Standardisation of Single-Breath Carbon Monoxide Uptake in the Lung PDF

Eur Respir J 2005; 26: 720–735 DOI: 10.1183/09031936.05.00034905 CopyrightßERS Journals Ltd 2005 SERIES ‘‘ATS/ERS TASK FORCE: STANDARDISATION OF LUNG FUNCTION TESTING’’ Edited by V. Brusasco, R. Crapo and G. Viegi Number 4 in this Series Standardisation of the single-breath determination of carbon...

Eur Respir J 2005; 26: 720–735 DOI: 10.1183/09031936.05.00034905 CopyrightßERS Journals Ltd 2005 SERIES ‘‘ATS/ERS TASK FORCE: STANDARDISATION OF LUNG FUNCTION TESTING’’ Edited by V. Brusasco, R. Crapo and G. Viegi Number 4 in this Series Standardisation of the single-breath determination of carbon monoxide uptake in the lung N. MacIntyre, R.O. Crapo, G. Viegi, D.C. Johnson, C.P.M. van der Grinten, V. Brusasco, F. Burgos, R. Casaburi, A. Coates, P. Enright, P. Gustafsson, J. Hankinson, R. Jensen, R. McKay, M.R. Miller, D. Navajas, O.F. Pedersen, R. Pellegrino and J. Wanger CONTENTS AFFILIATIONS For affiliations, please see Background............................................................... 721 Acknowledgements section Definitions............................................................... 721 Determinants of CO uptake................................................... 721 CORRESPONDENCE Gas analysers and general equipment.......................................... 722 V. Brusasco System design............................................................ 722 Internal Medicine University of Genoa Equipment requirements..................................................... 722 V.le Benedetto XV, 6 Performance standards for equipment.......................................... 722 Genova I-16132 Equipment quality control................................................... 723 Italy Infection control........................................................... 724 Fax: 39 0103537690 E-mail: [email protected] Single-breath testing technique standardisation issues............................. 724 Patient conditions for measurement............................................. 724 Received: Inspiratory manoeuvre....................................................... 724 March 23 2005 Accepted: Condition of the breath-hold and expiratory manoeuvre............................... 725 April 05 2005 Washout and sample collection volume.......................................... 725 Inspired gas composition.................................................... 725 Interval between tests....................................................... 726 Miscellaneous factors....................................................... 726 Calculations............................................................... 726 Calculating breath-hold time.................................................. 727 Calculating the alveolar volume................................................ 727 Inspired gas conditions...................................................... 728 CO2, H2O and temperature adjustment for VA calculations............................. 728 Evaluating the measurement of DL,CO........................................... 728 Acceptability, repeatability and number of tests..................................... 728 Adjustments to the measurement of DL,CO prior to interpretation......................... 729 Adjustment for haemoglobin................................................. 729 Adjustments for PA,O2...................................................... 730 Adjustment for COHb concentration and CO back pressure.......................... 730 Adjustment for lung volume................................................. 730 Reporting values........................................................... 730 Abbreviations.............................................................. 731 KEYWORDS: Alveolar-capillary permeability, carbon monoxide, carbon monoxide diffusing capacity of the lungs, carbon monoxide transfer factor of the lungs, gas exchange, inspiratory manoeuvres Previous articles in this series: No. 1: Miller MR, Crapo R, Hankinson J, et al. General considerations for lung function testing. Eur Respir J 2005; 26: European Respiratory Journal 153–161. No. 2: Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Respir J 2005; 26: 319–338. No. 3: Wanger J, Clausen JL, Coates Print ISSN 0903-1936 A, et al. Standardisation of the measurement of lung volumes. Eur Respir J 2005; 26: 511–522. Online ISSN 1399-3003 720 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION BACKGROUND Determinants of CO uptake This joint statement is based on the previous statements from The process of CO transfer from the environment to the the American Thoracic Society (ATS) and the European pulmonary capillary blood includes: 1) bulk flow delivery of Respiratory Society (ERS), and much of the material was taken CO to the airways and alveolar spaces; 2) mixing and diffusion from these statements [1, 2]. It has been updated according to of CO in the alveolar ducts, air sacs and alveoli; 3) transfer of new scientific insights and revised to reflect consensus CO across the gaseous to liquid interface of the alveolar opinions of both of these societies. This document is meant membrane; 4) mixing and diffusion of CO in the lung to function as a stand-alone document, but, for certain issues, parenchyma and alveolar capillary plasma; 5) diffusion across references will be made to the previous statements. Although the red cell membrane and within the interior of the red blood there are other ways to measure carbon monoxide (CO) uptake cell; and 6) chemical reaction with constituents of blood Hb (e.g. steady-state, intra-breath and rebreathing techniques) [3– [10–16]. 9], the following recommendations will be restricted to the single-breath technique, since this is the most common The process of CO uptake can be simplified into two transfer or methodology in use around the world. conductance properties: membrane conductivity (DM), which reflects the diffusion properties of the alveolar capillary The capacity of the lung to exchange gas across the alveolar- membrane; and the binding of CO and Hb. The latter can be capillary interface is determined by its structural and represented as the product of the CO–Hb chemical reaction functional properties [3–22]. The structural properties include rate (h) and the volume of Hb in alveolar capillary blood (Vc). the following: lung gas volume; the path length for diffusion in Since these are conductances in series , these properties are the gas phase; the thickness and area of the alveolar capillary related by: membrane; any effects of airway closure; and the volume of blood in capillaries supplying ventilated alveoli. The func- 1=DL,CO~(1=DM)z(1=hVc ) ð2Þ tional properties include the following: absolute levels of ventilation and perfusion; the uniformity of their distribution A number of physiological changes can affect DM or hVc to with respect to each other; the composition of the alveolar gas; influence DL,CO. As the lung inflates, DM increases (due to the diffusion characteristics of the membrane; the concentra- unfolding membranes and increasing surface area), while Vc tion and binding properties of haemoglobin (Hb) in the effects are variable (due to differential stretching and flattening alveolar capillaries; and the gas tensions in blood entering of alveolar and extra-alveolar capillaries) [10, 17–24]. The net the alveolar capillaries in that part of the pulmonary vascular effect of these changes is that DL,CO tends to increase as the bed which exchanges gas with the alveoli. lung inflates. Exercise, the supine position and Mueller manoeuvres (inspiratory efforts against a closed glottis) can Definitions all recruit and dilate alveolar capillaries, thereby increasing Vc The rate of CO uptake from the lungs is the product of alveolar and DL,CO [25–31]. Alveolar-capillary recruitment also occurs partial pressure of CO in excess of any back pressure in the blood in the remaining lung tissue following surgical resection, since (the driving pressure) and a rate constant. This is for CO in the the cardiac output now flows through a smaller capillary whole lung per unit of driving pressure. For practical reasons, network. This causes a less than expected loss of Vc for the using the single-breath method described below the CO uptake amount of lung tissue removed. In contrast, Valsalva man- from the lung (KCO) is measured as a concentration fall in oeuvres (expiratory efforts against a closed glottis) can reduce alveolar CO per unit time per unit CO driving pressure (PA,CO): Vc and thereby reduce DL,CO. KCO~D½CO
