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.

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 Volume in 1 Second

During FVC testing, several other measurements are also made. FEV₁ is a measurement of the volume exhaled in the first second of FVC (see Figure 19-2, A). To ensure validity of FEV₁, the measurement must originate from a set of three acceptable FVC trials. The first second of forced exhalation begins at the zero time point (see Figure 19-3). To ensure reliability of FEV₁, the largest FEV₁ and second largest FEV₁ from the acceptable trials should not vary by more than 0.150 L. Consistent with its definition, the largest FEV₁ (BTPS) measured is the patient’s FEV₁. The largest FEV₁ sometimes comes from a different trial than the largest FVC.

The %FEV₁/FVC, also called the forced expiratory volume in 1 second-to-vital capacity ratio (FEV₁/FVC), is calculated by dividing the patient’s largest FEV₁ by the patient’s largest VC and converting it to a percentage (by multiplying by 100). The two values do not have to come from the same trial; the VC should be the largest VC measured, even if measured as a slow VC or during inspiration.

Except for PEF rate, all other measurements that originate from FVC come from the “best curve”—these include forced expiratory flow between 200 ml and 1200 ml of FVC (FEF_200-1200); forced expiratory flow between 25% and 75% of FVC (FEF_25%-75%); forced expiratory flow between 75% and 85% of FVC (FEF_75%-85%); and instantaneous FEF_25%, FEF_50%, and FEF_75%. The best test curve is defined as the trial that meets the acceptability criteria and gives the largest sum of FVC plus FEV₁. The validity and reliability of these other measurements of pulmonary mechanics are based on their origin from a valid and reliable 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).

Peak Expiratory Flow

PEF is difficult to identify on a volume-time graph of FVC. The peak flow is the slope of the tangent to the steepest portion of the FVC curve. PEF is easy to identify on a flow-volume graph as the highest point on the graph (see Figure 19-2, B).²³ PEF is sometimes measured independently of FVC with a peak flowmeter. These devices are designed to indicate only the greatest expiratory flow rate. The validity of PEF rate is based on a preceding inspiration to TLC and a maximal effort. The FVC principles of ensuring reliability should apply to measurements of PEF rate. The two largest repeated measurements should agree within 5%.

In addition to PEF rate, the other instantaneous flow rates, such as forced expiratory flow at 25% (FEF_25%) of FVC, forced expiratory flow at 50% (FEF_50%) of FVC, and forced expiratory flow at 75% (FEF_75%) of FVC, during FVC are graphed on a flow-volume curve. When FVC is followed by a forced inspiratory VC, a flow-volume loop is produced (see Figure 19-2, B). On the flow-volume loop, the maximal forced inspiratory flow rate at 50% (FIF_50%) of VC can be measured and compared with FEF_50%.

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.

Quality Assurance

For comprehensive quality assurance of spirometry, the accuracy and precision of the volume or flow measuring device must be verified with a 3.0-L syringe using multiple full strokes at various injection speeds to mimic fast and slow flow rates. The average volume should meet the ±3% standard; the standard deviation should be small, and the 95% confidence interval or expected performance range should be determined. Subsequent individual checks of accuracy with the 3.0-L syringe should be compared with the ±3% standard and the expected performance range to ensure that the device is measuring consistently. The performance of the technologist to coach subjects during spirometric testing should be observed and reviewed periodically. Technologists successfully achieving testing acceptability and repeatability criteria should be monitored and feedback to the technologist should be provided periodically.

Significance

The normal values for the spirometric measurements of pulmonary mechanics are based on height, age, gender, and ethnicity. Table 19-4 provides common regression equations to predict normal values for the measurements of pulmonary mechanics for individuals of specific height (in centimeters), age (in years), and gender.^24–26 A positive correlation exists between measurements of pulmonary mechanics and height, and a negative correlation exists between measurements of pulmonary mechanics and age for patients older than 20 years. Male values are larger than female values when height and age are equal. The populations that were studied to determine the normal values of pulmonary mechanics were predominantly white. To account for ethnic differences of nonwhites, the predicted normal values for whites commonly are reduced by 12% to 15% when applied to nonwhites. Ethnic-specific equations for special populations, such as African-Americans and Mexican-Americans, have also been developed.²⁴

Although traditional textbooks suggest the typical normal VC is 4.80 L, the predicted normal FVC for a 20-year-old, 180-cm man approaches 5.60 L. A reduced FVC may occur with obstructive or restrictive impairments. Figure 19-7 shows FVC from volume-time spirometer tracings for normal, obstructive, and restrictive conditions. The FVC values in both the obstructed and the restricted curves are shown as reduced volumes compared with the normal curve. The primary difference between the curve in the restricted patient compared with the curve in the obstructed patient is the slope of the tracing; obstructive diseases produce flattened slopes and smaller FEV₁.

