Functional residual capacity (FRC) is the volume of gas remaining in the lungs at the end of a quiet breath. On a simple spirogram, this point is termed the end-expiratory level (see Figure 2-1). Residual volume (RV) is the volume of gas remaining in the lungs at the end of a maximal expiration regardless of the lung volume at which exhalation was started (see Figure 2-1). Total lung capacity (TLC) is the volume of gas contained in the lungs after maximal inspiration. FRC, TLC, and RV are reported in liters (L) or milliliters (mL), corrected to BTPS. The RV/TLC ratio defines the fraction of TLC that cannot be exhaled (RV), expressed as a percentage.

Thoracic gas volume (V_TG) is the absolute volume of gas in the thorax at any point in time and any level of alveolar pressure. V_TG is usually measured at the end-expiratory level and is then equal to FRC. It also may be measured at other lung volumes and corrected to relate to FRC. V_TG is always measured in a body plethysmograph, and, because it is used to measure the FRC, the term V_TG is often synonymous with FRC when FRC is measured by body plethysmography. The V_TG is reported in liters or milliliters, BTPS.

Technique

There are a variety of methods for measuring absolute lung volumes (Table 4-1). FRC is measured directly with the open-circuit multiple-breath N₂ washout, closed-circuit multiple-breath He dilution, and body plethysmographic techniques. Once FRC and VC have been measured, RV and TLC can be calculated. TLC can be estimated directly with the single-breath N₂ washout, and single-breath He dilution as part of the diffusing capacity (Dlco) test, and by chest imaging techniques. RV can only be measured indirectly once FRC or TLC has been determined.

Table 4-1

Methods for Measurement of Lung Volumes

Method	Lung Volume	Advantages/Disadvantages
Multiple-breath He dilution	FRC	Simple, relatively inexpensive; affected by distribution of ventilation in moderate or severe obstruction; multiple-breath; requires IC, ERV to calculate other lung volumes
Multiple-breath N₂ washout	FRC	Simple, relatively inexpensive; affected by distribution of ventilation in moderate or severe obstruction; multiple-breath; requires IC, ERV to calculate other lung volumes
Single-breath N₂ washout	TLC	Calculated from single-breath N₂ distribution test; may underestimate lung volume in the presence of obstruction
Single-breath He (or other inert gas; e.g., neon) dilution	TLC	Calculated as part of Dlco (V_A); may underestimate lung volume in the presence of obstruction
Plethysmography	V_TG (FRC_pleth)	Plethysmographic method more complex, but very fast; tends to be more accurate in the presence of airway obstruction than gas dilution techniques
Chest radiograph	TLC	Requires posterior-anterior and lateral chest x-ray films; must breath-hold at TLC; not accurate in the presence of diffuse, space-occupying diseases
Chest computerized tomography (CT)	TLC	Involves radiation exposure and increased cost; must breath-hold at TLC; underestimates lung volumes in the presence of airway obstruction
Magnetic resonance imaging (MRI)	TLC	No radiation exposure; very costly; research tool only

Dlco, Diffusing capacity; IC, inspiratory capacity; N₂, nitrogen; V_A, alveolar volume.

Open-Circuit, Multiple-Breath Nitrogen Washout

Determination of FRC with the open-circuit multiple-breath nitrogen washout technique (FRC_n2) is based on washing out the N₂ from the lungs while the patient breathes 100% O₂ for several minutes. At the start of the test, the N₂ concentration in the lungs is approximately 75%–80%. As the patient breathes 100% O₂, the N₂ in the lungs is gradually washed out, and the total expired volume is measured. At the end of the test, the N₂ concentration in the lungs is approximately 1%. The initial N₂ concentration, amount of N₂ washed out, and final N₂ concentration are measured and can then be used to calculate the volume of air in the lungs at the start of the test (FRC), using the following formula:

< ?xml:namespace prefix = "mml" />FRC=FEN2final×Expired Volume−N2tissueFAN2alveolar1−FAN2alveolar2

where:

F_EN_2final = fraction of N₂ in volume expired

F_AN_2alveolar1 = fraction of N₂ in alveolar gas initially

F_AN_2alveolar2 = fraction of N₂ in alveolar gas at end (from an alveolar sample)

N_2tissue = volume of N₂ washed out of blood/tissues

A correction must be made for N₂ washed out of the blood and tissue. For each minute of O₂ breathing, approximately 30–40 mL of N₂ is removed from blood and tissue. N_2tissue = 0.04 times T (where T is the time of the test). This value is subtracted from the total volume of N₂ washed out.

The original N₂ washout technique lasted 7 minutes. However, not all of the N₂ in the lungs may be washed out, even after 7 minutes of 100% O₂ breathing. The F_An_2alveolar2 is measured at the end of the test and subtracted from the initial N₂ concentration. Correction for the “switch-in” error should also be made (see Correcting for the Switch-in Error). The final FRC is then corrected to BTPS, and volume of the equipment dead space (including filters) must be subtracted (see the Sample Calculations on Evolve at http://evolve.elsevier.com/Mottram/Ruppel/).

To obtain RV, the ERV measured immediately after the acquisition of FRC as a “linked” maneuver (i.e., without the patient coming off the mouthpiece), is subtracted from the FRC:

RV=FRC−ERV(expiratory reserve volume)

To obtain TLC, the calculated value for RV is added to the “linked” inspiratory vital capacity (IVC): TLC = RV + IVC

Some available commercial systems use a rapid N₂ analyzer in combination with a spirometer to provide a “breath-by-breath” analysis of expired N₂ (Figure 4-1, A). An alternative approach is to use fast-response O₂ and carbon dioxide (CO₂) analyzers to calculate the concentration of N₂ in expired gas during the washout:

Figure 4-1 Open-circuit and closed-circuit FRC systems.
**(A)** Open-circuit equipment used for N₂ washout determination of FRC. The patient inspires O₂ from a regulated source and exhales past a rapidly responding N₂ analyzer into a pneumotachometer. FRC is calculated from the total volume of N₂ exhaled and the change in alveolar N₂ from the beginning to the end of the test (Figure 4-2 and open-circuit method); **(B)** Closed-circuit equipment used for He dilution FRC determination includes a directional breathing circuit with a volume-based spirometer, He analyzer, CO₂ absorber, O₂ source, and water absorber. A breathing valve near the mouth allows the patient to be “switched in” to the system after He has been added and the system volume determined. Tidal breathing and the He dilution curve are displayed on the computer.

N2=1−FEO2−FECO2

where:

F_Eo₂ = fraction of O₂ in expired gas (dry)

F_Eco₂ = fraction of CO₂ in expired gas (dry)

In these fast methods, the patient, wearing a noseclip, breathes through a mouthpiece-valve system. Precisely at end-expiration, a valve is opened to allow 100% O₂ breathing to begin. Each breath of 100% O₂ washes out some of the residual N₂ in the lungs. Analog signals proportional to N₂ concentration and volume (or flow) are integrated to derive the volume of N₂ exhaled for each breath. Values for each breath are summed to provide a total volume of N₂ washed out (Figure 4-2). The test is continued until the N₂ in alveolar gas has been reduced to approximately 1% (Criteria for Acceptability 4-1). Some older systems terminate the test at 7 minutes. However, the O₂ breathing should be continued until alveolar N₂ falls to less than 1.5% for at least three consecutive breaths. A change in inspired N₂ concentration of greater than 1% or sudden large increases in expiratory N₂ concentrations indicate a leak, in which case the test should be stopped and repeated.

Criteria for acceptability 4-1 N₂ Washout FRC

1. The washout tracing or display should indicate a continually falling concentration of alveolar N₂.

2. The test should be continued until the N₂ concentration falls to 1.5%.

3. Washout times should be appropriate for the type of subject tested. Healthy subjects should wash out N₂ completely within 3–4 minutes.

4. The washout time should be reported. Failure to wash out N₂ within 7 minutes should be noted.

5. Multiple measurements should agree within 10%; the average FRC from acceptable trials should be used to calculate lung volumes. At least 15 minutes of room-air breathing should elapse between repeated trials.

Figure 4-2 Open-circuit (N₂washout) determination of FRC.
The concentration (or log concentration) of N₂ is plotted against time or against the volume expired as the patient breathes through a circuit (Figure 4-1, A). The volume of N₂ expired with each breath is measured by integrating flow and N₂ concentration to determine the area under each curve (see inset). FRC is determined by dividing the volume of N₂ expired by the change in alveolar N₂ from the beginning to the end of the test, with corrections, as described in the text.

At least one technically satisfactory FRC_n2 determination should be made. If additional washouts are performed, a waiting period of at least 15 minutes is recommended to allow normal concentrations of N₂ to be reestablished in the lungs, blood, and tissues. If more than one FRC measurement is obtained, the mean of the technically acceptable results that agree within 10% should be reported.

Some pulmonary function systems use pneumotachometers that may be sensitive to the composition of expired gas. These devices correct for changes in the viscosity of the gas as O₂ replaces N₂ in the expirate. Such corrections are easily

4-1 How To…

Perform Nitrogen Washout Maneuver

1. Tasks common to all procedures.

a. Calibrate and prepare equipment.

b. Review order.

c. Introduce yourself and identify patient according to institutional policies.

d. Describe and demonstrate procedure.

2. Arrange the chair so that the patient is at a comfortable height relative to the mouthpiece, sitting up straight.

3. Noseclips on; mouthpiece in mouth, tight seal with lips.

4. At the end of a normal exhalation, after about four steady breaths, open the valve to begin breathing 100% oxygen.

5. Continue relaxed breathing until end-tidal N₂ concentration is less than 1.5% for three consecutive breaths or for 7 minutes total.