=Dt=PA,CO ð1Þ The measurement of CO uptake is also affected by the distribution of ventilation with respect to DM or hVc (i.e. CO When KCO is multiplied by the volume of gas in the lung uptake can only be measured in lung units into which CO was containing CO (alveolar volume (VA)), the total uptake of CO inspired and subsequently expired) [15, 16, 32, 33]. This is by the lung per unit of time per unit driving pressure is particularly important in diseases such as emphysema, where obtained. This product, KCO6VA, has been termed transfer the inhaled CO may only go to the better-ventilated regions of factor of the lung for CO by the European community and the lung and the subsequently measured CO uptake will be diffusing capacity of the lung for CO (DL,CO) by the North determined primarily by uptake properties of those regions. American community. The former term recognises that the Under these conditions, the tracer gas dilution used to measurement of CO uptake reflects a number of processes (not calculate VA will also reflect primarily regional dilution and just diffusion), and is a submaximal value and, thus, not truly a underestimate the lung volume as a whole. The resulting ‘‘capacity’’. However, the latter term has considerable histor- calculated DL,CO should thus be considered to be primarily ical significance and, for the sake of uniformity, the ERS and reflecting the gas-exchange properties of the ventilated regions ATS agreed to use the expression DL,CO in this document. of the lung. The ERS recommends expressing DL,CO in the SI units In addition to these physiological and distributional effects mmol?min-1?kPa-1, while the ATS prefers the traditional on DL,CO, a number of pathological states can affect DM, units mL (standard temperature, pressure and dry hVc, or both, and thereby affect DL,CO (table 1) [5, 6, 34–43]. (STPD))?min-1?mmHg-1. In fact, this is not an important issue, Measurement of DL,CO is indicated when any of these providing the same set of units is used throughout all pathological processes are suspected or need to be ruled out. calculations. Values in SI units should be multiplied by 2.987 to obtain values in traditional units. Moreover, measuring changes in DL,CO over time in these processes is a useful way of following the course of disease. c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 721 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. TABLE 1 Physiological and pathological changes that affect the carbon monoxide diffusing capacity of the lung (DL,CO) Extrapulmonary reduction in lung inflation (reduced VA) producing changes in DM or hVc that reduce DL,CO Reduced effort or respiratory muscle weakness Thoracic deformity preventing full inflation Diseases that reduce hVc and thus reduce DL,CO Anaemia Pulmonary emboli Other conditions that reduce hVc and thus reduce DL,CO Hb binding changes (e.g. HbCO, increased FI,O2) Valsalva manoeuvre (increased intrathoracic pressure) Diseases that reduce (in varying degrees) DM and hVc and thus reduce DL,CO Lung resection (however, compensatory recruitment of hVc also exists) Emphysema Interstitial lung disease (e.g. IPF, sarcoidosis) Pulmonary oedema Pulmonary vasculitis Pulmonary hypertension Diseases that increase hVc and thus increase DL,CO Polycythaemia Left-to-right shunt Pulmonary haemorrhage (not strictly an increase in hVc, but effectively an increase in lung Hb) Asthma Other conditions that increase hVc and thus increase DL,CO Hb binding changes (e.g. reduced FI,O2) Muller manoeuvre (decreased intrathoracic pressure as in asthma, resistance breathing) Exercise (in addition, a possible DM component) Supine position (in addition, possibly a slight increase in DM) Obesity (in addition, a possible DM component) VA: alveolar volume; DM: membrane conductivity; h: carbon monoxide (CO)–haemoglobin (Hb) chemical reaction rate; Vc: volume of pulmonary capillary blood; FI,O2: inspired fraction of oxygen; IPF: idiopathic pulmonary fibrosis; Hb: haemoglobin. GAS ANALYSERS AND GENERAL EQUIPMENT different gas compositions, concentrations or pulsatile flow System design changes created by demand valves. All devices should Descriptions of the apparatus and general instructions for maintain the required volume accuracy, regardless of the gas performing the single-breath diffusing capacity manoeuvre are mixture, direction of gas flow (e.g. inhaled or exhaled), or available elsewhere [2, 44–48]. Equipment in clinical use varies pulsatile flow pattern. 2) Gas-analyser accuracy is important in widely in complexity, but the basic principles are the same. All some circumstances, such as measuring CO ‘‘back pressure’’ systems have a source of test gas (bag-in-box, spirometer, (the expired fraction of CO when no CO has been inhaled). compressed gas cylinder), a method for measuring inspired However, in calculating DL,CO, only the ratios of alveolar to and expired volume over time (spirometers with kymographs, inhaled CO and tracer gas are needed. Thus, the analysers pneumotachometers near the mouthpiece or near a bag-in- must primarily be able to produce an output for measured box), and gas analysers (single-sample analysers or continuous exhaled CO and tracer gas that is a linear extrapolation high-speed analysers). Single-sample gas-analyser systems between the inhaled (test gas concentrations) and zero (no CO usually display only volume over time (fig. 1a). Continuous or tracer gas present in the analysers) [51, 52]. This is often gas-analyser systems also provide a continuous tracing of CO referred to as a linear response. Since measured DL,CO is very and tracer gas concentrations during the test (fig. 1b). sensitive to errors in relative gas concentration, nonlinearity for the analysers should not exceed 0.5% of full scale (i.e. once the Equipment requirements analysers have been adjusted to zero, with no test gas present Performance standards for equipment and scaled to full scale using test gas concentrations, system The performance standards for equipment are as follows nonlinearity on measurements of known dilutions of test gas (table 2). 1) The volume-measurement accuracy should be the should be no more than 0.5% of full scale). For example, if same as that established by the ATS/ERS for spirometry ; 0.300% CO is used for the test gas, then the maximum error on that is, ¡3% volume accuracy (¡3.5% accounting for 0.5% any dilution should be no more than ¡0.0015%. 3) The gas testing syringe error) over an 8-L volume range with test gases analysers should have only minimal drift in zero and gain, so present in concentrations likely to be encountered during that output is stable over the test interval. Manufacturers are DL,CO tests. Pneumotachometer devices for sensing flow and encouraged to provide a display of the measured gas volume during the DL,CO manoeuvre may be sensitive to concentrations so that stability can be confirmed. If significant 722 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION a) 5 gas-analyser performance. Furthermore, water vapour-perme- able tubing has a limited life expectancy. One method of # 4 checking water vapour-permeable tubing is to compare gas- concentration measurements made with both dry and humi- ¶ dified test gas, and make adjustments described as follows. 3 Volume L Manufacturers should provide a replacement schedule for water vapour-permeable tubing and/or a method for checking 2 its function. The second remedy for CO2 and/or H2O analyser interference is to characterise the effect of these gases on 1 analyser output, and then adjust the output of the analysers for the presence of the interfering gas species. Two approaches are often employed as follows: assume constant concentrations of 0 b) 100 the interfering gases and apply a fixed correction factor across all tests; or directly measure the CO2 and/or H2O for each test Gas concentration % full scale and make proportional adjustments in the analyser output based on the measured concentrations for CO2 and/or H2O (see CO2, H2O and temperature adjustment for VA calculations section). 5) Circuit resistance should be ,1.5 cmH2O?L-1?s-1 at 6 L?s-1 flow. If a demand-flow regulator is used on a compressed test gas cylinder, the maximal inspiratory pressure required for 6 L?s-1 inspiratory flow through both circuit and valve should be ,10 cmH2O. 6) The timing device in the DL,CO apparatus should be accurate to within 1% (100 ms over 10 s). The timing technique used for calculation should be identified. 0 If an instrument provides automatic data computation, the 0 2 4 6 8 10 12 14 16 Time s accuracy of breath-hold time computation should be docu- mented. 