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Understanding Pulmonary Function Tests H. T. MILHORN. JR.. M.D.. PH.D. University of Miaiiaippi Medical Center. Jackson. Mississippi Lung volume measurements include vital capacity, inspiratory capacity, expiratory reserve volume, functional residual capacity, residual volume, total lung capacity and tidal volume. Minute ventilation and forced vital cavacity help to determine the patient's ability to move air. Other tests measure parameters such as diffusion across the alveolocapillary membrane and gas distribution. Combined with blood gas measurements, these tests are invaluable in the study of dyspneic patients and patients undergoing major surgery, in monitoring of pulmonary disease and in selection of therapy. The primary function of the lungs is gas ex change: oxygenation of the blood and re moval of carbon dioxide. Pulmonary function studies help to evaluate how well the respira tory apparatus is able to accomplish this task. Pulmonary function studies are indicated to determine the presence and/or extent of lung disease, to ascertain the type of lung disease (obstructive, restrictive or diffusion defect), to monitor the course of pulmonary illness, to determine the extent of respiratory disability, including evaluation for surgery, and to aid in the selection of therapy. Basically, pulmonary function tests mea sure (1) lung volumes and capacities. (2) the ability to move air (airflow), (3) a combina tion of volume and flow, (4) diffusion across the pulmonary membrane. (5) gas distribution and (6)'ventilation-blood flow effectiveness. Measurements to determine these parameters are listed in Table 1. Normal values of the pulmonary function tests are a function of predicted values that have been determined in large groups of normal individuals; the predicted values are often modified by age, sex and height. Lung Volumes and Capacities The various lung volumes andcaparities are illustrated in Figure 1. ~o determine these. AFP / November 1981 the patient breathes in and out of a spirom eter, the bell of which is connected to a recording device so that the inspiratory and expiratory excursions are recorded on cali brated paper. During quiet resting breathing, the excursion represents tidal volume (TV). The patient is then instructed to take as deep a breath as he possibly can. and to exhale maximally. He subsequently resumes normal breathing. All volumes and capacities can be deter mined by spirometry except residual volume (RV), functional residual capacity (FRO and total lung capacity (TLC). Volumes such as inspiratory reserve volume (1RV) are single entities, whereas capacities, such as total lung capacity, are composed of two or more volumes and/or capacities. Changes in vital j/) capacity (VC) and inspiratory capacity (IC)^ TABLE 1. Pulmonary Parameters and cn Corresponding Pulmonary Function Testes jtSJ 139 A ! IJ DOW 06754 Understanding Pulmonary Function Tests usually parallel each other. A decrease in VC to less than 80 percent of the predicted value is considered abnormal; decreases in these measurements are caused by pathologic states which decrease the amount of disten sible lung tissue, or interfere with respiratory excursion in any manner. Changes in expira tory reserve volume (ERV) are not usually of diagnostic significance. FRC. RV and TLC cannot be measured by spirometry because a given amount of air, RV. always remains in the lungs, even at the end of a maximum exhalation. A closedcircuit helium dilution technique is usually applied to determine these three values. A spirometer is filled with a known volume of oxygen to which helium has been added, yielding an approximately 10 percent mix ture. The volume and exact concentration of helium are determined before the test begins. Initially, all of the helium is in the spirometer and none is in the patient's lungs. The patient then breathes room air in a normal fashion. A. valve is thrown at the end of a normal FIGURE 1. Lung volumes and capacities. fTLCm total lung capacity: VC "vital capacity: RV"teeidua! volume: lCmmtpi'alory capacity: FRC " functional residual capacity: IRV"tnipiraiory reserve volume: TV"tide! volume: ERV"expiratory reserve volume) 240 expiration to initiate his breathing through the spirometer. Since FRC is the volume cr gas in the lungs at the end of a normal ex piration. FRC is the capacity determined by this method. The patient continues to re breathe the gas in the spirometer until the concentration of helium falls to a stable leve: (equilibrium is reached between the lungs and the spirometer). This is usually accom plished within three minutes. The final helium concentration is measured and recorded. FRC is calculated from the fact that a known volume of helium was initially in the spirometer but became distributed between the spirometer and the patient's Jungs, with an equal concentration in each. Residual volume is determined by subtracting ERV-from FRC (RV -- FRC - ERV). An increase in FRC indicates hyperinflation, which is considered pathologic. Hyperinflation may result from emphysema, asthma or bronchial