6. Perform SVC maneuver from FRC; either ERV to RV followed by inhaled VC to TLC, or IC to TLC followed by exhaled VC to RV.

7. At least one technically acceptable trial is required; if needed, you may repeat after a resting period on room air of at least 15 minutes; FRC should agree within 10%.

8. Review data, and note comments about test quality.

accomplished by software or electronic correction of the analyzer output.

Closed-Circuit, Multiple-Breath Helium Dilution

FRC can also be determined by equilibrating the gas in the lungs with a known volume of gas containing He (closed-circuit, multiple-breath helium dilution [FRC_he]). A spirometer is filled with a known volume of air, and then a volume of He is added so that a concentration of approximately 10% is achieved (see Figure 4-1, B). The exact concentration of He and spirometer volume is measured and recorded before the test is begun. The patient breathes through a valve that allows a connection to a rebreathing system. The valve is opened at the end of a quiet breath (i.e., the end-expiratory level). Then the patient rebreathes the gas in the spirometer, with a CO₂ absorber in place, until the concentration of He falls to a stable level (Figure 4-3). A fan or blower mixes the gas within the spirometer system. O₂ is added to the spirometer system to maintain the Fio₂ near or above 0.21 and to keep the system volume relatively constant.

Figure 4-3 Closed-circuit (He dilution) determination of FRC.
At the beginning of the test, the patient’s lungs contain no He. The spirometer contains a known concentration of He in a known volume (see text). The patient then rebreathes the He mixture from this system (Figure 4-1, B). He is diluted until equilibrium is reached. At the end of the test, the known volume of He has been diluted in the rebreathing system and the lungs. FRC is calculated from the change in He concentration and the known system volume. The patient must be switched from breathing air to the He mixture at the end-expiratory level for accurate measurement of FRC. RV is derived by subtracting the ERV. (Modified from Comroe JH Jr, Forster RE, Dubois AB, et al. The lung: Clinical physiology and pulmonary function tests. 2nd ed. St. Louis: Mosby; 1962.)

An older method (i.e., the bolus method) added a large volume of O₂ to the spirometer at the beginning of the test. The patient then rebreathed and gradually consumed the O₂. Because of the possibility of equilibrium not being attained before the added O₂ was depleted, this method is no longer used.

Equilibration between normal lungs and the rebreathing system takes place in approximately 3 minutes when a 10% He mixture in a system volume of 6–8 L is used (Figure 4-4). The final concentration of He is then recorded. The system volume is computed first. System volume is the volume of the spirometer, breathing circuitry, and valves before the patient is connected. It can be calculated as follows:

Figure 4-4 Computer-generated recording of closed-circuit, multiple-breath FRC determination in a healthy patient.
Graph shows He concentration from the beginning of rebreathing until equilibrium is achieved *(upper line)*. System volume of the spirometer and the patient’s tidal breathing is also shown (*lower line*). A CO₂ absorber removes carbon dioxide produced by the patient. A computerized valve system replaces O₂ to keep the system volume constant. Measurements of He concentrations and system variables are also displayed.

System volume(L)=Headded(L)FHe initial

where:

He_added = volume of He placed in the spirometer in liters (L)

F_{he initial} = % He converted to a fraction (%He/100)

When the system volume is known, FRC can be computed as follows:

FRC=%Heinitial−%Hefinal%Hefinal×System volume

Either percent or fractional concentration of He may be used because the FRC calculation is based on a ratio.

Some automated systems use a similar method to calculate the system volume; a small amount of He is added to the closed system, followed by a known volume of air. The change in He concentration after the addition of the air is used to determine the system volume. Rebreathing is continued until the He concentration changes by no more than 0.02% in 30 seconds (Criteria for Acceptability 4-2).

Criteria for acceptability 4-2 He Dilution FRC

1. A tracing or display of spirometer volume should indicate that no leaks are present (system baseline flat). He concentration should be stable before testing.

2. The rebreathing pattern should be regular. If recorded, successive tidal breaths should show a gradually falling end-tidal level as O₂ is consumed. The addition of O₂ should return breathing to close to the system baseline.

3. The test should be continued until the He readings change by less than 0.02% in 30 seconds or until 10 minutes have elapsed.

4. The addition of O₂ should be appropriate for quiet tidal breathing (i.e., 200–400 mL/min).

5. The He equilibration curve, if plotted or displayed, should show a smooth and regular fall of He concentration until equilibrium is achieved.

6. Multiple measurements of FRC should agree within 10%; the average of acceptable multiple measurements should be reported.

4-2 How To…

Perform Helium Dilution Maneuver

1. Tasks common to all procedures.

a. Calibrate and prepare equipment.

b. Review order.

c. Introduce yourself and identify patient according to institutional policies.

d. Describe and demonstrate procedure.

2. Arrange the chair so that the patient is at a comfortable height relative to the mouthpiece, sitting up straight.

3. Noseclips on; mouthpiece in mouth, tight seal with lips.

4. At the end of a normal exhalation, after approximately four steady breaths, open the valve to begin closed-circuit rebreathing.

5. Continue relaxed breathing until helium concentration stabilizes at a minimum level, no more than 0.02% in 30 seconds or a maximum of 10 minutes.

6. Perform SVC maneuver from FRC; either ERV to RV followed by inhaled VC to TLC, or IC to TLC followed by exhaled VC to RV.

7. At least one technically acceptable trial is required; if needed, you may repeat after a resting period on room air of at least 5 minutes; FRC should agree within 10%. (See Criteria for Acceptability Box 4-2).

8. Review data, and note comments about test quality.

At least one technically satisfactory measurement should be obtained. If additional dilutions are performed, a waiting period of at least 5 minutes is recommended between repeated tests. If more than one measurement of FRC_he is obtained, it is recommended that the mean of the technically acceptable results that agree within 10% be reported.

Although a small volume of He dissolves in the blood during the test, it results in a negligible increase in FRC, and it is recommended that no correction be made. The volume of the equipment dead space (including filters) must also be subtracted from the measured FRC.

Correcting for the “Switch-In” Error

Most manufacturers provide “switch-in” error correction when the patient begins the test at a point either above or below the actual end-expiratory level (FRC). Depending on the patient’s breathing pattern, a volume difference of several hundred milliliters may result. The effect of the switch-in error may be insignificant, especially with the closed-circuit method. Equilibrium does not occur instantaneously at switch-in. The total volume of spirometer and lungs is constantly changing with tidal breathing, removal of CO₂, and the addition of O₂. If the switch-in error is large or the end-expiratory level appears to change during the maneuver, the test may need to be repeated.

Additional Comments on FRC by Gas Dilution Techniques

In the gas dilution techniques, RV is measured indirectly as a subdivision of the FRC. This method is preferred because the resting end-expiratory level depends less on patient effort than on maximal inspiration or expiration. The end-expiratory level (and the ERV) must be accurately measured. If tidal breathing is irregular, ERV may be overestimated or underestimated. Subtraction of an ERV value that is too large from the FRC will cause the RV to appear smaller than it actually is. Similarly, a small ERV will produce a larger than actual RV. The patient’s tidal breathing pattern must be carefully monitored during the VC measurement.

The accuracy of the gas dilution techniques depends on all parts of the lung being well ventilated. In patients who have an obstructive disease, some lung units are poorly ventilated. In these patients, it is often difficult to wash N₂ out or mix He to a stable level in poorly ventilated parts of the lungs. Thus, FRC, RV, and TLC may all be underestimated, usually in proportion to the degree of obstruction. Extending the time of these tests improves their accuracy. However, prolonging the test may not measure completely trapped gas, as found in bullous emphysema.

The graphic method of displaying breath-by-breath N₂ washout provides a means of quantifying the evenness of ventilation. Some systems plot the logarithm of the N₂ concentration against time or volume exhaled. The slope of the washout curve is determined by the FRC, tidal volume, dead space volume, and frequency of breathing. If N₂ is washed out of the lungs evenly, the log N₂ plot appears as a straight line. Because the lung is not perfectly symmetric, the washout curve is slightly concave. The deviation from the expected curve indicates the extent to which ventilation is uneven. Washout should be complete within 3–4 minutes in healthy patients. The time to reach He equilibrium during the closed-circuit FRC determination can also be used as an index of distribution of ventilation. By simply recording the time to reach equilibrium and plotting the dilution curve, an estimate of the evenness of ventilation is obtained. Use of gas dilution techniques to assess the distribution of ventilation is more commonly performed using radioisotope imaging and CT scanning.

PF Tip 4-1

The gas dilution techniques of measuring lung volumes usually underestimate lung volumes in the presence of moderate or severe obstruction. Both methods (N₂ washout and He dilution) are also subject to leaks in their respective breathing circuits. Leaks usually cause the measured lung volume to be overestimated.

In either of the gas dilution techniques, a leak will cause erroneous estimates of FRC. Leaks may occur in breathing valves or circuitry, or at the patient connection. Some patients have difficulty maintaining an adequate seal at the mouthpiece throughout the test. Failure to properly apply noseclips can also result in a leak. Leaks usually result in an overestimate of lung volume. A leak in the open-circuit N₂ washout system allows room air to enter, increasing the volume of N₂ washed out. A leak in the closed-circuit He dilution system allows air to dilute the He concentration or He to escape. Each situation causes the test gas concentration to change more than it should. Leaks during the N₂ washout can usually be identified by an inspection of the graphic display or recording (Figure 4-5). Inaccuracy or malfunction of the gas analyzers in either method often causes errors. Leaks or analyzer problems should be considered whenever FRC values are inconsistent with spirometry results (See Interpretive Strategies Box 4-1).