7) Dead space volume (VD) for both inspired test gas FIGURE 1. Schematic of lung volume (a) and gas concentrations (b) during and the alveolar sample should be known, and their role in all the single-breath diffusing capacity of the lung for carbon monoxide. The gas- data-computation algorithms identified and documented. For sampling period occurs between the two dotted lines. -----: tracer gas; – – –: carbon adults, the VD of the valve, filter and mouthpiece should total monoxide. #: dead space washout; ": sample collection. Modified from. ,0.350 L. Smaller VD volumes may be needed for paediatric applications. 8) The system must be leak free. This is drift is present over the time scale of a test (,30 s), then particularly important for DL,CO systems that aspirate gas adjustment algorithms should be devised to compensate for samples at subatmospheric pressure through the gas analysers. the analyser drift from measured data. Gas-analyser stability When samples are aspirated, leaks in tubing, fittings and other should be ¡0.001% absolute for CO and ¡0.5% of the locations allow room air to be drawn into the gas circuit, full-scale reading for the tracer gas. 4) If CO2 and/or H2O diluting the sample and reducing the concentrations of test interfere with gas-analyser performance, there are two gases. remedies. First, the CO2 and/or H2O can be removed from the test gases before passage through the gas analysers. H2O Equipment quality control is commonly absorbed by anhydrous CaSO4 or by other The considerations for equipment quality control are as follows products. Absorption of CO2 can be achieved with either (table 3). 1) Prior to each test, gas analysers should be zeroed. Ba(OH)2 or NaOH. Both generate H2O when combining with After each test, a new zeroing procedure should be carried out CO2. Therefore, if a CO2 absorber is used, it must precede the to account for analyser drift during the test. 2) Each day, there H2O absorber in the gas-analyser circuit. Selectively permeable should be a volume calibration with a 3-L syringe. tubing can also be used to remove water vapour; however, this Technicians should also note significant discrepancies between tubing may only reduce the water vapour to near ambient inspired volume (VI) and vital capacity (VC), or VA and total levels, and remaining H2O can still interfere with the lung capacity (TLC) that might suggest volume-calibration TABLE 2 Equipment specifications Volume accuracy ATS/ERS standards (currently 3.5% accuracy over an 8-L volume using test gases, with a testing syringe accuracy of 0.5%) Gas analysers Linear from zero to full span within ¡0.5% of full span. Stable over the duration of the test with drift ,¡0.5% of a measured gas Circuit resistance ,1.5 cmH2O?L-1?s-1 at a flow of 6 L?s-1 Demand-valve sensitivity ,10 cm H2O required for 6 L?s-1 flow through valve and circuit (if compressed gas source used) Timer ¡1.0% over 10 s (100 ms) Apparatus/valve filter VD ,0.350 L ATS: American Thoracic Society; ERS: European Respiratory Society; VD: dead space volume. c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 723 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. To minimise variability as much as possible, the following TABLE 3 Equipment quality control recommendations for the standardisation of testing techniques Gas-analyser zeroing Done before/after each test are offered. Volume accuracy Tested daily Standard subject or simulator testing Tested at least weekly Patient conditions for measurement Gas-analyser linearity Tested every 3 months Factors that affect Vc (e.g. exercise, body position, and Hb Timer Tested every 3 months affinity for CO, such as alveolar oxygen partial pressure (PA,O2), and carboxyhaemoglobin (COHb)) should be standar- dised. If clinically acceptable, the subject should not breathe problems. 3) Each week, or whenever problems are suspected, supplemental oxygen for 10 min prior to a standard test. When the following procedures should be carried out. First, leak using exercise or the supine position to assess the ‘‘recruit- testing should be done if it is appropriate to the instrument ability’’ of DL,CO [15, 25–28], the level of exercise and/or the being used. Secondly, a DL,CO test with a calibrated 3.0-L duration of the supine position should be noted. syringe should be used, which is performed by attaching the Before beginning the test, the manoeuvres should be demon- syringe to the instrument in the test mode. Test gas is strated and the subject carefully instructed. The subject should withdrawn from the DL,CO machine by the syringe and then be seated comfortably throughout the test procedure. The test reinserted at the end of the breath-hold. The measured DL,CO should be performed at a stable comfortable temperature should be near zero and the measured VI should be ,3.3L within manufacturer’s equipment specifications. (3.0 L6the body temperature, ambient pressure, saturated with water vapour (BTPS) factor). This procedure checks the COHb produces an acute and reversible decrease in DL,CO [57– inhaled volume accuracy in the DL,CO test mode, which 60], largely due to the effects on CO back pressure and the may be in error when spirometry measurements are not. ‘‘anaemia effect’’ from decreased Hb binding sites for CO from Thirdly, a test could be performed on a ‘‘standard subject’’ the test gas. As cigarette smoking is the most common source (biological control) or simulator. Standard subjects are of COHb, subjects should be asked to refrain from smoking or healthy nonsmokers (e.g. healthy laboratory personnel). If other CO exposures on the day of the test. The time of the last the DL,CO in a standard subject varies.10% from known cigarette smoked should be recorded and noted for the previous values, the test should be repeated. If the repeat interpretation. A correction for CO back pressure should be made for recent or heavy cigarette smoking (see Adjustment test confirms the finding, the DL,CO system should be for carboxyhaemoglobin concentration and CO back pressure evaluated carefully for the possibility of leaks, nonlinear section). Manufacturers are encouraged to provide the cap- analyser function, volume and time inaccuracy, etc. When ability to do this easily. sufficient data on a standard individual are obtained, laboratories should establish their own outlier criteria to serve as indicators of potential problems with their DL,CO Inspiratory manoeuvre systems. Manufacturers are encouraged to develop automated Once the mouthpiece and nose clip are in place, tidal breathing quality-control systems to assist and enhance the utility of should be carried out for a sufficient time to assure that the these steps. 4) Gas-analyser linearity should be assessed every subject is comfortable with the mouthpiece. Deep inspirations 3 months. A straightforward approach is to measure known should be avoided during this period as they can increase serial dilutions of the test gas , or measure the concentra- subsequent CO uptake. The DL,CO manoeuvre begins with tion of a separate high-precision test gas having a certificate of unforced exhalation to residual volume (RV). In obstructive analysis. At least one intermediate concentration should be lung disease, where exhalation to RV may require a prolonged used to check linearity. Manufacturers should be encouraged period, a reasonable recommendation is that this portion of the to automate this function. In addition, the timer should be manoeuvre should be limited to 6 s, a time consistent with assessed for accuracy every quarter. 