obstruc tion of any etiology. FRC and RV usually in crease together. TLC is usually calculated by adding FRC and 1C (TLC " FRC + IC). TLC is decreased in pulmonary edema, atelectasis, neoplasms, interstitial fibrosis, pulmonary congestion, pneumothorax and thoracic restriction. Be cause of an increase in FRC in diseases which cause air trapping. TLC may be concomitantly increased. The RV/TLC ratio is often used. In healthy young adults, this ratio, when expressed as a percentage, may vary from 20 to 35 percent. Values greater than 35 percent in the presence of an increased TLC may be indicative of chronic air trapping. Airflow Measurements to determine ability to move air include minute ventilation (V), forced vital capacity (FVC). forced expiratory volume in one second (FEVt), maximum midexpiratory flow rate (MMFR) and maximum voluntary ventilation (MW). These values also can be determined by use of a spirometer. volume 24. number 5 / AFP DOW 06755 msooois j MINUTE VENTILATION V is the amount of air inhaled or exhaled in one minute during quiet breathing. It is determined by multiplying tidal volume (TV) by the average number of breaths per minute as counted from the spirometer trac ing. Normal V ranges from 5 to 7 L per min ute. It is increased in patients with large dead spaces and by hypoxia, hypertarbia, acidosis and anxiety. It is decreased by severe airway obstruction, hypocarbia, alkalosis and re spiratory center depression caused by drugs or COi narcosis. Since V is the sum of minute dead space and alveolar ventilation, it is not a measure of effective ventilation for gas ex change. It is of most value when used in con junction with arterial blood gases. FORCED VITAL CAPACITY FVC is the volume of gas that can be ex pelled as rapidly as possible after a maximum inspiration. False-low valves can be obtained in uncooperative patients if maximum effort is not achieved. Normally, FVC is equal to VC. The forced expiration, however, does cause higher than normal transpulmonary pressures, so that bronchiolar collapse, ob structive lesions and air trapping are exagger ated. Thus, FVC may be smaller than VC in patients with obstructive lung disease such as emphysema and asthma. Both FVC and VC are decreased in restrictive disease such as interstitial fibrosis. FORCED EXPIRATORY VOLUME IN ONE SECOND FFVt is the volume of air expelled in the first second of an FVC maneuver (Figure 2). It is an indication of the degree of airway obstruction. FEVt is most useful when it is ex pressed as a percent of FVC (FEVi/FVC X 100). A normal individual should be able to expel at least 80 percent of his FVC in one second. A patient who has obstructive disease usually shows a reduced FEV| value, whereas a patient with restrictive disease has a normal value despite having a lower than normal FVC. MAXIMUM MID-EXPIRATORY FLOW RATE The MMFR, also known as forced expira tory flow 25 to 75 percent (FEF,,.%), is a measurement of airflow during the middle portion of an FVC maneuver. It is determined according to Figure 3. An abnormal MMFR is indicative of disease in small to medium-sized airways. It is usually decreased in obstructive lung disease. The MMFR complements the FFV,; normal values for healthy young men average 4.5 L. per second. 'CUR!L Dtttrmiimtwn of REV,. AFP / November 1981 MAXIMUM VOLUNTARY VENTILATION The MW is the largest volume per minute that can be breathed by voluntary effort. It is measured by having the patient breathe as deeply and rapidly as possible for 12 seconds. The volume breathed in this period is multi plied by five to give MW in liters per minute. MW measures the status of the respiratory muscles, the compliance of the lungs and thorax and the resistance of the airways and tissues. Healthy young men may average 170 141 ooul 0675k Understanding Pulmonary Function Tests FIGURE 3. Determination of MMFR. (MMFR - A/8) FICURE i. The expiratory portion of the fiote-oohune loop. 142 L. per minute. Values less than 80 percent or predicted values are considered abnormal. MW is reduced greatly in patients with ob structive disease, but may be within normal limits for patients with restrictive disease. The MW maneuver depends considerably on patient effort. Combined Flow and Volume (Flow-Volume Loop) The flow-volume loop is a graphic record ing of volume versus its first derivative, air flow, during a forced expiratory maneuver followed by a forced inspiratory maneuver. This type of recording requires a special spirometer capable of generating an electrical signal from which volume and flow can be determined. In actuality, only the expiratory portion of the loop is used, as illustrated in Figure 4. The excursion, in terms of lung volumes, goes from TIC to RV. FVC can be determined from the tracing as shown. Other*-''* parameters obtained from the flow-volume""* loop are peak flow (PF) and flow rates at'3 specific expired volumes (MEF,>*, MEF*%.