Interpretive strategies 4-1 Dilutional Lung Volumes

1. Was the FRC determination performed acceptably? Were multiple trials performed? If so, were they within 10%?

2. Was the VC maneuver acceptable? Were the ERV and IC measurements within 5% or 60 mL?

3. Were other lung volumes calculated appropriately (TLC, RV)?

4. Are the reference values appropriate? Age, height, sex, race?

5. Is the TLC less than the lower limit of normal? If so, restriction is present. Are other lung volumes (FRC, RV) reduced in similar proportion?

6. Is the TLC greater than the upper limit of normal? If so, suspect hyperinflation.

7. Is the RV/TLC ratio greater than predicted (approximately 35%)? Is the TLC normal or increased? If both are true, suspect air trapping.

8. Are lung volumes consistent with spirometric findings in regard to obstruction or restriction? Are they consistent with the history and physical findings?

9. Are additional tests indicated (plethysmographic lung volumes)?

Figure 4-5 Open-circuit, multiple-breath N₂ washout tracings.
**(A)** A computer-generated recording of a nitrogen (N₂) washout test in a healthy patient. The tracings show a continuous decrease in end-tidal N₂ concentration with successive breaths. The test is continued until the N₂ concentration falls to less than 1%. **(B)** A similar plot of N₂ washout from a healthy patient, but, in this instance, a leak occurs during the test. Leaks may occur if the patient does not maintain a tight seal at the mouthpiece. Leaks are usually easy to detect because room air enters the system and causes an abrupt increase in N₂ concentration.

Body Plethysmography

FRC measured with the body plethysmograph (FRC_pleth) (Figure 4-6) refers to the volume of intrathorax gas measured when airflow occlusion occurs at end-expiration (FRC) during shutter closure. The technique is based on Boyle’s law relating pressure to volume. A volume of gas varies in inverse proportion to the pressure to which it is subjected if the temperature remains constant (isothermal). The patient has an unknown volume of gas in the thorax at the end of a normal expiration (i.e., the FRC). The airway is occluded momentarily at or near FRC; the patient is asked to gently pant at a frequency between 0.5 and 1.0 Hz (0.5–1.0 cycles per second), allowing the air in the chest to be compressed and decompressed. This causes a change in volume and pressure. The changes in pressure are easily measured at the mouth (P_mouth) with a pressure transducer. Mouth pressure theoretically equals alveolar pressure when there is no airflow. Changes in pulmonary gas volume are estimated by measuring pressure changes in the plethysmograph. The pressure in the plethysmograph is measured by a sensitive transducer. This transducer is calibrated by introducing a small, known volume of gas into the sealed box and relating the pressure change to the known volume (P_box). The calibration factor is then applied to measurements made on human patients.

Figure 4-6 Components of the body plethysmograph used to measure thoracic gas volume (V_TG).
Boyle’s law states that volume varies inversely with pressure if temperature is held constant. A pressure-type (constant volume) plethysmograph, with pressure transducers for measurements of box pressure and mouth (alveolar) pressure, is shown. A pneumotachometer measures flow to track lung volumes. The mouth shutter occludes the airway momentarily so alveolar pressure can be estimated. The patient pants gently against the closed shutter. Gas in the lungs is alternately compressed and decompressed. Changes in lung volume are reflected by changes in box pressure. These changes are displayed as a sloping line on a computer display. When the original pressure (P), the new pressure (P +ΔP), and the new volume (V +ΔV) are known, the original volume (V or V_TG) can be computed from Boyle’s law (see the section on the technique of thoracic gas volume and see Evolve at http://evolve.elsevier.com/Mottram/Ruppel/).

The display of the panting maneuver is a graph with P_mouth and P_box. P_mouth is plotted on the vertical axis, and P_box is plotted on the horizontal axis (Figure 4-7). The resulting figure appears as a sloping line equal to DP/DV, where DP equals change in alveolar pressure and DV equals change in alveolar volume. Change in alveolar volume is measured indirectly by noting the reciprocal change in plethysmograph volume.

Figure 4-7 Normal and abnormal plethysmograph recordings.
*Top left,* Normal closed-shutter maneuver in which mouth pressure is plotted against box pressure. The loop should be closed, or nearly so. If thermal equilibrium has not been achieved, the loop (shown on the *top right*) tends to be open and to drift across the screen. *Bottom left,* Normal open-shutter measurement in which flow is plotted against box pressure. If the patient pants near FRC, the loop takes on a nearly closed S-shaped appearance. If the patient pants too rapidly or too deeply, the tracing becomes open and flattened (shown on the *bottom right*). Thermal drift can also cause the open-shutter tracing to resemble excessive patient effort.

The V_TG can then be obtained from the slope of the tracing by applying a derivation of Boyle’s law:

VTG=PBλVTG×PBOXcalPMOUTHcal×K

where:

V_TG = thoracic gas volume

P_B = barometric pressure minus water vapor pressure

^λV_TG = slope of the displayed line equal to DP/DV

P_boxcal = box pressure transducer calibration factor

P_mouthcal = mouth pressure transducer calibration factor

K = correction factor for volume displaced by the patient

For the complete derivation of the equation and sample calculations, see Evolve (http://evolve.elsevier.com/Mottram/Ruppel/).

Computerized plethysmograph systems permit the monitoring of tidal breathing in conjunction with the V_TG maneuver. Instantaneous changes in

4-3 How To…

Perform Body Plethysmography (“Body Box”) Maneuvers

1. Task common to all procedures

a. Calibrate and prepare equipment.

b. Review order.

c. Introduce yourself and identify patient according to institutional policies.

d. Describe and demonstrate procedure.

2. Place the patient in a body box; adjust height of mouthpiece so the patient is sitting up straight; noseclips on; mouthpiece in mouth, tight seal with lips.

3. Patient can practice breathing and panting maneuvers.

4. Close door; allow temperature stabilization.

5. Have the patient place hands on cheeks and breathe normally.

6. At end of a normal exhalation, after approximately four steady breaths, begin open shutter panting; frequency = 1.5-2.5 breaths per second. Demonstrate and say “in-out-in-out” or “1-2-1-2-1-2.” This will allow the determination of Raw.

7. Warn the patient that the shutter is about to close, then close the shutter and continue to coach panting at a slightly slower rate (0.5-1 breath per second). This will allow the determination of TGV and sGaw.

8. Alternatively, after recording open then closed shutter panting, repeat closed shutter panting alone to obtain TGV.

9. Open shutter, perform SVC maneuver from end-exhalation; either ERV to RV followed by inhaled VC to TLC (ATS-ERS preferred method), or IC to TLC followed by exhaled VC to RV (ATS-ERS alternative method).

10. Repeat maneuver until you obtain 3-5 technically acceptable trials; FRC (TGV) should agree within 5%. (See Criteria for Acceptability 4-3)

11. Open door; test ends.

12. Review data, and make adjustments in slopes if needed.

13. Note comments about test quality.

lung volume can be monitored by continuously integrating the flow through the plethysmograph’s pneumotachometer. The end-expiratory level can be determined from tidal breathing. The patient then pants with the mouth shutter open. The computer records the change in lung volume above or below the resting level (FRC). When asked to pant, most patients do so slightly above FRC. The shutter then closes automatically, the patient continues panting, and V_TG is measured as described. The computer then adds or subtracts the change in volume from the end-expiratory level (before panting began) to calculate the true FRC. This computerized technique allows the patient to pant at the correct frequency and depth before the shutter is closed. It also eliminates the necessity of closing the shutter precisely at end-expiration. Raw and sGaw can also be measured simultaneously during the open-shutter panting (see following text). It should be noted that the correct panting frequencies for measuring Raw and V_TG are slightly different. Raw measurements should be made with the patient panting at about 1.5–2.5 Hz (1.5–2.5 cycles per second, or 90–150 breaths/min), whereas V_TG should be measured with the patient panting at a slower rate of 0.5–1 Hz. Many computerized systems display the panting frequency so the technologist can coach the patient to achieve the correct rate.

A VC maneuver along with its subdivisions (ERV and IC) should be performed immediately after the acquisition of FRC_pleth. The ATS-ERS (American Thoracic Society-European Respiratory Society)Task Force recommends that a linked ERV maneuver to RV followed by an IC maneuver to TLC be performed. An alternative method is to perform linked maneuvers with an IC first to TLC followed by a VC breath to RV. Most plethysmograph systems allow these measurements using the built-in pneumotachometer. The same standards for accuracy should be applied to a slow VC measured in the plethysmograph as for any other spirometer. When FRC_pleth has been determined, the remaining lung volumes can be calculated as described for the gas dilution techniques.

Measurement of FRC_pleth is a complex procedure. Each patient must be carefully instructed in the required maneuvers. Allowing the patient to sit in the box with the door open is helpful. A few patients may experience claustrophobia in the plethysmograph. The panting maneuver should be demonstrated by the technologist and then practiced by the patient. The patient should be instructed to place both hands against the cheeks. This prevents unwanted pressure changes in the mouth when the patient pants against the closed shutter. If practical, the shutter may be closed so that the patient knows what to expect during the test. The door of the plethysmograph can then be closed. The patient should understand that the plethysmograph can be opened if he or she becomes uncomfortable. Most systems allow the patient to open the door from within the box. If the plethysmograph is equipped with a communication device, it should be adjusted so the patient can hear instructions.