5) Records of equipment using the forced expiratory volume in six seconds manoeuvre as a surrogate for VC. At RV, the subject’s mouthpiece is checks and standard subject tests should be dated, signed and connected to a source of test gas, and the subject inhales kept in a laboratory log book. Manufacturers are encouraged to rapidly to TLC. provide software and test equipment options for quality- control measurements and quality-control data management. A submaximal inspired volume (i.e. less than the known VC) can affect CO uptake, depending upon whether it is a result of Infection control an initial suboptimal exhalation to RV (test performed at TLC) The major goal of infection control is to prevent the or whether it is due to a suboptimal inhalation from RV (test transmission of infection to patients and staff during pulmon- performed below TLC) [19–22]. In the former case, the ary function testing. The recommendations in the ATS/ERS calculated VA and DL,CO will accurately reflect lung volume documents for spirometry and general considerations for and the CO uptake properties of the lung at TLC. In the latter pulmonary function testing also apply to DL,CO equipment case, the VA will be reduced and DL,CO measurement will be and procedures [49, 56]. affected (see Adjustment for lung volume section). SINGLE-BREATH TESTING TECHNIQUE Due to these effects, it is important that the VI be as close to the STANDARDISATION ISSUES known VC as possible. Data from a large patient population The single-breath determination of DL,CO involves measuring have shown that the VI during DL,CO measurements averages the uptake of CO from the lung over a breath-holding period. ,90% of the VC , but that as many as 32% of subjects may 724 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION fall below this target. A more recent study of.6,000 DL,CO measurements in a university laboratory demonstrated that 72, 86 and 92% of these patients could achieve VI targets of 90, 85 and 80%, respectively, of the known VC. Since it appears that VI reductions of as much as 15% of the known VC will reduce the DL,CO ,5% , a VI target of 85% of the largest- Volume known VC seems both reasonable and attainable. The inspiration should be rapid, since the DL,CO calculations assume ‘‘instantaneous’’ lung filling [24, 64–70]. Slower lung filling decreases the amount of time the lung is at full inspiration with a consequent reduction in CO uptake. Although various sample timing techniques address the issue of lung filling and emptying time, it is still reasonable to expect Time that 85% of VI should be inspired in ,4.0 s. If longer inspiratory times are needed to achieve the 85% VI goal, this FIGURE 2. Potential problems with the single-breath diffusing capacity of the should be noted on the test report. lung for carbon monoxide breathing manoeuvre that can lead to measurement errors. ???????: stepwise inhalation or exhalation; – - –: exhaled gas leak; – -- –: Condition of the breath-hold and expiratory manoeuvre inhalation too slow; – – –: exhaled volume larger than inhaled volume; ------: Valsalva (expiratory efforts against a closed airway) and transient overshoot from high flows and changing gas temperatures. Adapted Muller manoeuvres (inspiratory efforts against a closed from. airway) during the breath-hold, by decreasing and increasing thoracic blood volume, respectively, will decrease and increase DL,CO, respectively [29, 71, 72] The intrapulmonary pressure after VD washout will be a good reflection of the lung as a during the breath hold should thus be near atmospheric, and whole. However, in subjects with poor gas mixing or marked this is best accomplished by having the subject voluntarily sequential emptying of various lung regions, the gas sample maintain full inspiration using only the minimal effort collected will only reflect the properties of the regions necessary. The breath-hold time should be 10¡2 s, a target contributing to that sample. VS collection time will also affect easily achieved in the vast majority of subjects. the measurement of breath-hold time (see below). In order to standardise the collection process, a VS of 0.50–1.00 L should As with inspiration, the DL,CO calculation assumes instanta- be collected for analysis. In patients with VC ,1 L, a VS ,0.50L neous lung emptying [24, 64–69]. Although various sample may be used if it can be assured that the VD has been cleared. timing techniques address the fact that emptying is not instantaneous, it is still reasonable to expect that the expiratory If continuous analysers with graphical displays are used, manoeuvre should be smooth, unforced, without hesitation or computerised or visual inspection of the expired CO and tracer interruption, and total exhalation time should not exceed 4 s gas curves may be used to adjust washout and the VS if needed (with sample collection time ,3 s). In subjects who require a (fig. 1). These adjustments may be useful in subjects with longer expiratory time to provide an appropriate alveolar VC ,1 L who are unable to meet the minimum VD washout gas sample, the expiratory time should be noted in the and VS recommended previously (e.g. paediatric patients, or test report. Common errors that can occur during the adult patients with severe restrictive processes). These adjust- inspiration, breath-hold and expiration manoeuvres are given ments may also be useful in subjects with a large VD in whom in figure 2. the recommended value range of 0.75–1.0 L is inadequate. For these adjustments to be achieved properly, the displays must Washout and sample collection volume represent actual gas concentrations that occurred at the mouth, The DL,CO calculations (see Calculations section) require synchronised for delays in gas transport and adjusted for gas- alveolar gas samples. During expiration, a volume of gas must analyser response. In making such adjustments, the start of the be expired and discarded to clear anatomic and mechanical VD VS (end of the washout) must clearly be at a point where the before the alveolar sample is collected (fig. 1). Contamination tracer gas has started to plateau after the immediate fall from of the alveolar gas sample with VD gas will cause an its inspiratory concentration, and the CO curve has ceased its underestimation of true CO uptake. In general, the washout immediate fall and started a smooth gradual decline (fig. 1). volume should be 0.75–1.0 L (BTPS). If the patient’s VC is Furthermore, reports must indicate that manual adjustments ,2.00 L, the washout volume may be reduced to 0.50 L. Newer were used to select washout volumes and VS, so the interpreter devices can provide a graphical display of exhaled gas can review and verify the adjustments. concentrations to assure that VD gas is not present in the alveolar sample (fig. 1). Using such an analyser, HUANG et al. Inspired gas composition showed that the standard approach noted above ade- The test gases used to calculate DL,CO include a tracer gas to quately cleared VD in.90% of adults. measure VA, as well as CO. The remainder of the test gas mixture includes O2 and N2. The sample gas volume (VS) is the volume of gas used to analyse alveolar CO and tracer gas concentrations at the end of The tracer gas should be relatively insoluble and chemically the breath-hold. In subjects with good gas mixing and uniform ventilation and CO uptake properties, virtually any gas sample and biologically inert. Since the tracer gas is used to determine the initial alveolar CO concentration, as well as the VA from c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 725 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. which CO uptake is occurring, its gaseous diffusivity should By measuring DL,CO at several different levels of PA,O2, the be similar to CO. It should not interfere with the measurement two components of DL,CO (DM and Vc) can be distinguished. of CO concentration. The tracer gas should not ordinarily be This is accomplished by using the Roughton–Forster relation- present in alveolar gas or else be present at a known, fixed ship noted previously (equation 2) and varying h (the concentration (e.g. argon). reaction rate of O2 and Hb) by altering the PI,O2. Subsequently, 1/DL,CO is plotted against 1/h at the different Commonly used tracer gases are helium (He) and methane PI,O2 levels. The slope of this relationship is 1/Vc and the (CH4). While He meets most of the previous criteria, its intercept is 1/DM. gaseous diffusivity is considerably higher than CO. CH4 is commonly used as a tracer gas for systems that continuously Interval between tests sample expired gas. Its gaseous diffusivity is closer to CO, but At least 4 min should be allowed between tests to allow an it has a slightly higher liquid solubility than He. As new tracer adequate elimination of test gas from the lungs. The subject gases are introduced, manufacturers should demonstrate that should remain seated during this interval. In patients with they produce VA and DL,CO values equivalent to those obstructive airway disease, a longer period (e.g. 10 min) should measured using He, as this is the tracer gas that is used to be considered. Several deep inspirations during this period derive most of the available reference equations. may help to clear test gases more effectively. If continuous The inspired CO should nominally be 0.3%. However, as ratios monitoring of expired gas concentrations is available, the washout of tracer gas from the previous test may be confirmed are more important than absolute values, exact concentrations by observing end-tidal gas concentrations before beginning the are not critical. The assumption in calculating CO uptake is next test. that capillary blood does not contain CO. Thus, corrections are needed in patients who have significant COHb (see Adjust- Miscellaneous factors ment for COHb concentration and CO