmeter. Results are used to help make self-management decisions and incorporated into personal action plans. Typically, these action plans identify 80% to 100% of the patient’s personal best peak flow as the green zone, or normal (no special action required); 50% to 80% of the patient’s personal best as the yellow zone (requiring self-administration of bronchodilator plus possibly oral steroids); and below 50% of the patient’s personal best as the red zone (requiring self-administration of bronchodilator, contacting the doctor, and calling 9-1-1).
Most electronic spirometers also can display and record the FVC maneuver as a plot of flow versus volume, typically referred to as a flow-volume loop (Fig. 9-3). To produce a complete flow-volume loop, the patient must be instructed to take a full forced inspiration to TLC before and after the forced exhalation maneuver. The spirometer then records both the inspiratory and expiratory efforts. The expiratory component of the maneuver, called the maximal expiratory flow-volume curve (MEFV), is plotted above the horizontal zero flow line, with the maximal inspiratory flow-volume curve (MIFV) recorded below the zero flow line. The FVC equals the maximal width of the loop along the zero flow line. The PEF equals the maximal positive deflection on the flow axis, with the peak inspiratory flow (PIF) represented as maximal negative deflection on this axis. The FEF at any point in the FVC can also be measured directly. Typically, spirometers record the FEF at 25%, 50%, and 75% of the FVC. For example, the FEF50% mark on the flow-volume loop in Figure 9-3 represents the instantaneous forced expiratory flow at 50% of the FVC. Differences in flow between any of these points can also be computed, with the most common being the FEF25-75%, which is equivalent to the same measure plotted on a volume-versus-time spirogram.
None of the expiratory measures obtained from a flow-volume loop have proved superior to the FEV1, FVC, FEV1/FVC and FEF25-75% obtained by volume-versus-time plots. On the other hand, the shape of the MEFV and MIFV curves can be helpful in detecting certain patterns of disease, as discussed subsequently.
The MVV is the maximal volume of air a subject can breathe over a specified period of time, usually 12 seconds. The 12-second volume then is multiplied by 5 to extrapolate what could be achieved in 1 minute, with the value expressed in liters per minute. The MVV is affected by the strength of the respiratory muscles, compliance of the lungs and thorax, inspiratory and expiratory airway resistance, and patient motivation and effort. Reference values vary widely, ranging from more than 170 L/minute for young adult males to less than 90 L/minute for elderly females.
Because one can estimate a patient’s MVV by multiplying the measured FEV1 by a factor of 40, this demanding test is no longer included in most standard spirometry protocols. However, the MVV is still used to assess conditions in which ventilatory capacity may be impaired by mechanisms different from those affecting FEV1. For example, some patients with neuromuscular disorders exhibit much larger than expected decreases in their MVV relative to that predicted from their FEV1. The MVV also is used in exercise testing to estimate a subject’s breathing reserve (discussed later in this chapter).
To obtain valid spirometry data requires accurate instrumentation. However, because FVC maneuvers are technique dependent, obtaining accurate data also depends on the subject being able to perform the maneuvers in an acceptable and reproducible way. Thus, there are two major components to spirometry quality assurance: equipment calibration and technique validation.
The American Thoracic Society (ATS) guidelines specify that spirometers should be capable of measuring volumes of 8 L or more and capturing exhalation maneuvers for at least 15 seconds. Volume accuracy should be at least ±3.5% or ±0.065 L, with the measured flow range between 0 and 14 L/second (−14 to + 14 L/second for flow-volume loops). Flow measurements should be accurate within ±5% of the true value over a range of −14 to +14 L/second with a sensitivity (minimal detectable flow) of 0.025 L/second. Spirometers should be able to produce printouts of both volume-time and flow-volume plots.
To assure spirometry accuracy, regular calibration tests are required. Table 9-3 summarizes these tests as recommended by the ATS and European Respiratory Society (ERS).
|Volume||Daily||Calibration check with a calibrated 3-L syringe|
|Volume linearity||Quarterly||1-L increments with a calibrated syringe over entire volume range|
|Flow linearity||Weekly||Test at least three different flow ranges|
|Software||New versions||Log installation date and perform test using biologic controls and known subject|
Volume calibration should be performed daily using a calibrated 3-L syringe. The syringe itself must have an accuracy of ±0.5% (±15 mL for a 3-L syringe), which is incorporated into the volume accuracy standard (3.0% + 0.5% = 3.5%). To assess volume linearity, the syringe should be discharged in three 1-L steps, with each successive volume meeting the 3.5% accuracy standard. To assess flow linearity, the syringe volume should be injected at three different flows, ranging between 0.5 and 12 L/second. With a 3-L syringe, this would equate to injection times between 6 seconds and 0.5 second. The volume at each flow should meet the accuracy standard of ±3.5%.