Depending on the construction of the plethysmograph, venting to the atmosphere is usually required to establish thermal equilibrium. Equilibrium takes about 30-60 sec and can be presumed when the flow-volume recording stabilizes (i.e., does not drift). The patient is then instructed to pant. When the correct panting frequency and depth have been established, the shutter may be closed. Some plethysmograph systems require tidal breathing before shutter closure to establish the patient’s end-expiratory level. In either type of system, the patient should pant against the closed shutter until a stable tracing is produced. Two to four pants are usually sufficient. If the patient pants too hard, the tracing may drift or appear as an “open” loop (see Figure 4-7). The recorded pressure changes should be within the range over which the transducers were calibrated. The entire tracing should be visible on the display. If the tracing goes off-screen, the pressure changes probably exceed the calibration ranges. Three or more technically satisfactory maneuvers should be recorded, each followed by ERV and IVC maneuvers (Criteria for Acceptability 4-3). At least three FRC_pleth values that agree within 5% should be obtained and the mean value reported. The technologist’s comments should note the acceptability of the maneuvers. Most computerized plethysmographs automatically measure the slope of the DP/DV tracings. This is done by using the least-squares method to calculate a “best-fit” line through the recorded data points. The technologist may need to correct computer-generated tangents, depending on data quality from the panting maneuvers.

Criteria for acceptability 4-3 FRC_pleth

1. The panting maneuver shows a closed loop without drift or other artifact.

2. Pressure changes are within calibration ranges; the tracing does not go off-screen.

3. Panting frequency should be between 0.5 and 1.0 Hz.

4. A series of three to five technically satisfactory panting maneuvers should be recorded.

5. At least three FRC_pleth values that agree within 5% should be obtained.

6. Reported FRC_pleth is averaged from the three acceptable and repeatable panting maneuvers.

Additional Comments on FRC by Plethysmography

The plethysmographic method is a quick and accurate means of measuring lung volumes. It can be used in combination with simple spirometry to derive all lung volume compartments. The plethysmograph’s primary advantage is that it measures all gas in the thorax, whether in ventilatory communication with the atmosphere or not. The plethysmographic measurement of FRC is often larger than that measured by He dilution or N₂ washout. This is the case in emphysema and other diseases characterized by air trapping, as well as in the presence of an uneven distribution of ventilation. When gas dilution tests are continued for more than 7 minutes, the results for FRC determinations approach the V_TG value. See Interpretive Strategies 4-2.

Interpretive strategies 4-2 FRC_pleth

1. Were the panting maneuvers performed acceptably? Was the panting frequency appropriate (0.5–1.0 Hz)? If not, interpret results cautiously.

2. Were at least three maneuvers averaged to obtain FRC? Were individual values repeatable (within 5%)? If not, interpret cautiously.

3. Are reference values appropriate? Age, height, weight, race? Were they obtained plethysmographically?

4. Were other lung volumes (TLC) calculated appropriately? Were VC, ERV, and IC acceptable? If not, evaluate FRC only.

5. If the TLC is less than the lower limit of normal, suspect restriction.

6. If the TLC is greater than the upper limit of normal, suspect hyperinflation.

7. Are lung volumes consistent with spirometric findings? If not, evaluate carefully for combined obstruction and restriction.

PF Tip 4-2

Plethysmography offers several advantages over other methods of measuring lung volumes. FRC_pleth is less affected by the distribution of ventilation. Multiple measurements can be made quickly and averaged. It provides a more accurate estimate of lung volumes in patients who have airway obstruction. In addition, Raw, sGaw, and sRaw can be measured in the same testing session.

It is often useful to compare FRC values obtained by plethysmography with values obtained by gas dilution methods, particularly in patients with obstructive disease. The ratio of FRC_pleth/FRC_n2 or FRC_pleth/FRC_he can be used as an index of gas trapping. This ratio is usually near 1.0 in patients with normal lungs, or even those with a restrictive lung disorder. Values greater than 1.0 indicate gas volumes detectable by the plethysmograph but hidden to the gas dilution techniques. Care must be taken that lung volumes determined by the two separate methods are reliable before the values can be expressed as a ratio. Patients with severe bullous emphysema may have a difference in TLC of more than 1 L between FRC_pleth and the gas dilutional methods of measuring FRC.

Some evidence suggests that in severe airway obstruction, FRC may actually be overestimated when the plethysmographic technique is used. This occurs primarily because P_mouth (measured when the shutter is closed) may not equal alveolar pressure if the airways are severely obstructed. Rapid panting rates aggravate this inaccuracy. Care should be taken that patients with spirometric evidence of obstruction pant at a rate of 0.5–1 Hz.

Spirometry (e.g., FVC, FEV₁, and VC) may be performed with the patient in the plethysmograph. The pneumotachometer must be capable of accurately measuring the entire range of gas flows required (i.e., up to 12 L/sec).

Two varieties of body boxes are commonly used: constant-volume and flow-based (see Chapter 11). For constant-volume plethysmographs, spirometry is done with the door open. Flow-based plethysmographs have the advantage of allowing forced spirometry with the door closed. Flow boxes also allow a slightly different type of flow-volume curve to be recorded. Normal spirometry plots airflow at the mouth against volume at the mouth (as detected by the spirometer). With the patient in a flow box, flow at the mouth can be plotted against actual lung volume changes as detected by the box. This may be particularly useful in patients with severe airway obstruction. It is possible to detect a significant compression volume during forced expiration and plot it against the flow generated. Spirometry, lung volumes, and airway resistance can all be obtained in a single sitting with either type of plethysmograph.

Total Lung Capacity and Residual Volume/Total Lung Capacity Ratio

TLC is calculated combining other lung volume measurements once FRC is measured by either a gas dilution method or body plethysmography. The two most common methods to calculate TLC are as follows:

TLC=RV+VC

TLC=FRC+IC

Each method requires accurate measurement of the subdivisions of the VC. TLC can also be calculated with single-breath techniques (i.e., single-breath He dilution or single-breath N₂ washout). Single-breath measurements of lung volumes are usually done as part of other tests, such as the diffusing capacity (Dlco) test (see Chapter 3). Single-breath lung volumes correlate well with multiple-breath techniques in healthy patients. However, single-breath lung volumes tend to underestimate true values in moderate to severe obstruction. TLC can also be measured from standard chest x-ray films, as well as from CT scans of the thorax.

The RV/TLC ratio is calculated by dividing the RV by the TLC. This ratio is expressed as a percentage. Either ATPS (ambient temperature, pressure, saturation) or BTPS values may be used in the ratio, but both RV and TLC must be expressed in the same units.

The FRC, RV, and TLC should be reported in liters or milliliters, BTPS. Barrier filters may be used during lung volume determinations, particularly in rebreathing systems. If a filter is used, its volume must be subtracted from the lung volume measured.

Significance and Pathophysiology

FRC varies with body size, with change in body position, and with time of day (i.e., diurnal variation). However, unlike TLC and RV, FRC is an effort-independent maneuver that is determined by the balance of lung and chest wall recoil at relaxed end-expiration. As with other lung volumes, normal FRC may be affected by racial or ethnic background (Interpretive Strategies 4-1 and 4-2). Equations for calculating predicted FRC can be found on Evolve (http://evolve.elsevier.com/Mottram/Ruppel/).

Increased FRC is considered pathologic. FRC values greater than approximately 120% of predicted values represent air trapping. Air trapping may result from emphysematous changes or from obstruction caused by asthma or bronchitis. Compensation for surgical removal of lung tissue or thoracic deformity can also cause increased FRC. Elevated FRC usually results in muscular and mechanical inefficiency of the respiratory apparatus. As lung volume increases, the chest wall and lungs themselves become “stiffer.” This causes an increase in the work of breathing. FRC can increase dynamically; patients with airway obstruction may increase their end-expiratory lung volume (EELV) during exercise. This change in lung volume with increased ventilatory demand often results in a sensation of breathlessness.

The TLC is the maximal volume of inspired air. TLC is determined by the balance of lung recoil plus chest wall recoil on the one hand, and muscle strength and effort on the other hand. Thus, diseases that reduce lung recoil will result in increased TLC; those that increase lung recoil may reduce TLC. Likewise, muscle weakness or suboptimal effort may reduce TLC.

The RV is the volume left in the lungs after the VC is exhaled. An increased RV indicates that, despite maximal expiratory effort, the lungs contain a larger volume of gas than normal. Increased RV often results in an equivalent decrease in VC (Figure 4-8). Elevated RV may occur during an acute asthmatic episode but is usually reversible. Increased RV is characteristic of emphysema and bronchial obstruction; both may cause chronic air trapping. RV and FRC usually increase together. As RV becomes larger, increased ventilation is needed to adequately exchange O₂ and CO₂ in the lung. This requires an increase in tidal volume, respiratory rate, or both. Because of altered pressure-volume characteristics of the lung, work of breathing is also increased. Patients with increased RV often display gas exchange abnormalities such as hypoxemia or CO₂ retention. Because RV is effort-dependent, elevated RV may also be caused by muscle weakness or suboptimal effort.

Figure 4-8 Lung volumes in normal, restrictive, obstructive, and neuromuscular weakness patterns.
A comparison of changes in lung volume compartments and VC *(superimposed)* shows the following: in restrictive patterns FRC, RV, and VC are all decreased proportionately, resulting in a decrease in the TLC, which defines restriction (see text). In obstruction (with air trapping), FRC and RV are both increased at the expense of the VC, and therefore TLC remains relatively unchanged. Similar increases in RV and FRC may occur without reduction of VC, in which case the TLC increases (hyperinflation). In neuromuscular weakness, the FRC is normal, but the TLC is reduced and the RV/TLC is too high.

Measuring lung volumes is critical to understanding changes in FVC. A normal FVC almost always excludes significant restriction. But, because decrements in FVC can be from restriction, neuromuscular weakness, suboptimal effort, or severe obstruction, knowledge of TLC, FRC, and RV is invaluable in sorting out these possibilities.