back pressure section). There may be diurnal variation in DL,CO, since one study has Since PA,O2 fluctuates over the ventilatory cycle and can found that DL,CO fell 1.2–2.2% per hour throughout the day affect CO uptake by affecting h, a more stable PA,O2 during the. The reason for the change was not clear and was not DL,CO manoeuvre would seem desirable and, theoretically, can explained by CO back pressure or changes in VA, VI or breath- be achieved with a test gas fraction of inspired oxygen (FI,O2) of hold time. One explanation is a combination of changes in CO 0.17. Most current systems use either a FI,O2 of 0.21 (with back pressure and diurnal variation in Hb concentration. fractional concentrations of tracer gases such as CH4 of ,0.01), A 13% change in DL,CO during the menstrual cycle has been or gas mixtures containing CO and 10% He with ‘‘balance air’’ reported. The highest value was observed just before the (an effective FI,O2 of 0.19). Since DL,CO will increase 0.31 to menses, and the lowest was on the third day of menses. It is 0.35% for each 0.133 kPa (1 mmHg) drop in PA,O2 [73, 74], the not clear, however, if this is simply a Hb effect or whether it increase in DL,CO that would be expected as the FI,O2 is reflects other physiological processes (e.g. hormonal changes decreased from 0.21 to 0.17 (PA,O2 decreased ,3.7 kPa on pulmonary vascular tone). Ingestion of ethanol has been (,28 mmHg)) is 8–9%. It is recommended that laboratories reported to decrease DL,CO. The mechanisms involved are use gas mixtures with inspired oxygen partial pressure (PI,O2) not clear, although it is known that some fuel-cell CO analysers values similar to the reference set used in the interpretation are sensitive to exhaled ethanol and ketones. In obstructive (table 4) [75–82], or make appropriate adjustments of mea- lung disease subjects, after administration of a bronchodilator, sured or predicted DL,CO for the PI,O2. DL,CO may increase up to 6%. Bronchodilators can affect VA, vasomotor tone, etc., and their use prior to testing could conceivably optimise these factors. Use of a bronchodilator should be noted in the interpretation. TABLE 4 Inspired gas mixtures used during measurements CALCULATIONS of normal carbon monoxide (CO) uptake for commonly used reference equations The transfer factor or diffusing capacity for a gas in the lungs (DL) equals its rate of exchange across the lung divided by its Author [Ref.] Gas mixture# transfer gradient: DL~rate of gas uptake=transfer pressure gradient ð3Þ TECULESCU 1.5% He, balance air (FI,O2 0.20) VAN GANSE 14–15% He, balance air (FI,O2 0.18) The rate of gas uptake is expressed in mL STPD?min-1, and the FRANS 10% He, 18% O2 transfer gradient (the difference between alveolar and pul- CRAPO 10% He, 25% O2 (comparable to 21% at sea level) monary capillary pressures) in mmHg. Thus, DL,CO has PAOLETTI 10% He, 20% O2 traditional units of mL STPD?min-1?mmHg-1 (SI units of KNUDSON 10% He, 21% O2 mmol?min-1?kPa-1). For CO, the pulmonary capillary CO ROCA 13% He, 18% O2 tension is near zero and thus: HUANG 0.3% CH4, 0.3% C2H2, balance air (FI,O2 0.20) MILLER 10% He, ?balance air DL,CO~total CO uptake over time=PA,CO ð4Þ ~D½CO
|VA =Dt=PA,CO He: helium; FI,O2: inspired oxygen fraction; CH4: methane; C2H2: acetylene. #: in addition to 0.3% CO. The single-breath DL,CO technique assumes that both CO and the tracer gas (Tr) are diluted comparably on inspiration. Thus, 726 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION the initial alveolar partial pressure of CO (PA,CO,0) can be of empirically accounting for the effects of inspiratory and calculated by knowing the inspired tracer gas fraction (FI,Tr) expiratory time. This method has also been shown to and fraction alveolar tracer gas (FA,Tr): adequately address inspiratory flows as low as 1 L?s-1, breath-hold times as short as 5 s, and expiratory flows as FA,CO,0~FI,CO|FA,Tr=FI,Tr ð5Þ low as 0.5 L?s-1 in normal subjects. With the approach taken by JONES and MEADE , breath-hold PA,CO,0~PB|FA,CO,0 ð6Þ time equals the time starting from 0.3 of the inspiratory time to the middle of the sample collection time. As in spirometry, the where FA,CO,0 is the initial alveolar inspired CO fraction, FI,CO back-extrapolation technique should be used to establish is the inspired CO fraction, PB is the barometric pressure and time zero [48, 49]. The time when 90% of the VI has been FA,CO,0 is the initial alveolar CO fraction. inspired is a reasonable end point for defining inspiratory time Tracer gas dilution is also used to determine the effective VA as (fig. 3). described below. Solving for DL,CO thus yields the equation: A theoretically more accurate way to account for volume DL,CO~(VA =(t=60|(PBPH2 O))|ln((FA,Tr|FI,CO)= changes over time during inspiration and expiration is to use ð7Þ three separate equations for DL,CO during inspiration, breath (FI,Tr|FA,CO)) hold and expiration (the ‘‘three-equation’’ technique) [24, 64]. This algorithm is commercially available and may be particu- where VA is in mL STPD, t is breath-hold time in seconds, and larly useful in subjects unable to rapidly fill or empty their PH2O is water vapour pressure. lungs. However, clinical experience with this approach is limited. Calculating breath-hold time The ‘‘breath-hold time’’ or time of transfer during which CO Other breath-hold timing algorithms may be appropriate changes from its initial to final concentration is in the in maintaining consistency (e.g. longitudinal studies), but denominator of the DL,CO equation (equation 7). As noted these measurements should be recognised as less suitable previously, the single-breath measurement of CO uptake recommendations. assumes an ‘‘instantaneous’’ lung filling and emptying process. However, both inspiration and expiration require up Calculating the alveolar volume to several seconds, and these periods of changing gas volume VA represents an estimate of lung gas volume into which CO is in the lung must be accounted for in the calculations. For distributed and then transferred across the alveolar capillary purposes of standardisation, the method by JONES and MEADE membrane [3, 4]. Thus, it is critical in the measurement of (fig. 3) is recommended, since it has the theoretical appeal DL,CO. As noted previously, VA is measured simultaneously with CO uptake by calculating the dilution of an inert Tr. For Inhalation Breath holding Exhalation normal subjects, this calculated single-breath determination of 6 VA (VA,sb) plus estimated VD closely matches TLC determined tI TLC # by plethysmography [19, 70]. However, poor gas mixing 5 90% ¶ in patients with maldistribution of inspired volume (e.g. VI obstructed airways patients) can markedly reduce Tr dilution VI 4 and, thus, lead to values for VA,sb that are markedly less than a VA determined from the actual total thoracic gas volume (VTG). Volume L 3 The observed CO uptake is also affected by poor gas mixing 1 2 VI under these conditions, and will primarily reflect the CO transfer properties of the regions into which the test gas is 2 distributed. It has been suggested that a separately determined 0.3 tI VA from a more accurate technique (e.g. multiple-breath 1 technique (VA,mb) or plethysmography (VA,plethys)) could be RV substituted for VA,sb under these conditions to ‘‘correct’’ for 0 the effects of maldistribution. However, the DL,CO calculation 0 2 4 6 8 10 12 (equations 4 and 7) is based on the volume of gas into which Time s the Tr (and CO) distributes, and not the total VTG. Moreover, substituting a larger, separately determined VA,mb or VA,plethys FIGURE 3. Schematic illustration of different methods of measuring breath- assumes that DM and Vc properties in the unmeasured lung hold time for the single-breath diffusing capacity of the lung for carbon monoxide. regions are similar to those in the measured lung regions, an The method by OGILVIE (––––––) measures breath-hold time from the beginning assumption that is difficult to justify. Due to these considera- of inspiration to the beginning of alveolar sample collection. The method by JONES tions, a separately measured VA,mb or VA,plethys should not be and MEADE (???????????) includes 0.70 of inspiratory time and half of sample time. substituted for VA,sb. Instead, when the VA,sb is markedly less The Epidemiologic Standardization Project (– – – –) measures breath-hold time from than a separately determined VA,mb or VA,plethys, this should the time of 50% of inspired volume (VI) to the beginning of alveolar sample be reported and the ratio of VA,sb to VA,mb or VA,plethys collection. tI: time of inspiration (-----; defined from the back-extrapolated time 0 to