^ PEAK HOW CD PF is the maximum flow rate obtainecLn during an FVC maneuver (Figure 4). In healthy young men, PF may exceed 10 L. per second. Peak flow rates, however, are of limited value. Patients with obstructive disease, for instance, may develop an initially high flow rate before airway closure occurs. The value obtained is also dependent on patient effort. FLOW RATES AT SPECIFIC EXPIRED VOLUMES Flow rates at specific expired volumes have been shown to be more sensitive measure ments of airway resistance than peak flow. MEFrs* is the flow rate at 75 percent of FVC, MEFm is the flow rate at 50 percent of FVC and MEFU is the flow rate at 25 percent of FVC (Figure 4). Because the pres sures exerted on the alveoli and the small air ways become equal during a forced expire tion. volume 24. number S / AFP DOU 06757 I p lWTF T t * "n p fy ir n \' r.:n t T t l , V I * 1 MEF and MEFU* are independent of patient effort. Normal values of MEF, MEFm and MEFm for healthy young men average about 8.5, 6.5 and 3.5 L per second, respectively! Examples of flow-volume patterns with normal function, upper airway obstruction and small airway obstruction are illustrated in Figun 5. It is evident that PF, MEF%, TABLE 2. Capadties and Flows in Obstructive and Restrictive Disease My be decreased in severe rmtrictioe FIGURE J. Typical flow-volume anm for (A) a normal individual. (B) upper airway obstruction tuck as tracheal itmous and 1C) small airway obstruction, such at occult with chronic obstructive pulmonary diet AFP / November 1981 MEFm* and MEFu* will vary in these condi tions. Obstructive vs. Restrictive Disease Pulmonary function studies allow the physidan to distinguish between obstructive and restrictive lung disease (Table 2). Ob structive diseases, in general, reduce the abil ity to move air; lung volumes and capadties are either normal or increased. Obstructive diseases indude emphysema, chronic bron chitis and asthma. Restrictive diseases, in gen era], reduce volumes and capadties without interfering with airflow. Restrictive dis eases indude atelectasis, neoplasms, pneu mothorax, surgical excision and pulmonary fibrosis. A restrictive pattern may also be seen in certain conditions not related to the lungs. such as respiratory center depression, neuro muscular disease, limitation of diaphr^ggi movement by pregnancy or asdtes and luaf- tation of thoradc movement by kvphotey- liosis or pain. a Determination of Therapy <3 .^ Pulmonary function studies aid in the*i- termination of therapy, particularly in 98- eases which have an obstructive component. After determination of baseline volumes, capadties and flow rates, a bronchodilator challenge (usually inhaled isoproterenol) is administered and the volumes, capadties and flows are redetermined. Significant reduction in FRC and increases in FEVt, MMFR, MW and other flow parameters suggest that use of oral bronchodilators will be beneficial, where as lack of response or minimal changes suggest that chronic oral bronchodilation will be of no value. An increase in TLC of greater than IS percent in restrictive disease indicates im provement will occur with the use of oral bronchodilators. Diffusion Tests. .The ability of gases to diffuse across the pulmonary membrane is expressed as the diffusing capacity for carbon monoxide 143 I low u l r (lllc it |H1 M l m ult DOW 06758 I Understanding Pulmonary Function Tests I' * * f :I :ii 4 1 144 (DLco). Carbon monoxide is used because of its tremendous affinity for hemoglobin, thus reducing the partial pressure of carbon mon oxide in the blood of alveolar capillaries to essentially zero. The principle of determina tion for the steady-state technique is as fol lows: in the steady state, the rate of uptake by mouth of a low concentration of carbon dioxide (0.3 percent) is equal to the rate of its diffusion across the pulmonary membrane. The rate of diffusion across the pulmonary membrane is proportional to the difference in partial pressure of carbon monoxide in alveolar gas and alveolar capillary blood. Hence: Rate of uptake at mouth -- K (Pa -- Pc) Pa is the partial pressure of carbon monoxide in alveolar gas. Pc is its partial pressure in alveolar capillary blood and K is a propor tionality constant equivalent to DLco* Since Pc is approximately zero, rearranging the equation yields: DLco - Rate of uptake at mouth/PA The rate of uptake of carbon monoxide at the mouth can be measured, and alveolar carbon monoxide partial pressure can be approxi mated from end-tidal samples; DLco can then be calculated. Normal values for DLco average about 21 mL. per minute per mm. Hg. Diffusing capacity is decreased in em physema, interstitial fibrosis, sarcoidosis, the The Author H. T. MllHORN JR.. u.o.. m.d. h aonunt protestor of family mediant end director of the Division of Research at the University of Mississippi School of Medicine. Jackson. He is also professor of physiology and biophytia. Dr. Milhom received a Ph.D. in physiology and biophysics in 1964 and an M.D. in 1973 horn the University of Mississippi School of Medicine. He completed a family practice residency in 1980 at the tame insti tution. He hat served on the faculty of the medical school since 19M. A diplomats of the American Board of Family Practice. Dr. Milhom is a member of the Academy. He is the author of more than 100 publications, including a book. The Application of Control Theory to Physielopctl Systems. pneumoconioses, oxygen toxicity and pul monary edema, states that are usually cate gorized as 'diffusion defects.