FRC, RV, and TLC are typically decreased in restrictive diseases. Decreased lung volumes are seen in interstitial diseases associated with extensive fibrosis (e.g., sarcoidosis, asbestosis, and complicated silicosis). Restrictive disorders affecting the chest wall include kyphoscoliosis, neuromuscular disorders, and obesity. Diseases that impair the diaphragm often result in reduced lung volumes, particularly TLC. Lung volumes may also be decreased in diseases that occlude many alveoli, such as pneumonia. Congestive heart failure causes pulmonary congestion, which can also reduce lung volume. Any disease process that occupies volume in the thorax can reduce lung volume. Examples include tumors and pleural effusions.

Table 4-2 lists comparative lung volumes for a healthy adult male and for patients with air trapping (as in emphysema), hyperinflation, restriction (as in sarcoidosis) and neuromuscular weakness (as in amyotrophic lateral sclerosis–ALS).

Table 4-2

Comparative Lung Volumes for a Healthy Adult Male and Patients with Air Trapping, Hyperinflation, Restriction, and Neuromuscular Weakness

Value	Normal	Air Trapping	Hyperinflation	Restriction	Neuromuscular Weakness
VC (L)	4.80	3.00	4.80	3.00	3.50
FRC (L)	2.40	3.60	3.60	1.50	2.40
RV (L)	1.20	3.00	3.00	0.75	1.50
TLC (L)	6.00	6.00	7.80	3.75	5.00
RV/TLC (%)	20	50	38	20	30

Restrictive processes usually cause lung volumes to be reduced equally. Proportional relationships between lung volume compartments, such as the RV/TLC ratio, may be relatively normal in restrictive diseases. In various forms of extrapulmonary restriction, such as obesity, kyphoscoliosis, or pleural effusion, RV is often less reduced than TLC, resulting in a relatively elevated RV/TLC ratio, despite a low TLC.

In obstruction, two different patterns may be observed. RV is usually increased. This increase may be at the expense of a reduction in VC (see Figure 4-6), with TLC remaining close to normal. In other cases, RV may increase while VC is preserved, so TLC is greater than predicted. The term air trapping is sometimes used to describe an increase in FRC and RV, and the term hyperinflation is used to describe the absolute increase in TLC. TLC may be either normal or increased in obstructive processes such as asthma, chronic bronchitis, bronchiectasis, cystic fibrosis, and emphysema. TLC does not appear to change dynamically, even though FRC may increase acutely during exercise.

In neuromuscular weakness, FRC is typically normal, because lung and chest wall recoil is not specifically affected. However, TLC is too low, and RV is too high, relative to FRC, because of the reduced muscle strength required to reach these upper and lower limits of lung volume, respectively.

Processes that occupy space in the lungs such as edema, atelectasis, neoplasms, or fibrotic lesions may decrease TLC. Other diseases that commonly result in decreased TLC include pulmonary congestion, pleural effusions, pneumothorax, or thoracic deformities. Pure restrictive defects show

PF Tip 4-3

Total lung capacity (TLC) is an important diagnostic tool in both obstructive and restrictive lung diseases. In restriction, the TLC is usually less than 80% of the predicted value, or below the lower limit of normal (LLN). In obstruction, the TLC is either normal or increased (hyperinflation).

proportional decreases in most lung compartments as described for FRC and RV. When the TLC value is less than 80% of predicted, or less than the 95% confidence limit, a restrictive process is present. Reduced VC, along with a normal or increased FEV₁/FVC ratio, is suggestive of restriction, but a measurement of TLC is needed to confirm the diagnosis of a restrictive defect.

The RV/TLC ratio describes the percentage of total lung volume that cannot be emptied during expiration. In healthy adults, the RV/TLC ratio may vary from 20% in young adults to 35% in older patients. Values greater than 35% may result from absolute increases of RV (as in emphysema) or from a decrease in TLC because of a loss of VC. A large RV/TLC in the presence of increased TLC is often indicative of hyperinflation. An increased RV/TLC with a normal TLC indicates that air trapping is present. As an indicator of air trapping, the RV/TLC ratio is a weak but statistically significant indicator of outcome after lung volume reduction surgery (LVRS).

A less common pattern is mixed obstructive-restrictive lung disease, characterized by a low FEV₁/FVC ratio and a low TLC. This is suggested by simple spirometry when both FEV₁/FVC and FVC are reduced, but can only be confirmed by measuring TLC. Measuring the reduction in TLC may allow for more accurate characterization of the concomitant degree of airway obstruction. Another interesting PFT pattern is the so-called non-specific pattern, characterized by a reduced FEV₁ and FVC, normal FEV₁/FVC ratio and normal TLC. This pattern may represent lung volume decruitment occurring in the presence of airways disease or thoracic restriction, such as obesity. Obesity is typically characterized by larger decrements in FRC and ERV than in TLC.

Airway resistance and conductance

Description

The forces governing maximal airflow are the elastic recoil pressure of the lung and airway resistance upstream from the equal pressure point. This section addresses the measurement of airway resistance, which is accomplished by examining the relationship between alveolar pressure and flow during open loop panting in the body plethysmograph.

Airway resistance (Raw) is the pressure difference per unit flow as gas flows into or out of the lungs. Raw is the difference between mouth pressure and alveolar pressure, divided by flow at the mouth. This pressure difference is caused primarily by the friction of gas molecules in contact with the airways. Raw is recorded in centimeters of water per liter per second (cm H₂O/L/sec).

Gaw is the flow generated per unit of pressure drop across the airways. It is the reciprocal of Raw (1/Raw) and is recorded in liters per second per centimeter of water (L/sec/cm H₂O). Gaw is not commonly reported because it changes with lung volume. Instead, specific airway conductance (sGaw), which is Gaw divided by the lung volume (in liters) at which the measurement was made, is usually calculated. It is reported in liters per second per centimeter of water per liter of lung volume (L/sec/cm H₂O/L). The lung volume at which Raw is measured is determined during the closed shutter maneuver to calculate V_TG. However, the closed shutter maneuver may be difficult to perform in some individuals, especially children, so V_TG cannot be measured. In these cases, flow can be related to the small shifts in lung volume that occur during tidal breathing to derive specific airway resistance, sRaw. Mathematically, sRaw = Raw × V_TG. Because sRaw encompasses lung volume, and Raw and lung volume vary inversely, sRaw is relatively stable with respect to changes in lung volume (i.e., as lung volume decreases, Raw increases, so the product sRaw remains the same, and vice versa). In this way, sRaw is similar to sGaw because it takes into account lung volume. However, whereas sGaw reflects Raw only, sRaw reflects both Raw and lung volume.

Technique

Raw can be measured as the ratio of alveolar pressure (P_A) to airflow (). Gas flow at the mouth is measured with a pneumotachometer (see Chapter 11). P_A is measured in the body plethysmograph (Figure 4-9, A). For gas to flow into the lungs during inspiration, P_A must fall below atmospheric pressure (mouth pressure). During expiration, P_A rises above atmospheric pressure. Changes in are plotted against plethysmograph pressure changes. Changes in plethysmograph pressure are proportional to alveolar volume changes. The patient pants with a small V_T at a rate of about 1.5–2.5 breaths/sec (1.5–2.5 Hz). Shallow, rapid breathing produces an S-shaped pressure-flow curve (see Figure 4-9, B). A tangent (the slope) is measured from this curve. Traditionally, the tangent is drawn to pass through zero flow and connects the +0.5 L/sec and –0.5 L/sec flow points. The slope of this line is the ratio of / P_box. is flow at the mouth, and P_box is plethysmograph pressure.

Figure 4-9 Measurement of airway resistance (Raw) using the body plethysmograph.
**(A)** diagrammatic representation of airway resistance measurement:
Raw = Atmospheric pressure-alveolar pressure/Flow
Flow () is measured directly by means of the pneumotachometer. As the patient pants with the shutter open, flow is plotted against box pressure (/P_box) as it occludes the airway momentarily, usually at end-expiration, and a sloping line representing the ratio of mouth pressure to box pressure (P_mouth/P_box) is recorded in a manner similar to that used for measurement of V_TG. In this example, the flow tracing (shutter open) and volume tracing (shutter closed) are superimposed on the computer display. The P_mouth/P_box tangent is measured as for the V_TG; **(B)** The flow tangent is measured from the steep portion of the flow tracing, from -0.5 to + 0.5 L/sec. Airway resistance is then calculated as the ratio of these two tangents, using appropriate calibration factors (see text and Evolve at http://evolve.elsevier.com/Mottram/Ruppel/).

Problems with the open-shutter panting technique are illustrated by the loops seen in Figure 4-10. The most common problems are thermal drift, or panting at too high or low a frequency or volume.

Figure 4-10 Normal and abnormal open shutter tracings.
**(A)** Normal tracing, showing narrow loop and consistent slope on both inspiration and expiration; **(B)** Small, narrow loop caused by panting frequency that is too high or panting volume that is too small; **(C)** Wide, large loop (hysteresis) caused by panting frequency that is too low or panting volume that is too large; **(D)** Thermal drift caused by the failure of body plethysmograph to come to a steady temperature before obtaining panting loops; **(E)** High inspiratory resistance, causing hysteresis and flattening of inspiratory portion of panting loop. Notice that separate inspiratory and expiratory tangents (slopes) can be obtained; **(F)** High expiratory resistance, causing hysteresis and flattening of expiratory portion of panting loop; **(G)** High inspiratory and expiratory resistance caused by fixed airway obstruction, causing flattening of both inspiratory and expiratory portions of panting loops; **(H)** Typical loop seen in asthma, with slight widening and flattening of the loop, indicative of overall increased resistance.