reported. For the subsequent interpretation of DL,CO, it should the time that 90% of the VI has been inhaled); TLC: total lung capacity; RV: residual volume. #: dead space washout; ": sample collection. Adapted from. then be noted that the maldistribution of inspired gas probably contributed to any observed reduction in measured DL,CO. c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 727 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. The volume of distribution for the tracer gas can be determined before the single-breath manoeuvre so that it will not contain from values for VI, FI,Tr and FA,Tr, and knowing the conditions expiratory gas from a previous subject. VSRV should be ,2% of of the inspired and expired gases. Since the amount of tracer the VS or 10 mL, whichever is larger. gas in the lung (alveolar plus dead space) equals the amount of inspired tracer gas, and the dead space tracer gas fraction is the Inspired gas conditions same as the inspired fraction (all expressed at BTPS): Though inspired gas is often assumed to be measured at ambient temperature and pressure, saturated with water V I|FI,Tr~V A|FA,TrzV D|FI,Tr ð8Þ vapour conditions, this is only true in systems in which the test gas is transferred to a water-sealed spirometer before it is V A~VI V D|(FI,Tr=FA,Tr) ð9Þ inspired. In most cases, the test gas inspired from a bag-in-box Although VA is usually expressed under BTPS conditions, it system, through a pneumotachometer from a bag, or a compressed gas cylinder with a demand valve is a dry gas must be converted to STPD conditions to calculate DL,CO in (,10 ppm H2O) and, thus, at ambient temperature and equation 7. pressure, dry conditions. The inspired volume needs to be It is essential that VD is considered in the calculation of VA. converted to BTPS conditions to use in equations 7, 8 and 9. It VD occurs in two areas: instrument VD (i.e. volume of the is recommended the VI (BTPS) be reported, and manufacturers mouthpiece, filters and connections within the valving should specify and document inspired gas conditions for each system); and anatomic VD (i.e. the volume in the conducting instrument. airways that does not participate in gas exchange). Instrument VD should be specified by the manufacturer, but may vary as CO2, H2O and temperature adjustment for VA calculations the user alters the system (e.g. addition of a filter). Exhaled gas contains CO2 and H2O, which were not present in the test gas mixture. As noted previously, some systems There are various methods to estimate anatomic VD. Examples remove one or both of these if they interfere with analyser include a fixed value of 150 mL (although this does not function, and this will raise both CO and tracer gas concentra- work well for small adults or children), and another of tions. Under these circumstances, adjustments are required for 2.2 mL6kg body weight (although this does not work the increase in FA,Tr to calculate VA (table 5). However, no well for very obese subjects). In studies deriving the commonly adjustment for the increase in alveolar inspired CO fraction at used reference equations (table 4), the most commonly used time t (FA,CO,t) and FA,Tr is necessary in calculating the rate of technique was to assume 2.2 mL6kg body weight. However, CO uptake, since the concentration factor appears in both some investigators ignored anatomic VD [79, 80, 82], and one the numerator and the denominator of the expression used age+2.2mL6kg body weight. If the body mass index (FA,CO,0/FA,CO,t) and therefore cancels. is ,30, the current authors recommend using an estimate for anatomic VD of 2.2 mL6kg body weight. In more obese Exhaled gas is initially at body temperature. Some systems subjects or if the weight is unknown, VD (mL) can be estimated allow this to cool (gas volume contracts), whereas others will using the following equation: provide heat to maintain the temperature. Adjustments to BTPS conditions may be required depending upon the system V D~24|height|height=4545 ð10Þ design (table 5). where height is measured in cm, or: All of these adjustments should be documented by the manufacturer for their particular system. V D~24|height|height=703 ð11Þ EVALUATING THE MEASUREMENT OF DL,CO where height is measured in inches. Acceptability, repeatability and number of tests Acceptable tests are defined in table 6. Repeatability describes In single-sample systems, the sample-bag residual volume the variability on repeated testing with no change in test (sometimes called a sample-bag dead space) dilutes the sample conditions [90, 91]. In a large university-based laboratory gas and alters the measured concentrations of expired gases. study, a coefficient of variation of repeated measurements in The size and direction of the error depends on VS, the residual normal subjects was 3.1%, and this increased only slightly volume of the sample bag and its connectors (VSRV), and VSRV (from 4.0 to 4.4%) in patients with abnormal spirometry gas content. VSRV could contain test gas, room air or expired patterns. In contrast, an inter-session DL,CO variability of gas from a subject (after a DL,CO test). When VSRV contains up to 9% (reproducibility) has been documented in normal room air, its effect is to reduce the measured concentrations of individuals in repeated measurements over a period of 1 yr expired gases. The following equation adjusts for this:. Adjusted FA,Tr~measured FA,Tr|(V S=(V SV SRV)) ð12Þ Since most intra-session variability is technical rather than physiological, the mean of acceptable tests is reasonable to Estimates of the potential change in DL,CO in existing systems report. In this report, there should be at least two acceptable when no adjustment is made for sample-bag dead space range tests that meet the repeatability requirement of either being from 0.3–8%, depending on sample-bag size and VSRV. within 3 mL CO (STPD)?min-1?mmHg-1 (or 1 mmol?min-1?kPa-1) Manufacturers should report instrument and sample-bag dead of each other or within 10% of the highest value. In a large space. Both of these must be flushed with room air (or, if DM university-based laboratory study,.95% of the patients and Vc are to be calculated, appropriate levels of oxygen) could meet this criteria. 728 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION TABLE 5 Corrections for barometric pressure (PB), ambient water vapour pressure (PH2O), partial pressure of CO2 and temperature H2O removed from sampled gas; CO2 does not interfere with analysers VA,BTPS5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6(PB/(PB–47))6(310/(273+T)) VA,STPD5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6(PB/760)6(273/(273+T)) H2O and CO2 removed from sampled gas VA,BTPS5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr(1+FA,CO2)/FS,Tr)6(PB/(PB–47))6(310/(273+T)) VA,STPD5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr(1+FA,CO2)/FS,Tr)6(PB/760)6(273/(273+T)) If no measurement of FA,CO2 is available, then it may be assumed to be 0.05 H2O in sampled gas equilibrated to room air; CO2 does not interfere with analysers. If FI,Tr is read by the analysers, the equations are the same as for when H2O is removed from sampled gas. If tank values (i.e. dry gas concentrations) are used for FI,Tr, then the following equations are used VA,BTPS5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6((PB–PH2O)/(PB–47))6(310/(273+T)) VA,STPD5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6((PB–PH2O)/760)6(273/(273+T)) Neither H2O nor CO2 removed from sampled gas, no interference with analysers, heated sample tubing to prevent condensation VA,BTPS5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6(310/(273+T)) VA,STPD5(VI,ATPD–VD,INST–VD,ANAT)6(FI,Tr/FS,Tr)6((PB–47)/760)6(273/(273+T)) In these calculations, room temperature (T) is measured in Celsius and gas pressures are measured in mmHg. In all four cases, the inspired volume (VI) is the measured volume of inhaled dry gas and, thus, is considered under ambient temperature, ambient pressure, and dry (ATPD) conditions. The conversion to body temperature, ambient pressure, saturated with water vapour (BTPS) and standard temperature, pressure and dry (STPD) may require factors to compensate for the diluting or concentrating effects of adding or deleting H2O or CO2 at the gas sampling site. Therefore, standard gas condition conversion formulae must be adjusted as described previously. VA: alveolar volume; VD,INST: instrument dead space; VD,ANAT: anatomic dead space; FI,Tr: fraction of tracer (Tr) gas in the inspired test gas; FS,Tr: fraction of the Tr gas in the alveolar sample, which may differ from the fraction of alveolar Tr gas, depending