Figure 19-8 displays the FVC from flow-volume tracings for obstructive and restrictive conditions. The shapes of these tracings are different; obstructive diseases produce lower peaks and lower flow rates at all lung volumes. Forced inspiratory flow rates sometimes are useful for identifying extrathoracic airway obstructions. In moderate and severe obstructive lung diseases, the FVC is reduced if weakened bronchioles collapse and trap air in the lungs, creating an increase in RV. Some laboratories compare the volumes of slow vital capacity (SVC) and FVC to identify air trapping. VC is reduced in restrictive lung diseases because the patient’s inhaled volume is reduced.

Forced expiratory volume in half of a second (FEV_0.5) is an indicator of patient effort during the initial phase of the FVC maneuver. With good effort, a patient should exhale at least 50% of his or her VC in the initial half of a second.

Although FEV₁ is measured as a volume, FEV₁ is considered a flow rate. The predicted normal FEV₁ for a 20-year-old, 180-cm man approaches 4.70 L. FEV₁ may be reduced with obstructive or restrictive impairments. For patients with airway obstruction, FEV₁ measures the general severity of airway obstruction. For patients with restrictive impairment, FEV₁ may be reduced when the patient’s VC is smaller than the predicted FEV₁.

The FEV₁/FVC ratio separates patients with airway obstruction from individuals with normal pulmonary function and from patients with restrictive impairment. The LLN can be determined or the predicted normal %FEV₁/FVC can be calculated by dividing the predicted normal FEV₁ by the predicted normal VC. Generally, individuals without airway obstruction are able to exhale at least 70% of their VC in the first second, and individuals with airway obstruction exhale less than 70% of their VC in the first second.

To interpret other flow rates, a generalization may be helpful. Gas exhaled during the early portion of the FVC reflects the resistance in the larger airways, and gas exhaled during the later portion of the FVC reflects the resistance in the smaller airways. As exhalation of FVC proceeds, flow decreases, and the airways reflected in the measurements get smaller. Any flow measured in the first half of the FVC reflects on the bronchi; any flow measured beyond 50% of the VC reflects on the bronchioles.

PEF, FEF_200-1200, and FEF_25% occur near the onset of FVC. Typical normal values are similar; PEF is 9.5 L/sec, FEF_200-1200 is 8.5 L/sec, and FEF_25% is 9.0 L/sec. A reduced PEF rate, FEF_200-1200, or FEF_25% may occur as a result of a large airway obstruction and from lack of sufficient effort to inhale maximally and exhale forcibly. FEF_25%-75% and FEF_50% occur in the middle of FVC. Because FEF_25%-75% is an average of half the VC and FEF_50% is an instantaneous flow, the typical normal values are less similar; FEF_25%-75% is 4.5 L/sec, and FEF_50% is 6.5 L/sec. Reduced FEF_25%-75% or FEF_50% may occur because of small airway obstruction and from lack of effort to sustain a maximal exhalation. FEF_75% and FEF_75%-85% occur late in FVC and reflect on the smallest airways. Typical values are 3.5 L/sec for FEF_75% and 1.5 L/sec for FEF_75%-85%. Sometimes patients who are asymptomatic for cough, sputum production, or dyspnea may have reduced flow in the small airways. A singular reduction in small airway flow may indicate nothing at all or may be an early indicator of obstruction.

The shape of the flow-volume loop and the FEF_50%/FIF_50% ratio provide additional information about upper airway obstruction. Compared with the normal flow-volume loop, a fixed upper airway obstruction produces a curve that appears box-shaped. In Figure 19-9, both expiratory and inspiratory flows are decreased and limited by the solid obstruction; the FEF_50%/FIF_50% ratio remains normal. Variable upper airway obstructions produce two different shapes depending on the site of the obstruction. Because the intraairway pressure during a forced inspiration is less than atmospheric outside the thorax, a variable extrathoracic upper airway obstruction limits inspiratory flow, and the FEF_50%/FIF_50% ratio is greater than 1.0. Because the intraairway pressure during a forced inspiration is greater than atmospheric pressure inside the thorax, a variable intrathoracic upper airway obstruction limits expiratory flow, and the FEF_50%/FIF_50% ratio is less than 1.0.