Regular testing using biologic controls also is recommended to assess spirometer performance and accuracy of software computations. A biologic control is a healthy subject for whom quality control data are available. Such data typically include the mean and standard deviation of 8 to 10 reproducible FVC maneuvers measured at different times.
If volume or flow inaccuracy is detected, no tests should be conducted until the cause of the measurement error is corrected. As with any quality assurance program, logs should be maintained to document spirometer calibration, maintenance, and any corrective actions taken.
FVC maneuvers are highly technique dependent. For this reason, accurate measurements require proper patient performance of the procedure. Providing clear instructions and eliciting a maximal effort are the keys to assuring a valid FVC maneuver. In addition, the clinician must be able to recognize and correct any observed errors in technique.
Box 9-1 outlines the key elements in the measurement of FVC using an electronic bedside spirometer. Most elements also pertain to spirometry measurement in the pulmonary function laboratory.
The goal is to obtain at least three acceptable and error-free maneuvers that are repeatable. An acceptable maneuver must be free from artifacts, exhibit a good and forceful start, and achieve complete exhalation. To assure repeatability, the therapist must confirm that the key volume measures (FVC and FEV1) are free of significant variability. Although many computerized spirometers automatically perform validity checks on each FVC maneuver, the clinician still must manually inspect each plot to confirm that it is acceptable and that the multiple measures are repeatable. A summary of the acceptability and repeatability criteria recommended by the ATS/ERS for FVC maneuvers (plotted as volume vs. time) appear in Box 9-2.
The most common error in obtaining an acceptable FVC is failure to achieve a rapid start to the forced exhalation. This typically is evident when the plotted FVC-versus-time curve is S shaped (like the oxyhemoglobin curve). Alternatively, one can change the spirometer display from volume versus time to flow versus volume and inspect the expiratory curve to determine whether the start of test was satisfactory. On a flow-volume curve, the PEF should be achieved with a sharp rise and occur close to the point of maximal inflation.
Spirometry software programs typically identify a slow start to forced exhalation by detecting a delay in time to peak flow (>120 msec) or by calculating the back-extrapolated volume. Figure 9-4 demonstrates how the back-extrapolated volume is calculated, normally by the spirometer’s microprocessor. If the back-extrapolated volume is more than 5% of FVC or more than 150 mL, the FVC is unacceptable, and a repeat maneuver is required. On the other hand, if the back-extrapolated volume falls below this quality threshold, the FVC breath still can be used, but with the breath starting point adjusted to the new extrapolated zero time point.
If no acceptable maneuvers can be obtained, results should not be reported; rather, all specific errors encountered should be documented. Only if three acceptable maneuvers can be obtained, meeting both repeatability criteria, should results be reported. If the initial results are acceptable but are not repeatable, additional testing should be conducted until either the repeatability criteria are met or the patient cannot continue. All reports should contain the therapist’s overall assessment of test quality.
Interpretation of spirometry test results normally is performed by a trained physician. Assessment involves comparison of the individual patient’s data with reference values generated from samples of healthy subjects and review of the applicable plots.
Although other spirometry reference values are available, the preferred prediction equations for subjects 8 to 80 years of age are those established by the National Health and Nutrition Examination Survey (NHANES) III. These equations provide reference ranges for all spirometry parameters based on the subject’s height, gender, age, and race.
The pulmonary function community has adopted ±2 standard deviations (SD) from the predicted mean as the normal reference range. With ±2 SD encompassing 95% of the values in a normal distribution (Fig. 9-5), one can define the boundaries of this range as cut-off points, outside of which results are abnormal. These cut-off points are called the lower limit of normal (LLN) and (if applicable) the upper limit of normal (ULN).
For interpretation of tests in which either abnormally low or high values apply, the LLN occurs at the 2.5th percentile and the ULN at the 97.5th percentile. If only abnormally low values are the focus (as with spirometry measures), then only the LLN is applicable, which corresponds to values in the distribution below the 5th percentile.
Figure 9-6 outlines the basic process for interpreting spirometry test results. After ensuring that the test results are valid (three acceptable maneuvers, with two of them being repeatable), one compares the patient’s FVC, FEV1, and FEV1/FVC to the predicted reference ranges. If all three values fall within the reference ranges, the results are deemed normal.