on the effects of CO2 and H2O as noted; FA,CO2: fraction of CO2 in the alveolar sample. TABLE 6 Acceptable test criteria for diffusing capacity of the lung for carbon monoxide Use of proper quality-controlled equipment VI of.85% of largest VC in ,4 s# A stable calculated breath hold for 10¡2 s. There should be no evidence of leaks, or Valsalva or Mueller manoeuvres Expiration in ,4 s (and sample collection time ,3 s)#, with appropriate clearance of VD and proper sampling/analysis of alveolar gas VI: inspired volume; VC: vital capacity; VD: dead space. #: tests outside these timing limits might still have clinical utility, but these deviations from standard acceptability criteria should be noted and possible impact/correction factors considered. The average of at least two acceptable tests that meet this Adjustment for haemoglobin repeatability requirement should be reported (i.e. outliers Since CO–Hb binding is such an important factor in CO excluded). While it is recommended that at least two DL,CO transfer, DL,CO changes can be substantial as a function of Hb tests should be performed, research is needed to determine the concentration [93–97]. The empirical change in DL,CO with Hb actual number of tests required to provide a reasonable change closely matches what is expected from a theoretical estimate of average DL,CO value for a given person. As noted approach using the relationship in equation 2, with h assumed to below, five tests will increase COHb by ,3.5% , which will be proportional to the Hb, DM/hVc is assumed to be 0.7 , and decrease the measured DL,CO by ,3–3.5%. Thus, more than the ‘‘standard’’ Hb value is assumed to be 14.6 g?dL-1 (9 mmol?l-1 five tests are not recommended at the present time. SI) in adult males and adolescents and 13.4 g?dL-1 (8.26 mmol?l-1 SI) in adult females and children ,15 yrs. Using these relation- Adjustments to the measurement of DL,CO prior to ships and expressing Hb in g?dL-1, the equation for adjusting interpretation predicted DL,CO in adolescents and adult males is: DL,CO depends upon a number of physiological factors. Besides varying with age, sex, height and possibly race, DL,CO,predicted for Hb~ DL,CO also changes with Hb, lung volume, COHb, PI,O2 (e.g. ð13Þ DL,CO,predicted|(1:7 Hb=(10:22zHb)) altitude), exercise and body position. Although these effects may cause changes in DL,CO in opposite directions , all The equation for adjusting predicted DL,CO in children ,15 yrs should be considered in interpreting the observed CO uptake. of age and females is: Moreover, specific adjustments for three of these factors (Hb, COHb and PI,O2) should always be made to ensure appropriate DL,CO,predicted for Hb~DL,CO,predicted|(1:7 Hb=(9:38zHb)) ð14Þ interpretation (see below). Consideration could also be given to adjust for a submaximal inspiration resulting in a less than expected VA. Results from a more recent study in patients with a wide range of Hb abnormalities showed a slightly greater and more c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 729 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. linear relationship, but corrected values were generally following equation empirically reduces predicted DL,CO by 1% consistent with equations 13 and 14. for each per cent COHb.2%: Adjustments for PA,O2 DL,CO,predicted for COHb~DL,CO,predicted|(102%  COHb%) ð18Þ As noted previously, PA,O2 affects the measurement of DL,CO. An adjustment for COHb is not required, but is recommended PA,O2 changes will occur as a consequence of supplemental O2 for interpretative purposes when COHb is elevated/suspected. breathing (higher PA,O2) or performing DL,CO assessments at No adjustment is required if COHb ,2%, since reference altitude (lower PA,O2). As mentioned before, DL,CO will change equations already incorporate this. by ,0.35% per mmHg change in PA,O2 [73, 74] or by ,0.31% per mmHg decrease in PI,O2. Adjustments to the predicted Adjustment for lung volume DL,CO in a subject on supplemental O2 may be made using a As noted previously, DL,CO decreases as the lung deflates as a measured PA,O2 and assuming a normal PA,O2 on room air at a function of both membrane and capillary configuration sea level of 100 mm Hg, as follows: changes [17–24, 104–111]. The relationship is complex, how- DL,CO,predicted for elevated PA,O2~ ever, and is probably nonlinear [108, 110]. In normal subjects ð15Þ with experimental reductions in VI (and, thus, VA), adjustment DL,CO,predicted=(1:0z0:0035(PA,O2100)) equations for this effect have been derived [18, 19, 109, 111] and a recent representative example consists of the following: If the adjustment is being made for altitude, assuming a PI,O2 of 150 mmHg at sea level: DL,CO (at V Am)~DL,CO (at V Ap)|(0:58z0:42(V Am=V Ap)) ð19Þ DL,CO,predicted for altitude~ ð16Þ KCO (at V Am)~KCO (at V Ap)|(0:42z0:58=(V Am=V Ap)) ð20Þ DL,CO,predicted=(1:0z0:0031(PI,O2150)) where VAm represents measured VA and VAp represents Adjustment for COHb concentration and CO back pressure predicted VA at normal TLC. COHb can affect the measured uptake in the following two ways [98–100]. First, by occupying Hb binding sites, CO It should be noted that this DL,CO adjustment for a reduced VI produces an ‘‘anaemia effect’’. Secondly, CO partial pressure (and VA) from a submaximal effort is substantially less than a in the blood will reduce the driving pressure for CO transport 1:1 DL,CO/VA adjustment (i.e. the fall in DL,CO as lung volumes from alveolar gas to capillary blood. are reduced is much less than the fall in VA). As a consequence, the DL,CO/VA ratio will rise with a reduced VI from a Exposure to ordinary environmental CO and endogenous submaximal effort. Thus, if this ratio is used to adjust production of CO as a byproduct of Hb catabolism commonly (‘‘correct’’) DL,CO for the effects of a reduced VA from a results in measured COHb levels of 1–2%. The 1–2% submaximal VI, it will markedly ‘‘overcorrect’’. baseline COHb levels that are attributable to endogenous production of CO and ordinary environmental exposures are It is important to emphasise that the VA effects on DL,CO already incorporated into reference values based on healthy discussed above were derived from studies in normal subjects nonsmoking subjects. Cigarette smoke and other environmen- with submaximal VI. These VA effects (and consequent DL,CO tal sources, however, can produce measurable levels of CO adjustments for VA) have not been validated in lung diseases back pressure and COHb that may need to be considered in the where lung pathology has reduced CO uptake properties, as measurement of CO uptake. Small increases in COHb also well as VI and VA. In some of these diseases (e.g. status post- occur when CO is inspired in the DL,CO test. FREY et al. , for pneumonectomy), the reduction in DL,CO may be less than the example, found that COHb increased by ,0.7% with each reduction in VA (high DL,CO/VA); in others (e.g. pulmonary single-breath DL,CO test. vascular disease), the reduction in DL,CO may be greater than the reduction in VA (low DL,CO/VA). In many disease CO back pressure can be measured in expired gas before a states, however, the ratio of pathological reductions in DL,CO DL,CO manoeuvre or estimated using one of several available and VA may be quite variable and of unclear physiological or techniques [100–103]. For example, CO back pressure can be clinical significance. Thus, although the DL,CO/VA relationship calculated from COHb from the following equation: can be used to describe the relative reductions in CO uptake alveolar ½CO
~(COHb=O2 Hb)|(alveolar ½O2