Similar to other spirometric measurements of pulmonary mechanics, normal values of MVV are based on gender, age, and height. MVV is reduced in patients with moderate and severe airway obstruction. A measured value less than 75% of predicted is significant. The normal for men is approximately 160 to 180 L/min; it is slightly lower in women. In restrictive lung disease, MVV may be normal or only slightly reduced. Respiratory muscle strength is a primary determinant of MVV in patients with interstitial lung disease and an important determinant in patients with chronic obstructive pulmonary disease (COPD). Undernourished patients also may have a reduced MVV.²⁶

Reversibility

Based on the initial results of baseline spirometry, additional testing of pulmonary mechanics is often desirable. If the baseline test indicates airway obstruction, determining the reversibility of the obstruction is indicated. RTs also use the concept of reversibility when evaluating routine therapy by performing spirometry before and after therapy. In the laboratory, the FVC maneuver is often repeated after the patient has received a bronchodilator administered by small volume nebulizer or metered dose inhaler. This laboratory protocol is commonly known as spirometry before and after bronchodilator. Reversibility of the airway obstruction indicates effective therapy. Although improvement in other measurements of pulmonary function is sometimes used, reversibility is defined as a 15% or greater improvement in FEV₁ and at least a 200-ml increase in FEV₁. Improvement is determined using the percent change formula:

% Improvement=Post-FEV1−Pre-FEV1Pre-FEV1×100

Bronchial Challenge

When the patient’s history suggests episodic symptoms of hyperreactive airways and airway obstruction, such as seasonal or exercise-induced wheezing, and the results of baseline spirometry are normal, performing a bronchial provocation may be indicated.²⁷ Bronchial provocation testing uses an agent to stimulate a hyperreactive airway response and to create airway obstruction. Although several types of provocations are possible, such as inhaling histamine or cold air or exercising, provoking a hyperreactive airway response by inhaling methacholine is the most popular technique with the most predictable results. The procedure usually begins with the patient inhaling a normal saline aerosol and then repeating the FVC maneuver. Some very sensitive patients exhibit hyperreactive airways with saline alone; a positive response to saline is defined as a decrease in FEV₁ of 10% or greater. The methacholine provocation protocol systematically exposes the patient to increasing dosages of methacholine. Usually starting with a low dose of 0.03 mg/ml, patients inhale methacholine aerosol and then repeat the FVC maneuver. A positive response to methacholine is defined as a decrease in FEV₁ of 20% or greater (another example of percent change). If a positive response does not occur, the methacholine dose is doubled to 0.06 mg/ml, and the FVC maneuver is repeated. The process of “double-dosing” and performing FVC maneuvers continues until there is a positive response or until the full dose, 16 mg/ml, is given. If a positive response occurs, treatment with a fast-acting bronchodilator is indicated, and sometimes administering O₂ is helpful. The final test report should include the concentration of methacholine that caused the 20% decrease in FEV₁ in the form of PD%FEV₁. For example, PD₂₂FEV₁ = 4 mg/ml indicates that the provocation dose of 4 mg/ml resulted in a 22% decrease in FEV₁.

Helium Dilution

The helium dilution technique for measuring lung volumes uses a closed, rebreathing circuit (Figure 19-11).²⁹ This technique is based on the assumptions that a known volume and concentration of helium in air begin in the closed spirometer, that the patient has no helium in his or her lungs, and that an equilibration of helium can occur between the spirometer and the lungs. For the helium dilution procedure to be performed, a measurable volume of helium is added into the spirometer circuit, and the initial concentration of helium (F_iHe) is measured. Next, the valve is turned to connect the patient to the breathing circuit usually at the resting expiratory level of the FRC. Starting the test at RV requires maximal expiratory effort by the patient and is not considered a reliable starting point. Although starting the test at the TLC level requires a maximal inspiration, TLC may be a reliable alternative beginning point.