To measure V_TG in association with Raw, the shutter at the mouthpiece is closed immediately after the open-shutter panting, and the patient continues to pant. The optimal rate for closed-shutter panting is about 0.5-1.0 Hz, slightly slower than open-shutter panting. Changes in P_box are then plotted against P_mouth. Because there is no flow into or out of the lungs, P_mouth equals P_A. A second tangent is measured from this curve. The slope of this line is P_A/P_box, where P_A equals alveolar pressure. Computerized plethysmographs usually calculate a “best-fit” line to measure the open-shutter and closed-shutter tangents. The technologist should visually inspect all computer-fitted lines. The system should allow the technologist to adjust computer-generated tangents manually. Problems with the closed-shutter panting technique are similar to those that plague the open-shutter panting measurement, and also include leak around the mouth or nose.

Raw is then calculated by taking the ratio of these two slopes, as follows:

Raw=PA/PBOXV·/PBOX×Mouth calFlow cal

where:

= airflow

P_A = alveolar pressure

P_box = plethysmographic pressure, measured with the shutter open and closed

Mouth cal = calibration factor for the mouth pressure transducer

Flow cal = calibration factor for the pneumotachometer

This formula shows how Raw is calculated from the ratio of P_A and , as P_box cancels out. Calibration factors for the flow and mouth pressure transducers are included in the previous equation. (See the Sample Calculations on Evolve (http://evolve.elsevier.com/Mottram/Ruppel/).

Panting eliminates a number of artifacts from the tracing. Small rapid breaths (1.5–2.5 per second) reduce thermal drift, both in the box and in the pneumotachometer. Panting helps keep the glottis open, allowing measurement of alveolar pressure. Panting also allows measurements to be made near FRC. The resistances of the mouthpiece and pneumotachometer are subtracted from the patient’s Raw.

Gaw can be calculated as the reciprocal of Raw. Specific conductance is calculated by dividing Gaw by the lung volume at which it was measured, which is V_TG calculated from the closed shutter maneuver. sGaw should be calculated separately for each Raw maneuver because the lung volume at which measurements are made influences Raw and Gaw. After three to five acceptable trials are obtained, calculated Raw and sGaw are averaged. Individual values should be within approximately 10% of the mean (Criteria for Acceptability 4-4).

Criteria for acceptability 4-4 Raw and sGaw

1. Pressure-flow loops should be closed; pressure and flow should be within the calibrated range of the respective transducers.

2. Thermal equilibrium should be established; no drift during recording.

3. Panting frequency should be 1.5-2.5 Hz for each maneuver.

4. Raw and sGaw should be calculated for each maneuver; do not average tangents.

5. Mean of three or more acceptable efforts should be reported; individual values should be within 10% of mean.

sRaw is calculated from the ratio of change in box volume (measured through changes in box pressure) to flow during tidal breathing, which is done in practice by taking the slope of the flow-pressure (or specific resistance) loop. The slope may be drawn through the +0.5 L/s and -0.5 L/s fixed flow rates, or may be drawn between the points of maximal change in box pressure. Measurements should be made at breathing frequencies of 30-45 breaths per minute, which is usually near spontaneous breathing frequency for very young children. The final value is taken as the median of five technically valid specific resistance loops

Computerized plethysmographs permit thoracic gas volume (V_TG), Raw, and sGaw to be measured from a combined maneuver. The patient breathes through the pneumotachometer with the plethysmograph sealed. Tidal breathing is recorded with the patient breathing near FRC. The computer stores this end-expiratory volume as a reference point. Then the patient pants, and the open-shutter slope of /P_box is recorded. The mouth shutter is then closed and P_mouth /P_box is recorded as described previously. The V_TG in this maneuver does not equal the FRC because the shutter is closed at a volume different from the FRC. However, the change in volume from the tidal breathing level was stored at the beginning of the maneuver. This volume is the “switch-in” volume described earlier and can be added to or subtracted from the V_TG to determine FRC. Most patients pant above their FRC, so the V_TG in this combined method is usually slightly greater (Figure 4-11). Because of this issue, a separate test to measure FRC for use in calculating absolute lung volumes may need to be done and is often recommended.

Figure 4-11 Difference in V_TG and FRC measured in a combined maneuver.
On the left, a combined open followed by closed shutter maneuver was performed to obtain both Raw and V_TG. Inhalation is down and exhalation is up. Because V_TG drifted above FRC (downward in the tracing) during the open panting maneuver, V_TG is higher than FRC. V_TG, in this case, is still necessary to calculate sGaw, but it is a poor approximation of FRC. On the right, only a closed shutter maneuver was performed to measure V_TG, which closely approximates FRC. In this case, a separate open shutter maneuver is needed to measure Raw, followed by a closed shutter maneuver to calculate sGaw.
FRC, functional residual capacity; RV, residual volume; V_TG, thoracic gas volume; TLC, total lung capacity.

Significance and Pathophysiology

Normal values of Raw in adults range from 0.6 to 2.4 cm H₂O/L/sec. Gaw in healthy adults is between 0.42 and 1.67 L/sec/cm H₂O. sGaw varies in a manner similar to Gaw. sGaw values less than 0.15 to 0.20 L/sec/cm H₂O/L are consistent with airway obstruction. Measurements are standardized at flow rates of ±0.5 L/sec, as described previously. Normal values for sRaw in children are 9.2-17.3 cm H₂O/sec.

Raw in healthy adults is divided across the airway as follows:

Nose, mouth, upper airway = 50%

Trachea and bronchi = 30%

Small airways = 20%

Small airways (less than 2 mm in diameter) contribute only approximately one fifth of the total resistance to flow. Significant obstruction can develop in the small airways with little increase in Raw or decrease in sGaw. Early or mild obstructive processes are not usually identified by abnormal Raw or sGaw. Raw may be increased in an acute asthmatic episode by as much as three times the normal values. Inflammation, mucus secretion, and bronchospasm all increase Raw in the small and medium airways. Raw is increased in advanced emphysema because of airway narrowing and collapse, especially in the bronchioles. Other obstructive diseases (e.g., bronchitis) may cause increases in Raw proportionate to the degree of obstruction in medium and small airways.

Lesions obstructing the larger airways (e.g., tumors, traumatic injuries, or foreign bodies) may also cause a significant increase in Raw. Large airway obstruction is often accompanied by increased work of breathing and dyspnea on exertion. Airflow in the trachea and mainstem bronchi is predominately turbulent. Any large airway obstruction can exaggerate this turbulent flow. For this reason, Raw and SGaw may be more sensitive to elevations in central airway resistance than FEV₁. Breathing low-density gas mixtures (e.g., helium + oxygen) reduces Raw and therefore the work of breathing. The shapes of the open panting loops may give clues as to the underlying pathophysiology, as seen in Figure 4-10.

Because some patients have changes in sGaw without changes in FEV₁ or FVC after a bronchodilator, and such changes are thought to be clinically significant, measuring bronchodilator response by sGaw may be more sensitive than relying on spirometric indices only. Part of the reason for this may be caused by the anatomic location of airway obstruction, and part may relate to the bronchodilating ability of the deep breath necessarily associated with measuring spirometry, which occurs in healthy subjects and those with mild airway obstruction. Because of this increased sensitivity, sGaw may be less specific for patients with asthma compared to healthy subjects.

Raw is decreased at increased lung volume. The airways (particularly large and medium airways) are distended slightly, and their cross-sectional area increases. For this reason, the V_TG is obtained with Raw to derive sGaw. This allows a comparison of values in different patients or in the same patient after treatment. sGaw is particularly useful for assessing changes in airway caliber after bronchodilator therapy or inhalation challenge. SGaw may change significantly after a bronchodilator or inhalation challenge, even though other measures of flow (i.e., FEV₁) vary only slightly. The primary site of airway obstruction (i.e., large versus small airways) may determine which parameters reflect changes in airway caliber.

Raw and sGaw measurements are not influenced by the degree of patient effort, so Raw and sGaw measurements may be useful for determining airway status in patients who are unable or unwilling to exert maximum effort. However, acceptable panting maneuvers in the plethysmograph require a certain degree of patient coordination. Not all patients may be able to perform these maneuvers. Patients with severe obstruction may produce pressure-flow curves during panting that are difficult to measure. Such curves may be flat (see Figure 4-10) or show widening (hysteresis). Inspiratory and expiratory flows may produce different resistances, causing the curve to appear as a loop (see Figure 4-10). In these cases, inspiratory flow resistance is usually reported.

In addition to resistance caused by flow through conducting airways, some frictional resistance is caused by the displacement of the lungs, rib cage, and diaphragm. In healthy patients, this tissue resistance is only approximately one fifth of the total resistance, and therefore total pulmonary resistance is approximately 20% greater than the measured Raw.

In addition to measuring Raw by use of the body plethysmograph, measurement of Raw by means of the forced oscillation technique is becoming increasingly common. For further discussion, see Chapter 10.

Gas distribution tests: single-breath nitrogen washout, closing volume, and closing capacity

Description

The single-breath nitrogen washout test (SBN₂) measures the distribution of ventilation. Distribution is analyzed by measuring the change in N₂ concentration during expiration of the VC after a single breath of 100% O₂. Evenness of distribution is assessed by two parameters: the change in percentage of N₂ between the 750- and 1250-mL portion of the SBN₂ test (D%N_{2 750–1250}) and the slope of phase III of the expiratory tracing. Each of these indices is recorded as a percent. Closing volume (CV) is the portion of the VC that can be exhaled from the lungs after the onset of airway closure. CV is also measured from the SBN₂ maneuver and is usually expressed as a percentage of the VC. A related measurement, closing capacity (CC), is the sum of the CV and RV. CC is expressed as a percentage of the TLC.