* A diffusion defect is suspected when DLco is less than 73 percent of the predicted value. DLco ia also decreased by anemia. Cas Distribution Helium equilibration time is determined during the closed-circuit measurement of FRC. The unevenness of ventilation deter mines the slope of the equilibration curve. If distribution is uniform, the curve is exponen tial. If ventilation is uneven, the slope is de creased, especially after the initial dilution of well-ventilated areas. A helium equilibration time of more than three minutes is abnormal. Ventilation-Blood Flow Effectiveness Ventilation-blood flow effectiveness can be determined by several methods, including radionuclide ventilation-perfusion scanning. The only indicator of ventilation-blood flow effectiveness used as part of routine pulmonary function studies is arterial blood gases '~~ (American Family Physician. March 1980). ~~ Body Plethysmography The body plethysmograph is not routinely used in small community hospitals, but is commonly used in larger institutions. It e= measures thoracic gas volume, airway resistance and specific conductance. To make the measurements, the patient sits inside an airtight box and breathes through a flow meter. Calculations are based on the relation ship between lung volume and the pressure within the box. THORACIC CAS VOLUME Body plethysmography measures all of the gas in the thorax, not just the portion freely exchangeable with the atmosphere. There fore, in a patient with large noncommunicat ing bullae or cysts, the thoracic gas volume (TCV) would be larger than its counterpart, the FRC. volume 24. number 5 / AFP DOU 06759 1'ST0005488 TABUX AIKWAY RESISTANCE Airway resistance (Raw) is defined as the pressure difference between the mouth and the alveoli, divided by the flow rate. Alveolar pressure, however, can only be measured indirectly; the body plethysmograph is one method of obtaining this measurement. Since airway resistance varies inversely with lung volume, reported values of Raw must be accompanied by the values of TCV at which the measurements were made. Raw is normally less than 2 cm. HaO per L. per second. SPECIFIC CONDUCTANCE Specific conductance (SC) is an indicator of how easily air flows in and out of the lungs. It is determined by dividing the reciprocal of Raw by the TCV at which it is measured: SC - (1/Raw)/TCV Normal values range between 0.14 and 0.35. All community hospitals that have a body plethysmograph do not use it routinely. Its application may be based on criteria dbtermined from routine pulmonary function studies. For instance, plethysmography may be used only if FVC is less than 80 percent of the predicted value, if FEV, is less than 70 percent of the predicted value or if MMFR is less than 50 percent of its predicted value. Evaluating Surgical Patients Pulmonary function studies are valuable in evaluating patients with shortness of breath and in patients undergoing major surgery. AFP / November 1981 Measurement of lung volumes, capacities and flow rates allows separation of lung dis- ease into restrictive and obstructive types. Distribution of ventilation tests indicates asynchrony in filling and emptying of lung units; they are, therefore, helpful in the evaluation of cysts and bullae in patients undergoing lung surgery. Diffusion tests aid in evaluating the amount of functioning pulmonary capillary bed. Arterial blood gases allow an overall assessment of pulmonary gas exchange from the atmosphere to the blood. Preoperative evaluation of pulmonary function is used to identify patients at in creased risk for morbidity and mortality. Several pulmonary functions do, however, change postoperatively. Total lung capacity and each of its subdivisions decrease after ab dominal surgery, but not after operations on extremities. This decrease is greater for upper than for lower abdominal surgery; VC. for instance, may decrease as much as 50 percent following upper abdominal surgery. PaO| may also decrease significantly following abdominal surgery, consistent with intrapul- monary shunting. Risk factors predisposing patients to postoperative pulmonary compli cations are listed in Table 3. Patients with these risk factors are generally considered candidates for preoperative pulmonary func tion studies. 0 SUGCE5TED READING Ayet* IN. Whipp 81. Zitncnt I. A guide to the in terpretation of pulmonary function tests. New York: Protect! in Health. Inc.. 1974. Gennaro MT. Preoperative evaluation ef pulmonary function. Am Rev Reap Dis 1979;119:293-310. Milhom HT Jr. Understandinf arterial blood lasts. Am Fam Physician 1980:21131:112-20. Ruppd C. Manual of pulmonary function testing. St. Louis: CV Motby. 197S. West J. Pulmonary pathophysiology. Baltimore: WUHama k Wilkins, 1977. Address tingle-copy reprint requests to H. T. Milhom. }r.. M.D.. Deportment of femily Medicine. University of Mittiutppt MedicoI Center. 2300 N. Stole St., focktott. MS 39216. 145 DOL) 06760