The patient is connected to the helium-air mixture, and the concentration of helium is diluted slowly by the patient’s lung volume. Wearing nose clips, the patient breathes normally in the closed circuit. Exhaled CO₂ is absorbed with soda lime, and O₂ is added at a rate equal to the patient’s O₂ consumption. A constant volume is maintained to ensure accurate helium concentration measurements. The patient rebreathes the gas in the system until equilibrium of helium concentration is established. In healthy patients and patients with a small FRC, equilibration occurs in 2 to 5 minutes. Patients with obstructive lung disease may require 20 minutes to equilibrate because of slow gas mixing in the lungs. The helium dilution time or the duration of the test is a gross index of the distribution of ventilation.

For FRC to be calculated using the helium dilution technique, several observations must be made: the volume of helium added (vol He) to the closed spirometer, the initial helium concentration (F_iHe) before the patient is connected to the breathing circuit, the final helium concentration (F_fHe) after helium equilibrium between the spirometer and patient is established, the spirometer temperature, and the time necessary for helium equilibration to occur. If the patient is connected to the circuit at the resting level, the FRC can be calculated with the following equation:

FRC=(vol He÷FiHe)×[(FiHe−FfHe)×FfHe]

Corrections for temperature and helium absorption are normally applied. All lung volumes and capacities must be reported under BTPS conditions. Volumes measured by spirometers are at ambient temperature, pressure, and saturated (ATPS) conditions and must be adjusted for the temperature difference between the spirometer and the patient’s body temperature. This ATPS to BTPS adjustment can increase volumes 5% to 10%, and the difference is large enough to invalidate the test results, unless the correction is made.

Although helium is an inert gas with a negligible solubility in plasma, another correction is sometimes applied. A small amount of helium is thought to diffuse across the alveolar-capillary membrane and is lost in the measurement of final helium concentration. To account for the loss, 30 ml of BTPS-corrected volume is subtracted for each minute of helium breathing, up to 200 ml for a 7-minute test.³⁰ Once these corrections are made, RV can be calculated by subtracting ERV from FRC according to the equation: RV = FRC − ERV.

Nitrogen Washout

The nitrogen washout technique uses a nonrebreathing or open circuit (Figure 19-12).³¹ The technique is based on the assumptions that the nitrogen concentration in the lungs is 78% and in equilibrium with the atmosphere, that the patient inhales 100% O₂, and that the O₂ replaces all of the nitrogen in the lungs. Similar to the helium dilution technique, the patient is connected to the system at either the resting expiratory level or the TLC. The patient’s exhaled gas is monitored, and its volume and nitrogen percentage are measured.

Generally, two types of circuits are used to measure lung volumes with this technique. In one type of circuit, all of the exhaled gases are collected in a large container, where the volume and concentration of nitrogen are measured. In the second type of circuit, the volume and concentration of each exhaled breath are measured separately and stored in a memory; the sum of the volumes and the weighted average of the nitrogen concentration are calculated by a computer.

Wearing nose clips, the patient breathes 100% O₂ until nearly all of the nitrogen has been washed out of the lungs, leaving less than 1.5% nitrogen in the lungs. When the peak exhaled concentration of nitrogen is less than 1.5%, the patient exhales completely, and the fractional concentration of alveolar nitrogen (F_AN₂) is noted. Similar to the helium technique, the time it takes to wash out the nitrogen is approximately 2 to 5 minutes in healthy individuals and longer in patients with obstructive lung disease. The test must occur in a leakproof circuit because the presence of any air increases the measured nitrogen percentages and results in grossly elevated measurements of lung volume.

For FRC or TLC to be calculated by the nitrogen washout technique, several measurements must be made: the total volume of gas exhaled during the test (V_E), the fractional concentration of exhaled nitrogen in the total gas volume (F_EN₂), the fractional concentration of nitrogen in the alveoli at the end of the test (F_AN₂), and the spirometer temperature. FRC (or TLC, if the test began at TLC) can be calculated with the following equation:

FRC=(VE×FEN2)÷(0.78−FAN2)

The calculated FRC (or TLC, if connection to the breathing circuit occurred after inspiring maximally) must be adjusted for the temperature difference between the spirometer and the patient’s body temperature using the BTPS correction factor. During the test, some nitrogen from the plasma and body tissues is usually excreted and exhaled with lung nitrogen. For this reason, another correction is needed. The volume of tissue nitrogen excreted (V_tis in milliliters) is directly related to the duration (t in minutes) of the test and weight (W in kilograms) of the patient. A correction for this extra nitrogen should be made according to the following formula: V_tis (ml) = (0.1209 − 0.0665) × (W ÷ 70). V_tis (ml) is subtracted from the BTPS-corrected lung volume. RV is the difference between ERV and FRC.