Technique

The test is performed with equipment similar to that used for the open-circuit FRC_n2 determination (see Figure 4-1, A). The patient exhales to RV, then inspires a VC breath of 100% O₂ from a reservoir or demand valve. The 100% O₂ dilutes the N₂ present in the lungs. Without holding the breath, the patient exhales slowly and evenly at a flow of 0.3–0.5 L/sec. The N₂ concentration is measured by an N₂ analyzer, while the exhaled volume is measured by the spirometer. Volume expired is plotted against N₂ concentration on a graph (Figure 4-12). This washout curve can be divided into four phases:

Figure 4-12 Single-breath nitrogen elimination (SBN₂).
A plot of increasing N₂ concentration on expiration after a single VC breath of 100% O₂. The curve is divided into four phases. Phase I is the extreme beginning of the expiration, when only O₂ is being exhaled. Phase II shows an abrupt rise in N₂ concentration as mixed bronchial and alveolar air is expired. Phase III is the alveolar gas plateau. N₂ concentration changes slowly as long as ventilation is uniformly distributed. Phase IV is an abrupt increase in N₂ concentration as basal airways close and a larger proportion of gas comes from the N₂–rich lung apices. Several useful parameters are derived from the SBN₂ tracing. The N_{2 750–1250} and slope of phase III are indices of the evenness of ventilation distribution. CV can be read directly from the onset of phase IV until RV is reached; VC can also be read directly. RV, TLC, and CC can be calculated if the area under the curve is determined either by planimetry or electronic integration (see text).

Phase I: upper airway gas from the anatomic dead space (V_Danat), consisting of 100% O₂

Phase II: mixed dead space gas in which the relative concentrations of O₂ and N₂ change abruptly as the V_Danat volume is expired

Phase III: a plateau caused by the exhalation of alveolar gas in which relative O₂ and N₂ concentrations change slowly and evenly

Phase IV: an abrupt increase in the concentration of N₂ that continues until RV is reached

The initial 750 mL of expired gas contains dead space gas from phases I and II and is not used to assess distribution of ventilation. The difference in N₂ concentration between the 750-mL and 1250-mL points is called the N₂ (%N_{2 750–1250}).

The slope of phase III is the change in N₂ concentration from the point at which 30% of the VC remains up to the onset of phase IV. It is recorded as %N₂ per liter of lung volume.

The volume expired after the onset of phase IV is the CV. CV may be added to the RV, if the RV has been determined, and expressed as the CC. CV is reported as a percentage of VC:

CVVC × 100

CC is recorded as a percentage of TLC:

CCTLC×100

TLC can be determined from the SBN₂ test by integrating the area of the washout curve. When the volume of N₂ is known, a dilution equation can be used to calculate RV. RV is then added to the measured VC to derive TLC. RV is calculated as follows:

RV=VC×FE¯ N2 N2FA N2−FE¯ N2

where:

F_EN₂= mean expired N₂ concentration determined by integration of the area under the curve

F_AN₂ = N₂ concentration in the lungs at the beginning of inspiration, approximately 0.75–0.79

This method is accurate only in patients who do not have significant obstruction or dead space–producing disease. CV and CC measurements may be in error if the patient does not perform an acceptable VC maneuver (Criteria for Acceptability 4-5). The inspired and expired VC should be within 5%. The VC during the SBN₂ should match the FVC or VC within 5% or 200 mL. Expiratory flow should be maintained between 0.3 and 0.5 L/sec.

Criteria for acceptability 4-5 SBN₂

1. Inspired and expired VC should be within 5% or 200 mL.

2. The VC during SBN₂ should be within 200 mL of a previously determined VC.

3. Expiratory flow should be maintained between 0.3 and 0.5 L/sec.

4. The N₂ tracing should show minimal cardiac oscillations.

Summary

• To measure TLC, the functional residual capacity (FRC) is measured first. FRC can be measured with N₂ washout (FRC_n2), He dilution (FRC_he), or body plethysmograph (FRC_pleth) techniques.

• TLC is calculated by adding the inspiratory capacity (IC, measured from simple spirometry) to the FRC.

• The expiratory reserve volume (ERV, also measured by simple spirometry) may be subtracted from FRC to derive RV.

• The addition of RV and VC (from spirometry) also provides an estimate of TLC.

• The plethysmographic technique is the preferred method because it is largely independent of gas distribution in the lungs.

• The plethysmographic technique also allows the measurement of airway resistance (Raw) and its volume-associated parameters, sGaw and sRaw.

• Gas dilution techniques may underestimate lung volumes in the presence of significant airway obstruction.

• Gas dilution techniques, along with the analysis of N₂ after a single breath of oxygen (SBN₂), provide a qualitative means of assessing gas distribution in the lungs.

Case Studies

Case 4-1

History

A 27-year-old male high school teacher whose chief complaint is dyspnea on exertion. He states that his breathlessness has worsened over the past several months. He has smoked one pack of cigarettes per day for 10 years (10 pack years). He denies a cough or sputum production. No one in his family ever had emphysema, asthma, chronic bronchitis, carcinoma, or tuberculosis. There is no history of exposure to extraordinary environmental pollutants.

Pulmonary function testing

Personal Data

Sex:	Male
Age:	27 yr
Height:	65 in.
Weight:	297 lb
Body surface area (BSA):	2.28 m²

Spirometry and Airway Resistance

	Before Drug	Predicted	%
FVC (L)	2.9	4.7	62
FEV₁ (L)	2.47	3.86	64
FEV_1% (%)	85	82	—
FEF_25%–75% (L/sec)	4.62	4.35	106
FEF_50% (L/sec)	4.94	5.82	85
MVV (L/min)	178	130	137
Raw (cm H₂O/L/sec)	2.33	0.6 – 2.4	—
sGaw (L/sec/cm H₂O/L)	0.23	0.12 – 0.50	—

Lung Volumes (by Plethysmograph)

	Before Drug	Predicted	%
VC (L)	2.9	4.7	62
IC (L)	1.96	2.91	67
ERV (L)	0.94	1.8	59
FRC (L)	1.87	3.29	57
RV (L)	0.93	1.49	57
TLC (L)	3.83	6.2	62
RV/TLC (%)	25	24	—

Technologist’s Comments

Spirometry results met all ATS/ERS acceptability and repeatability recommendations. Good subject cooperation and effort.

Questions

1. What is the interpretation of:

• Spirometry?

• Lung volumes?

• Airway resistance?

2. What is the cause of the subject’s symptoms?

3. What other tests might be indicated?

4. What treatment might be recommended based on these findings?

Discussion

Interpretation

All data from spirometry and lung volumes are acceptable. Spirometry shows a decreased FVC and FEV₁. The FEV_1% is normal. Flows are within normal limits, as is the maximal voluntary ventilation (MVV). Raw is near the upper limit of normal, but sGaw is normal. Lung volumes are decreased, with the RV/TLC ratio preserved.

Impression: Moderate restrictive lung disease without evidence of obstruction. The restrictive pattern may be related to the subject’s weight. Recommend arterial blood gas testing to evaluate gas exchange abnormalities.

Cause of Symptoms

This case is a good example of what might be considered a pure restrictive defect. A proportional decrease in all lung volumes, including FVC and FEV₁, is characteristic of a restrictive process. Flows such as FEF_25%–75% or FEF_50% show little or no decrease (see Figure in p. 126). In addition, characteristic of simple restriction is the well-preserved ratio of FEV₁ to FVC. The volume expired in the first second was in correct proportion to the VC, despite decreases in their absolute volumes. The MVV demonstrates the subject’s ability to move a normal maximal volume. This may be accomplished, despite moderately severe restriction by an increase in the rate rather than the tidal volume.

The explanation for the restrictive pattern lies in the subject’s weight of 297 pounds. His actual weight is approximately 200% of his ideal weight. Obesity commonly causes restrictive patterns.

Other Tests

The subject returned for analysis of arterial blood gases, which revealed resting hypoxemia and slightly elevated Paco₂. Blood gas measurements confirmed the degree of impairment caused by the moderately severe restrictive pattern. Other tests that might be considered include ventilatory response tests for hypoxemia or hypercapnia (Chapter 5). Studies to diagnose sleep apnea might be indicated if the subject had

**Flow-volume loop from the subject**.
Upward tick mark superimposed on the loop denotes the FEV₁, whereas downward tick marks show FEF_50% and FEF_25%, respectively. Small, shaded boxes represent the student’s reference values.

symptoms of disordered breathing during sleep or excessive daytime sleepiness. This subject’s borderline Raw suggests upper airway involvement, which might predispose him to obstructive sleep apnea.

Treatment

The subject was referred to a dietician for counseling in weight management.

Case 4-2

History

A 37-year-old pipe fitter whose chief complaint is shortness of breath at rest and with exertion. His dyspnea has worsened in the past 6 months, so much so that he is no longer able to work. Additional symptoms include a dry cough. He admits some sputum production when he has a chest cold. He has smoked one pack of cigarettes per day for 19 years (19 pack years). He quit smoking approximately 3 weeks before the tests. His father died of emphysema, and his mother of lung cancer. His brother is in good health. His occupational exposure includes working for the past 13 years in the assembly room of a boiler plant. He admits to seldom using the respirators provided at work, despite a dusty environment.