If, however, the FEV1 is reduced and the FEV1/FVC ratio falls below its LLN (below the 5th percentile of predicted), an expiratory flow limitation exists, and the patient is classified as having an obstructive ventilatory impairment. Often, the severity of obstruction is quantified according to magnitude of reduction in FEV1/FVC, as outlined in Table 9-4.
|Severity of Obstruction||FEV1 (% Predicted)|
The other major possibility is a reduced FVC, normal or reduced FEV1, and FEV1/FVC above the LLN (normal or higher than normal). An FEV1/ FVC above the LLN rules out an obstructive impairment. However, the presence of a reduced FVC suggests that the patient may have a reduction in lung volume, which would be classified as a restrictive ventilatory impairment
Interpretation is enhanced by review of the graphic plots obtained by spirometry. Figure 9-7 portrays typical volume-versus-time curves for a normal subject, one with an obstructive impairment and one with a restrictive disorder. Note that the obstructive pattern is characterized by a decrease in the slope of the curve (indicating reduction in expiratory flow) and a longer time to empty the lungs. On the other hand, in the restrictive pattern, the lung empties as fast or faster than normal but to a smaller volume.
Inspection of the patient’s flow-volume loop can aid in interpretation, especially in identifying the presence and location of large airway obstruction. Figure 9-8 provides six examples of flow-volume loops representing distinct patterns of abnormal function. Disorders causing generalized expiratory obstruction—like asthma and emphysema—mainly affect the MEFV curve, with both exhibiting a reduction in peak flow and FEF50%. Note also that the MEFV portion of the loop on the patient with emphysema is markedly concave and positioned left of the normal loop, indicating greater flow obstruction at lower lung volumes as well as air trapping and hyperinflation. On the other hand, a patient with a restrictive ventilatory impairment typically produces a flow-volume loop with the same shape as normal, but one that is markedly narrower on the volume axis, consistent with the smaller FVC.
The bottom three flow-volume loops in Figure 9-8 portray different types of large airway obstruction. The variable intrathoracic obstruction loop reveals a markedly reduced peak flow on expiration despite near-normal inspiratory flows. This typically is the result of expiratory flow obstruction in the large airways, as may occur with tracheomalacia or tumors of the trachea or bronchi. The opposite pattern is seen in variable extrathoracic obstruction, that is, reduced inspiratory flow and relatively normal expiratory flow. Vocal cord dysfunction and laryngeal edema are common causes of variable extrathoracic obstruction. An equal reduction in inspiratory and expiratory flows suggests a fixed large airway obstruction. Causes of fixed large airway obstruction include tracheal stenosis, tracheal tumors, and foreign body aspiration.
After the basic pattern is identified, additional testing may be indicated. If the spirometry results indicate that the patient has an expiratory obstructive impairment, the doctor typically will want to determine whether the obstruction is reversible with treatment, that is, if it responds to bronchodilator therapy (see Fig. 9-6). This is accomplished by measuring spirometry values before and after administration of the selected bronchodilator. Normally, the baseline spirometry should be conducted at least 4 hours after any prior use of a short-acting β-agonist (e.g., albuterol) and at least 12 hours after any administration of a long-acting bronchodilator (e.g., salmeterol).
Box 9-3 outlines the key elements in the procedure for assessing reversibility. If after bronchodilator administration, the FEV1 or FVC increases by more than 12% and at least 200 mL, the obstructive impairment is considered reversible. A lesser response indicates that airflow limitation is not reversible. However, some patients who do not meet the criteria for reversibility may still experience improvement after bronchodilator therapy, such as a decrease in dyspnea. This response likely is due to a decrease in hyperinflation occurring without a significant decrease in airway resistance.
As indicated in Figure 9-6, a reduced FVC in the presence of a normal or high FEV1/FVC suggests that the patient may have a reduction in lung volumes. However, this same pattern can occur among patients who fail to completely inhale or exhale during the maneuver. For this reason, confirmation of a reduction in lung volumes and the presence of a restrictive disorder requires measurement of static lung volumes (FRC, RV, TLC). Moreover, if it is suspected that the restrictive condition is due to an interstitial disease processes, most doctors will also request a diffusing capacity study (Dlco).
Static lung volume determination involves measurement of the resting lung volume or FRC. If the FRC is known and the inspiratory capacity (IC) and ERV also are obtained by spirometry, both the RV (FRC − ERV) and the TLC (FRC + IC) can be computed. In general, static lung volume testing is performed in the pulmonary function laboratory.
There are three different methods used to measure FRC: (1) closed-circuit helium dilution, (2) open-circuit nitrogen washout, and (3) body plethysmography. Currently, there is no firm evidence to recommend any one technique over the others. In most cases, the method used will be based on the available equipment and personnel.