The validity of the helium dilution and nitrogen washout techniques can be ensured by measuring known volumes accurately, such as a 3.0-L syringe, while recognizing that this method would not include O₂ consumption and O₂ titration for the helium method or tissue nitrogen excretion for the nitrogen method. The reliability of these techniques can be established by repeated measurements of known volumes or healthy patients that agree within 5%. Establishing reliability of these measurements on patients requires repeating tests after reestablishing baseline lung gases. For patients with severe obstructive lung disease, waiting up to 1 hour may be necessary. When the FRC is measured by either He dilution or N₂ washout, any leak anywhere in the patient, tubing, or gas analyzers would increase the measured FRC and may be falsely interpreted as hyperinflation.

Plethysmography

The plethysmography technique applies Boyle’s law and uses measurements of volume and pressure changes to determine lung volume, assuming temperature is constant.³² The plethysmography technique measures the volume of all compressible gas in the thorax, including gas trapped behind airway obstructions or in the pleural space. Gas in the abdomen may also be included in the measurement. The whole-body plethysmograph consists of a sealed chamber in which the patient sits (Figure 19-13). Pressure transducers (electronic manometers) measure pressure at the mouth and in the chamber. An electronically controlled shutter near the mouthpiece allows the airway to be occluded periodically, measuring airway pressure changes under conditions of no airflow. Without air flow, pressure changes measured at the mouth are pressure changes in the alveoli. According to Boyle’s law (V × P = k), when temperature is constant, volume changes in the thorax create volume changes in the chamber, which are reflected by pressure changes in the chamber. The pressure and volume changes are included in the equation:

VTG1×Palv1=VTG2×Palv2

When measurement of TGV is being done, the patient sits in the chamber and initially breathes normal tidal volumes through the mouthpiece. When the patient is near FRC, the shutter is closed at end expiration for 2 to 3 seconds. The patient holds his or her cheeks and performs gentle panting at 1 Hz or one pant per second.³³ During panting, changes in airway pressure (ΔP) and changes in chamber volume (ΔV) are measured. Because the panting maneuver occurs with small pressure changes around barometric pressure, the simplified equation used to calculate TGV is V_TG = P_B × (ΔV ÷ ΔP), where P_B is the barometric pressure in cm H₂O. A series of three to five panting maneuvers should be performed. After panting, the patient should exhale completely to record ERV and then inhale maximally to record the inspiratory vital capacity.

Because the body plethysmographic method of measuring FRC actually measures TGV, the value obtained for some patients may be larger than values resulting from either the helium dilution or nitrogen washout techniques. Such a difference occurs whenever there is gas in the thorax that is not in communication with patent airways, as might be the case in patients with pneumothorax, pneumomediastinum, or emphysema. The RV is the difference between TGV and ERV, and the TLC is the sum of RV and VC, or the sum of TGV and IC.

Quality Assurance

Comprehensive quality assurance for measuring lung volumes depends on the measuring technique. For helium dilution and nitrogen washout techniques, the accuracy and precision of the volume or flow measuring device must be ensured as in spirometry; the accuracy and linearity of the gas analyzer must be verified, and the leak test must be acceptable while monitoring change in volume and gas concentrations over at least 1 minute. Technologists successfully identifying the subject’s breath-to-breath resting level and complying with retest waiting periods should be monitored. Correctly measuring the volume of the 3.0-L syringe provides a quality control standard. For the plethysmographic technique, the box and mouth pressure transducers must be calibrated and accurate. The technologist’s choice of the best-fit line of the relationship between box and mouth pressures should be monitored. Measuring a known volume using a flask with squeeze bulb connected to the mouthpiece or using a consistent known subject provides a quality control standard. For all techniques, achieving testing acceptability and repeatability criteria should be monitored, and feedback to the technologist should be provided periodically.

Significance

Changes in lung volumes and capacities are generally consistent with the pattern of impairment. TLC, FRC, and RV increase with obstructive lung diseases and decrease with restrictive impairment. Some lung volumes provide valuable diagnostic information. For example, TLC is always reduced in restrictive lung disease, unless obstruction and restriction occur together. When obstruction and restriction occur together, the TLC may be a less sensitive measure of the restrictive impairment. Other volumes and capacities may remain normal with mild obstructive or restrictive disease. The pattern of lung volume changes and the proportion of FRC and RV to TLC are also important.