Pulmonary function tests

Personal Data

Sex:	Male
Age:	37 yr
Height:	69 in.
Weight:	143 lb

Spirometry

	Before Drug	Predicted	%	After Drug	%
FVC (L)	3.04	5.05	60	3.1	61
FEV₁ (L)	2.03	3.90	52	2.26	58
FEV_1% (%)	67	77	—	73	—
FEF_25%–75% (L/sec)	1.3	4.09	32	1.60	39
FEF_50% (L/sec)	2.12	5.78	37	2.42	42
FEF_25% (L/sec)	0.78	2.95	26	1.2	41
MVV (L/min)	83	141	59	91	65
Raw (cm H₂O)/L/sec)	2.51	0.6 – 2.4	—	2.47	—
sGaw (L/sec/cm H₂O/L)	0.14	0.11 – 0.44	—	0.15	—

Lung Volumes (by N₂ Washout)

	Before Drug	Predicted	%
VC (L)	3.04	5.05	60
IC (L)	1.62	3.18	51
ERV (L)	1.42	1.87	76
FRC (L)	2.75	3.81	72
RV (L)	0.33	1.94	69
TLC (L)	4.37	6.99	63
RV/TLC (%)	30	28	—

Technologist’s Comments

Spirometry results met all ATS-ERS recommendations, prebronchodilator and postbronchodilator. Lung volumes by N₂ washout: All maneuvers were performed acceptably. Duplicate measurements were averaged. Raw and sGaw efforts were performed appropriately.

Questions

1. What is the interpretation of:

• Prebronchodilator spirometry?

• Response to bronchodilator?

• Airway resistance and conductance?

• Lung volumes?

2. What is the cause of the subject’s symptoms?

3. What other tests might be indicated?

4. What treatment might be recommended, based on these findings?

Discussion

Interpretation

All data from spirometry and lung volumes are acceptable. Spirometry shows a reduced FVC and FEV₁. The FEF_25%–75%, FEF_50%, and FEF_25% are all reduced. The MVV is low. Raw and sGaw are close to their respective limits of normal. Response to bronchodilators is borderline, with an increase in the FEV₁ of 230 mL (11%). The FEF_25% improved somewhat more than the other flows. The subject’s lung volumes are all decreased. His RV/TLC ratio is normal.

Impression: There is moderate airway obstruction combined with a restrictive pattern. The response to inhaled bronchodilator medication is borderline. A trial of bronchodilators should be

**Open-circuit, multiple-breath N**₂**washout graph.**
Tracing shows a normal pattern of washout with N₂ concentration plotted against lung volume (FRC). The test was terminated when N₂ concentration fell to less than 1.5%.

considered. Arterial blood gas testing to evaluate possible hypoxemia is recommended.

Cause of Symptoms

He typifies a student who has combined obstructive and restrictive disease. His spirometry results indicate that a serious obstructive component is present, as revealed by his FEV₁ (52% of predicted) and the other flows. His FEV_1%, however, is close to normal because his FVC is also reduced. Airway narrowing as a result of restriction is sometimes responsible for decreased flows. This is particularly evident when restriction is severe. His symptoms of cough and sputum suggest a genuine obstructive process. His smoking and family history place him at risk. Because FVC can be reduced in either obstructive or restrictive processes, spirometry alone would not have adequately defined the subject’s disease.

Lung volume measurements (see Figure in p. 128) confirm the presence of a restrictive component (see Figure 4-8). All lung volumes are reduced in similar proportions. The reduction in VC matches decreases in FRC, RV, and TLC. The subject’s history and symptoms suggest the possibility of restrictive or obstructive disease or both. The obstructive component may be related to the student’s smoking history, but the etiology of the restrictive component is less clear. On further inquiry, it was learned that the student’s occupation involved exposure to asbestos, which can cause fibrosis and/or carcinoma. Chest radiographs revealed linear calcifications of the diaphragmatic pleura and pleural thickening, as well as fibrotic changes. These findings are all consistent with asbestos exposure.

Other Tests

A sputum examination was performed, and asbestos bodies were identified from the subject’s sputum. Open-lung biopsy was deferred because both the obstructive and restrictive components seemed to be appropriately identified. Measurement of pulmonary compliance (CL) could be used to document the severity of the fibrosis. Analysis of resting arterial blood gases is probably indicated because the subject has a significant degree of restriction. Diffusing capacity (Dlco) could also be used to identify possible gas exchange abnormalities.

Because of his dyspnea on exertion, the student returned for an exercise evaluation. He walked on a treadmill with an arterial catheter in place. His Pao₂ fell from 65 mm Hg (rest) to 55 mm Hg after only 2 minutes of walking at 2 miles per hour. The desaturation was corrected when the subject breathed O₂ by nasal cannula at 1 L/min.

Treatment

The subject was given a trial of bronchodilators and reported significant symptomatic improvement. He began using supplemental O₂ via a portable system and was able to return to work in a position modified to accommodate his abilities.

Case 4-3

History

A 68-year-old white woman, whose chief complaint is worsening dyspnea, was seen by a pulmonologist in the office. She admitted to having been a heavy smoker (90 pack years) but recently quit. She has a productive cough and trouble sleeping at night. She had worked in the meat department of a grocery store for many years, and there was no history of exposure to extraordinary environmental pollutants. She is 62 inches tall and weighs 110 pounds. Her chest radiograph had an emphysematous appearance with increased diameters.

Doctor’s office pulmonary function test

Spirometry

	Before Drug	Predicted	%	After Drug	%
FVC (L)	2.18	3.63	60	2.38	66
FEV₁ (L)	1.09	2.57	42	1.19	46
FEV_1% (%)	50	71	—	50	—
FEF_25%–75% (L/sec)	0.65	2.38	26	0.71	30

Lung Volumes (by N₂ Washout)

	Before Drug	Predicted	%
VC (L)	2.47	3.63	68
IC (L)	2.05	2.02	101
ERV (L)	0.50	0.89	56
FRC_n2 (L)	2.58	2.80	92
RV (L)	2.08	1.91	109
TLC (L)	4.63	4.82	96
RV/TLC (%)	45	40	—

After seeing the results of the office pulmonary function test, the doctor ordered another pulmonary function test at the area hospital.

Hospital pulmonary function test

Spirometry and Dlco

	Before Drug	Predicted	%	After Drug	%
FVC (L)	2.30	3.63	63	2.45	67
FEV₁ (L)	1.12	2.57	44	1.30	51
FEV_1% (%)	49	71	—	53	—
FEF_25%–75% (L/sec)	0.59	2.38	25	0.83	35
Raw (cm H₂O/L/sec)	2.89	0.6 – 2.4	—
sGaw (L/sec/cm H₂O/L)	0.06	0.12 – 0.50	—
Dlco(mL/min/mm Hg)	17.4	23.5	74
Dlco/V_A	3.66	4.88	75

Lung Volumes (by Plethysmography)

	Before Drug	Predicted	%
VC (L)	2.50	3.63	69
IC (L)	1.80	2.02	89
ERV (L)	0.86	0.89	97
FRC_pleth (L)	3.99	2.80	142
RV (L)	3.13	1.91	164
TLC (L)	5.79	4.82	120
RV/TLC (%)	54	40	—

Questions

1. What is the interpretation of the pulmonary function tests done in the doctor’s office?

2. What concerns did the doctor have that prompted sending the subject for another test?

3. What is the interpretation of the pulmonary function tests done in the hospital laboratory?

4. Why is there such a difference?

Discussion

Interpretation of Tests from Doctor’s Office

Pulmonary function test data from the doctor’s office reveal normal lung volumes (TLC, FRC, and RV) and a reduced SVC. The results also reveal severe airflow limitation with a small but clinically insignificant response to the bronchodilator (9.2% and 100-mL improvement in FEV₁). The most likely interpretation of these data would state a mixed obstructive-restrictive disorder.

Concerns About Office Data

The main concern by the office physician was the low lung volumes and lack of hyperinflation that would be consistent with this degree of obstruction and the chest radiograph interpretation.

Interpretation of Tests from Hospital Laboratory

Pulmonary function data from the hospital pulmonary function laboratory, which used a body plethysmograph, reveal severe airflow limitation, decreased diffusing capacity, but markedly increased lung volumes. There is a better bronchodilator response (16.1% and 180-mL improvement in FEV₁). The interpretation of these data is that of severe airflow limitation. There is no evidence of a mixed obstructive-restrictive disorder.

Why the Difference?

The major difference between the two pulmonary function tests is the measurement of FRC. In the doctor’s office, the FRC_n2 value was 2.58L. In the hospital laboratory, the body plethysmography was used in determining an FRC_pleth of 3.99L. The difference between these values can be explained by the presence of trapped gas or poorly communicating airways. In subjects with significant airways obstruction, the gas dilutional methods (i.e., N₂ washout and He dilution) will likely underestimate true lung volume, and the body plethysmograph is a better instrument to use.

Case 4-4

History

A 38-year-old man is referred to the PFT lab for testing for asthma. The student has classic symptoms of intermittent shortness of breath, especially after exercise, and bronchodilator relieves the symptoms, but spirometry in the primary care office reveals normal FEV₁, FVC, and FEV₁/FVC, with no bronchodilator response.

	Baseline			Post-Albuterol
	Actual	% Predicted	Predicted	Actual	% Predicted	% Change
FVC (L)	4.17	85	4.89	4.23	86	1
FEV₁ (L)	3.46	91	3.79	3.49	92	1
FEV₁/FVC (%)	83	107	77	83	107	0

The pulmonologist reviews these data and orders lung volumes and airway resistance to be measured pre- and postbronchodilator. The results are as follows:

	Baseline			Post-Albuterol
	Actual	% Predicted	Predicted	Actual	% Predicted	% Change
TLC (L)	6.13	91	6.76	5.55	82	-9
FRC (box) (L)	2.97	80	3.70	2.28	62	-23
RV (L)	2.09	112	1.87	1.44	77	-31
Raw (cm H₂O/L/sec)	2.30		0.2-2.5	1.18		-48
sGaw (L/sec/cm H₂O)	0.14		0.11-0.40	0.32		+128