Carbon Monoxide Diffusing Capacity (DLCO)

A. Definition

1. Carbon monoxide (CO) diffusing capacity (DLCO) provides an objective measurement of lung function. It is defined as the lung's ability to take up an inhaled nonreactive test gas, such as carbon monoxide (CO), which binds to hemoglobin. CO will bind to hemoglobin with such a high affinity; virtually all of the CO will reach the alveolar space. This will cause the carbon monoxide to cross the alveolar air-blood barrier, and thus reaching a red cell that will bind to hemoglobin, and will be removed with the exhaled gas.

   Carbon monoxide diffusing capacity (DLCO) is the rate of uptake of carbon monoxide (CO) per driving pressure of alveolar CO. The simplified equation is:

DLCO = VCO/PACO

VCO = the uptake of CO (milliliters per minute)

PACO = the mean alveolar pressure of CO (milliliters of mercury)

1. The test can be used for a wide variety of diseases, because it is relatively easy to measure or estimate the two determinants. The component resistances to DLCO include:

   1. The pulmonary membrane (pulmonary tissue and plasma layer).

   2. The red blood cell resistance, which is a function of the rate of CO uptake by hemoglobin and the pulmonary capillary blood volume.

B. Indications for Use

1. There are three major types of pulmonary disorders that cause a decrease in DLCO:

   1. Obstructive airway disease (particularly emphysema and possibly cystic fibrosis)

   2. Interstitial lung disease

   3. Pulmonary vascular disease

   DLCO should be measured whenever specific diseases causing these general pulmonary disorders are being considered. Many providers consider DLCO to be a routine part of a pulmonary function test.

C. Methodology

1. All physiologic methods for determination of DLCO involve measuring VCO and estimating CO driving pressure. The most widely used method is the single-breath, breath-holding technique. It is easy to perform, and is affected minimally by ventilation-perfusion abnormalities.

   Method:

   1. The subject inhales a volume of gas (approximately 10% He, 0.3% CO, and either 21% [in United States] or 17% [in Europe] O2, with balance N2) during an inspiration to total lung capacity (TLC), holds their breath for 10 seconds, and then performs a forced vital capacity (FVC) maneuver.

   2. An alveolar sample of gas is collected after an initial volume of exhaled gas washes out mechanical and anatomic dead space.

   3. The DLCO is calculated from:

      1. Total volume of the lung (alveolar volume [VA])

      2. Breath-hold time

      3. Initial and final alveolar CO concentrations.

   4. Helium dilution is used to calculate VA and the initial alveolar concentration of CO.

   At least two acceptable tests should be performed, and the mean value should be reported. An acceptable test is defined as:

   1. A rapid inspiration

   2. An inspired volume greater than 90% of the largest FVC

   3. A breath-hold time of 9-11 seconds

   4. Adequate dead-space washout volume (0.75 to 1.00 L)

   5. An appropriate sample size

D. Factors that Influence DLCO

1. Measurement of lung-diffusing capacity is influenced by three important factors:

   1. The ability of the test gas to reach the alveolar gas-exchanging surfaces

   2. The ability of the test gas to cross the alveolar septa

   3. The mass of red cells in the pulmonary capillary bed available to bind to the test gas.

   A defect in any one of the above components will influence measurement of lung-diffusion capacity.

   There are many other factors that influence DLCO. Age, gender, and height are independent factors that influence DLCO. Obesity is not a predictor variable until the weight-to-height ratio (kilometers per centimeter) exceeds 1.0. As lung volume (VA) increases, so does DLCO.

   Body position also affects DLCO. Changing from a standing to sitting position produces a 10% to 15% increase in DLCO, and moving from a sitting to a supine position increases DLCO by 15% to 20%. The reported variation in DLCO from morning to evening appears to be due to the minor decrease in the morning hemoglobin (Hb) concentration and the artifactual rise in carboxyhemoglobin (COHb) from repeated DLCO testing. Exercise leads to a rapid increase in DLCO by increasing pulmonary capillary blood volume. There is also a temporary increase in DLCO during the first trimester of pregnancy, presumably due to increased blood flow.

   Hemoglobin concentration will affect DLCO: polycythemia causes an increase, anemia causes a decrease. Elevations in COHb decrease DLCO by creating CO back-pressure in venous blood and by reducing available Hb binding sites (due to the high affinity of CO to Hb). Because PACO changes as a function of altitude, an adjustment in the measurement or calculation of DLCO is recommended for laboratories at altitudes other than sea level.

E. Normal Values

1. The normal values for CO diffusing capacity vary widely between laboratories, and both absolute values and their reproducibility are largely influenced by the measurement technique. Therefore, this measurement is most useful if the patient's lung function changes are followed consistently by the same laboratory.

F. Interpretation of Results

1. A DLCO value must always be considered in the context of other clinical, physiologic, and radiographic findings. Emphysema, interstitial lung disease, and pulmonary vascular disease are the three major disorders that result in reduced DLCO. There can also be a decrease in DLCO for individuals with infiltrating (interstitial) lung disease. Pulmonary vascular occlusive disease, especially pulmonary hypertension and pulmonary embolism, will lead to a reduced DLCO, but the patient's lung volumes and flow rates will remain normal. This emphasizes why it is important to exclude anemia, and then consider pulmonary vascular obstruction when all pulmonary function tests are normal, but the DLCO is decreased.

DLCO in various diseases

SOURCES

Bone, R. C., Dantzker, D. R., George, R. B., Matthay, R. A., & Reynolds, H. Y. (Eds.). (1998). Pulmonary & Critical Care Medicine (5th ed., pp. F6-16). St. Louis: Mosby.

Goldman, L., Bennett, J. C. (Eds.). (2000). Cecil Textbook of Medicine (21st ed., pp. 385-386). Philadelphia: W.B. Saunders Company.

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