The normal V_T is approximately 500 to 700 ml for an average healthy adult. In the normal population, great variation of tidal volumes and measurements beyond the normal range are not indicative of a disease process. Normal V_T is often observed in both restrictive and obstructive lung diseases. V_T alone is not a valid indicator of the type of lung disease.

The normal IC is approximately 3600 ml, with a significant variation in the normal population. IC may be normal or reduced in restrictive and obstructive lung diseases. A reduction of IC occurs in restrictive lung diseases because the patient’s inhaled volume is reduced, and there is a reduction in TLC. In mild obstructive lung diseases, IC is usually normal. In moderate and severe obstructive diseases, IC can be reduced because the resting expiratory level of FRC has increased owing to hyperinflation of the lungs. An increase in IC may occur when the patient inhales from below the resting expiratory level when the measurement is performed; athletes and musicians who play wind instruments may also have increased inspiratory capacities. RTs use the measurement of IC in clinical protocols to decide between methods of lung expansion therapy (see Chapter 39).

IRV is not commonly measured. Similar to V_T and IC, IRV can be normal in both restrictive and obstructive diseases and is not a useful diagnostic measurement. The normal value for IRV is 3.10 L.

The normal ERV is approximately 1.20 L and represents approximately 20% to 25% of the VC. It can be either normal or reduced in obstructive and restrictive lung diseases. ERV is subtracted from FRC to calculate RV.

The normal value of the VC is 4.80 L and represents approximately 80% of TLC. Normal values for VC can vary significantly depending on age, gender, height, and ethnicity. A reduction of VC occurs in restrictive lung diseases because the patient’s inhaled volume is reduced and there is a reduction in TLC. In mild obstructive lung diseases, the slow VC is usually normal if the patient exhales leisurely and has had enough time to exhale completely or if the VC is measured during inspiration. Measurements made from FVC provide valuable data for pulmonary mechanics.

RV, FRC, and TLC are the most important measurements of lung volumes. Age, height, gender, ethnicity, and sometimes weight or body surface area correlate with normal values for these lung volumes.³⁴ Table 19-5 provides common regression equations to predict the lung volumes for individuals of specific height (in centimeters), age (in years), and gender. A positive correlation exists between lung volumes and height, and a negative correlation exists between lung volumes and age for patients older than 20 years. Male values are larger than female values when height and age are equal.

The typical normal TLC is 6.00 L. The normal RV is approximately 1.20 L and represents approximately 20% of TLC. FRC is approximately 2.40 L, which represents approximately 40% of the TLC. RV and FRC are usually enlarged in acute and chronic obstructive lung diseases because of hyperinflation and air trapping (Figure 19-14).

TLC may also be enlarged in COPD. TLC is always reduced in restrictive lung diseases because of a loss of lung volume; RV and FRC are often reduced proportionately. Certain acute disorders, such as pulmonary edema, atelectasis, and consolidation, also cause a reduction of TLC and FRC.

Single-Breath Technique

There are several techniques to measure the diffusing capacity of the lung for CO, including steady-state, intrabreath, and rebreathing techniques, but the single-breath method (DLCOSB) is the most common measurement technique because it is quick and reproducible. Standards for measuring diffusing capacity of the lung were initially published in 1995 and updated in 2005; these standards focus primarily on the DLCOSB.36,37 The entire test can be performed in just slightly longer than 10 seconds. The patient exhales completely to RV, rapidly inspires a VC of a gas mixture containing 0.3% CO and an inert tracer gas such as He in air, maintains breath holding for 10 seconds, and then exhales rapidly at least 1.0 L. Inspiring at least 85% of VC measured during spirometry is expected. After a predesignated volume of 0.75 to 1.0 L is exhaled, a sample of alveolar gas is collected and analyzed for expired CO (F_eCO_t) and helium (F_eHe). Because the breath holding period (t) begins when inspiration of the gas mixture begins and the period ends when the alveolar sample is collected, inspiration and expiration should be rapid, and this period should not exceed 11 seconds. To regulate the breath holding period, some measuring systems close the mouthpiece with a timed shutter. The suitable breathing pattern requires some patient cooperation and coordination; some patients benefit from a timer as a visual aid.