)=210 ð17Þ properties and alveolar gas volumes in lung disease [17, 19, 107, 112], drawing more specific clinical or pathological DL,CO can then be recalculated after subtracting the estimated conclusions based upon VA (or any other volume) adjustments CO back pressure from both the initial and final alveolar CO. should be made with caution. This is especially true if the Units must be consistent before making the subtraction. adjustment leads to the implication that CO uptake properties However, this method will not adjust DL,CO for the ‘‘anaemia’’ of the lung are normal. Further study is clearly needed on the effect of COHb. interactions of CO uptake and alveolar gas volume in lung disease before more specific volume-adjustment recommenda- Several studies have evaluated both the empirical and tions can be made. theoretical effects of COHb on DL,CO and incorporated both the back pressure and the ‘‘anaemia’’ effects of COHb. In Reporting values general, a 1% increase in COHb reduces the measured DL,CO Several values are measured with the single-breath DL,CO and by ,0.8–1% from both effects [13, 14]. Using this approach, the many factors affect DL,CO. It is important that the report 730 VOLUME 26 NUMBER 4 EUROPEAN RESPIRATORY JOURNAL N. MACINTYRE ET AL. CO DIFFUSING CAPACITY STANDARDISATION includes the results needed for optimal interpretation. The TABLE 7 (Continued) average of at least two acceptable tests should be reported (i.e. IVC Inspiratory vital capacity outliers excluded). KCO Transfer coefficient of the lung (i.e.DL,CO/VA) The report should always include the unadjusted measured kg Kilograms DL,CO, the predicted and per cent predicted DL,CO, and the kPa Kilopascals predicted and per cent predicted DL,CO/VA (KCO). Any L Litres adjustments (e.g. for Hb, COHb, PI,O2, or lung volume) should L?min-1 Litres per minute also be reported along with the data used to make the L?s-1 Litres per second adjustment. The average VA should be reported along with the lb Pounds weight predicted VA (the predicted TLC minus predicted VD) and per MEFX% Maximal instantaneous forced expiratory flow where X% of the cent predicted VA. The average VI should also be noted. If a FVC remains to be expired separately measured VC is available, it should be reported to MFVL Maximum flow–volume loop serve as a reference for the adequacy of the VI. In addition, mg Milligrams comments relevant to the quality of the measurements should MIF Maximal inspiratory flow be included. mL Millilitres mm Millimetres ABBREVIATIONS MMEF Maximum mid-expiratory flow Table 7 contains a list of abbreviations and their meanings, ms Milliseconds which will be used in this series of Task Force reports. MVV Maximum voluntary ventilation PA,O2 Alveolar oxygen partial pressure PB Barometric pressure TABLE 7 List of abbreviations and meanings PEF Peak expiratory flow ATPD Ambient temperature, ambient pressure, and dry PH2O Water vapour partial pressure ATPS Ambient temperature and pressure saturated with water vapour PI,O2 Inspired oxygen partial pressure BTPS Body temperature (i.e. 37uC), ambient pressure, saturated with h (theta) Specific uptake of CO by the blood water vapour RT Rise time from 10% to 90% of PEF C Centigrade RV Residual volume CFC Chlorofluorocarbons s Seconds cm Centimetres STPD Standard temperature (273 K, 0uC), pressure (101.3 kPa, COHb Carboxyhaemoglobin 760 mmHg) and dry DL,CO Diffusing capacity for the lungs measured using carbon monoxide, TB Tuberculosis also known as transfer factor TGV (or VTG) Thoracic gas volume DL,CO/VA Diffusing capacity for carbon monoxide per unit of alveolar tI Time taken for inspiration volume, also known as KCO TLC Total lung capacity DM Membrane-diffusing capacity Tr Tracer gas DT Dwell time of flow.90% of PEF ttot Total time of respiratory cycle EFL Expiratory flow limitation TV (or VT) Tidal volume ERV Expiratory reserve volume VA Alveolar volume EV Back extrapolated volume VA,eff Effective alveolar volume EVC Expiratory vital capacity VC Vital capacity FA,X Fraction of gas X in the alveolar gas Vc Pulmonary capillary blood volume FA,X,t Alveolar fraction of gas X at time t VD Dead space volume FEF25–75% Mean forced expiratory flow between 25% and 75% of FVC VI Inspired volume FEFX% Instantaneous forced expiratory flow when X% of the FVC has VS Volume of the expired sample gas been expired mg Micrograms FEV1 Forced expiratory volume in one second FEVt Forced expiratory volume in t seconds FE,X Fraction of expired gas X FIFX% Instantaneous forced inspiratory flow at the point where X% of the FVC has been inspired ACKNOWLEDGEMENTS FI,X Fraction of inspired gas X N. MacIntyre: Duke University Medical Center, Durham, NC, FIVC Forced inspiratory vital capacity USA; R. Crapo and R. Jensen: LDS Hospital, Salt Lake City, UT, FRC Functional residual capacity USA; G. Viegi: CNR Institute of Clinical Physiology, Pisa, Italy; FVC Forced vital capacity D.C. Johnson: Massachusetts General Hospital and Harvard H2O Water Medical School, Boston, MA, USA; C.P.M. van der Grinten: Hb Haemoglobin University Hospital of Maastrict, Maastrict, the Netherlands; Hg Mercury V. Brusasco: Università degli Studi di Genova, Genova, Italy; F. Hz Hertz; cycles per second Burgos: Hospital Clinic Villarroel, Barcelona, Spain; R. IC Inspiratory capacity Casaburi: Harbor UCLA Medical Center, Torrance, CA, USA; IRV Inspiratory reserve volume A. Coates: Hospital for Sick Children, Toronto, ON, Canada; P. Enright: 4460 E Ina Rd, Tucson, AZ, USA; P. Gustafsson: c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 4 731 CO DIFFUSING CAPACITY STANDARDISATION N. MACINTYRE ET AL. Queen Silvias Children’s Hospital, Gothenburg, Sweden; J. 15 Michaelson ED, Sackner MA, Johnson RL. Vertical Hankinson: Hankinson Consulting, Inc., Valdosta, GA, USA; distribution of pulmonary diffusing capacity and capil- R. McKay: Occupational Medicine, Cincinnati, OH, USA; M.R. lary blood flow in man. J Clin Invest 1973; 52: 359–365. Miller: University Hospital Birmingham NHS Trust, 16 MacIntyre NR, Nadel JA. Regional diffusing capacity in Birmingham, UK; D. Navajas: Lab Biofisica I Bioenginyeria, normal lungs during a slow exhalation. J Appl Physiol Barcelona, Spain; O.F. Pedersen: University of Aarhus, Aarhus, 1982; 52: 1487–1492. Denmark; R. Pellegrino: Azienda Ospedaliera S. Croce e Carle, 17 Hughes JMB, Pride NB. In defense of the carbon Cuneo, Italy; J. Wagner: Pharmaceutical Research Associates, monoxide transfer coefficient KCO (TL/VA). Eur Respir Inc., Lenexa, KS, USA. J 2001; 17: 168–174. 18 Stam H, Versprille A, Bogaard JM. The components of the carbon monoxide diffusing capacity in man dependent REFERENCES on alveolar volume. Bull Eur Physiopath Respir 1983; 19: 1 American Thoracic Society. Single-breath carbon mon- 17–22. oxide diffusing capacity (transfer factor). Recommend- 19 Johnson DC. Importance of adjusting carbon monoxide ations for a standard technique: 1995 update. Am J Respir diffusing capacity (DLCO) and carbon monoxide transfer Crit Care Med 1995; 152: 2185–2198. coefficient (KCO) for alveolar volume. Respir Med 2000; 2 Cotes JE, Chinn DJ, Quanjer PH, Roca J, Yernault JC. 94: 28–37. Standardization of the measurement of transfer factor 20 Filley GF, MacIntosh DJ, Wright GW. Carbon monoxide (diffusing capacity). Report Working Party Standard- uptake and pulmonary diffusing capacity in normal ization of Lung Function Tests, European Community for subjects at rest and during exercise. J Clin Invest 1954; 33: Steel and Coal. Official Statement of the European 530–539. Respiratory Society. Eur Respir J 1993; 6: Suppl. 16, 41–52. 21 Leech JA, Martz L, Liben A, Becklake MR. Diffusing 3 Krogh M. The diffusion of gases through the lungs of capacity for carbon monoxide: the effects of different man. J Physiol (Lond) 1914; 49: 271–300. durations of breath-hold time and alveolar volume and of 4 Forster RE. Exchange of gases between alveolar air and carbon monoxide back pressure on calculated results. Am pulmonary capillary blood: pulmonary diffusing capa- Rev Respir Dis 1985; 132: 1127–1129. city. Physiol Rev 1957; 37: 391–452. 22 McGrath MW, Thomson ML. The effect of age, body size 5 MacIntyre NR. Diffusing capacity of the lung for carbon and lung volume change on alveolar-capillary perme- monoxide. Respir Care 1989; 34: 489–499. ability and diffusing capacity in man. J Physiol (Lond) 6 Crapo RO, Forster RE. Carbon monoxide diffusing 1959; 146: 572–582. capacity. Clin Chest Med 1989; 10: 187–198. 23 Newth CJL, Cotton DJ, Nadel JA. Pulmonary diffusing 7 Wilson AF, Hearne J, Brennen M, Alfonso R. capacity measured at multiple intervals during a single Measurement of transfer factor during constant exhala- exhalation in man. J Appl Physiol 1977; 43: 617–623. tion. Thorax 1994; 49: 1121–1126. 24 Graham BL, Dosman JA, Cotton DJ. A theoretical analysis 8 Leathart GL. Steady-state diffusing capacity determined of the single breath diffusing capacity for carbon by a simplified method. Thorax 1962; 17: 302–307. monoxide. IEEE Trans Biomed Eng 1980; 27: 221–227. 9 Meyer M, Scheid P, Riepl G, Wagner H-J, Piiper J. 25 Huang YC, Helms MI, MacIntyre NR. Normal values for Pulmonary diffusing capacities for CO2 and CO meas- single exhalation diffusing capacity and pulmonary ured by a rebreathing technique. J Appl Physiol 1981; 51: capillary blood flow in sitting, supine positions and 1643–1650. during mild exercise. Chest 1994; 105: 501–508. 10 Weibel ER. Morphometric estimation of pulmonary 26 Stam H, Kreuzer FJA, Versprille A. Effect of lung diffusion capacity. I. Model and method. Respir Physiol volume and positional changes on pulmonary diffusing 1971; 11: 54–75. capacity and its components. J Appl Physiol 1991; 71: 11 Forster RE, Fowler WS, Bates DV, Van Lingen B. The 147