The single-breath method (DLCOSB) is based on the diffusion decay curve described by Forster and colleagues³⁸ (Figure 19-15). When a bolus of CO gas is inhaled, the rate of gas diffusion declines logarithmically. The diffusing capacity of the lung is a function of lung volume (V_A) in STPD conditions exposed to the test gas,³⁹ the duration (60 ÷ t) that the test gas is in contact with the lung, the initial concentration of test gas in the lung (FaCO₀), and the final concentration of test gas in the lung (FaCO_t), according to the following equation:

where (P_B − 47) is ambient barometric pressure corrected for water vapor pressure at 37° C.

Helium or sometimes neon is in the gas mixture as a tracer gas for the dilution of the inspired CO concentration by RV and to measure the effective TLC by a single-breath helium dilution. The inspired CO concentration of 0.3% is not the concentration of CO received by the lungs because the inspired volume is diluted by RV of the patient. The dilution of CO is reflected by the dilution of helium, and the FaCO₀ can be calculated according to the following equation:

The FaCO₀ is the concentration of CO in the lung at “zero time” before any diffusion occurs. The single-breath technique distributes the gas mixture through unobstructed airways to an alveolar volume that is also called the effective total lung capacity. The effective total lung capacity (V_A) can be calculated according to the following equation:

VA=VC×(FiHe÷FeHe)

The V_A is necessary to calculate the DLCO, and it is used in the determination of the diffusing capacity of the lung-to-alveolar volume ratio (DLCO/V_A).

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. In patients with obstructive airway disease, a longer period (e.g., 10 minutes) may be necessary. The actual DLCO reported should be the mean of 2 acceptable tests. An acceptable test is defined as one that is reproducible to within 10% or 3 ml of the CO/min/mm Hg value, whichever is greater. (See Clinical Practice Guideline 19-4.)

Quality Assurance

For comprehensive quality assurance for measuring diffusing capacity, the accuracy and precision of the volume or flow measuring device must be ensured as in spirometry; the accuracy and linearity of the gas analyzer must be verified as in measuring lung volumes. Technologists successfully achieving testing acceptability and repeatability criteria should be monitored, and feedback should be provided to the technologist periodically. Measuring the diffusing capacity of the 3.0-L syringe or using a consistent known subject provides a quality control standard.

Significance

Normal values for the DLCO using the single-breath technique are based primarily on a patient’s age, height, and gender (Table 19-6). A typical normal value for a 20-year-old healthy man is 40 ml/min/mm Hg.⁴⁰ Before interpretation of the test result, corrections for abnormal Hb level, breathing supplemental O₂ or testing at altitude, and elevated COHb levels may be necessary, if these conditions apply. Initially, all Hb levels were corrected to 15 g/dl.⁴¹ More recent and more specific corrections have been advocated. For men and adolescents, if the Hb level varies from 14.6 g/dl, the predicted normal value should be adjusted using the following equation:⁴²

Predicted DLCO for Hb=Predicted DLCO×(1.7Hb/10.22+Hb)

For women and children, the normal Hb level is 13.4 g/dl, and the equation is⁴²:

Predicted DLCO for Hb=Predicted DLCO×(1.7 Hb/9.38+Hb)

If the patient’s PaO₂ differs from 100 mm Hg because of breathing supplemental O₂ or performing the test at altitude, the DLCO would be affected by approximately 0.31% to 35% per mm Hg difference from 100 mm Hg.43,44 The predicted normal value should be adjusted using the following equations:

Predicted DLCO for altitude=Predicted DLCO/[1.0+0.0031(PiO2−150)]

The DLCO may be reduced from the predicted normal in patients with obstructive or restrictive lung diseases. With destruction of alveoli in pulmonary emphysema, with small lung volumes, and with fibrosis of alveoli in asbestosis, the DLCO may be less than normal. Pulmonary embolism also may decrease the DLCO. The DLCO may be useful in identifying which patients with obstructive impairment are likely to desaturate during exercise and which may benefit from O₂ therapy. The DLCO may be increased in patients with polycythemia, congestive heart failure (resulting from an increase in pulmonary vascular blood volume), and elevated cardiac output. Factors that can alter the DLCO above or below the normal value are summarized in Box 19-2.

The diffusing capacity of the lung-to-effective total lung capacity ratio (DLCO/V_A) differentiates between diffusion abnormalities caused by having a small lung volume compared with diffusion abnormalities caused by alveolar-capillary membrane pathologies. Patients whose only problem is small lungs would have a decreased DLCO, but their DLCO/V_A ratio would be normal. Patients with pulmonary emphysema or fibrosis would have a decreased DLCO and a decreased DLCO/V_A ratio.