DLCO

This photo is of Pulmonary Function Testing equipment. There is a hose that goes from the computer to the patient with a mouth apparatus like a scuba mouthpiece and the nose is pinched closed so the breathing only goes through the computer.

That took a while to get used to. During one test, the door to the glass box is closed and pressure is used to measure lung capacity. During the test for the DLCO, gas is inhaled and is monitored to determine how long it takes for it to clear.

The full bank of tests takes between 60-90 minutes as each test has to be done three times. If there is a variable, the test has to be repeated until there are three similar results. It is exhausting.

My biggest issues are DLCO and the Single-Breath test. The Single-Breath test measures the downward progress of my disease.

I found a web site that describes the DLCO test and why it is so important. It is a bit "medical” and may be a bit of a bore but some people might be interested.

It won’t hurt my feelings if you don’t want to read or try to understand it. It even took me awhile to understand it all. Please don’t miss the comments at the end.

Diffusing Capacity

Understanding gas diffusion through the lungs requires recognizing the basics of the gas exchange interface and of the various forces at work by which oxygen and carbon dioxide move by molecular diffusion. Diffusion is limited by the surface area in which diffusion occurs, capillary blood volume, hemoglobin concentration, and the properties of the lung parenchyma that separate the alveolar gas from the red blood cell with the capillary (alveolar-capillary membrane thickness, presence of excess fluid in the alveoli)

Because all lung volume is not exchanged, most gas exchange occurs as a function of diffusion independent of bulk flow. The role of ventilation is to reset concentration of the bulk flow of gas with the ambient air and to provide a constant gradient for oxygen and carbon dioxide. As spirometry measures the components of this bulk flow exchange, diffusing capacity measures the forces at work in molecular movement with its concentration gradient from the alveolar surface through to the hemoglobin molecule. The clinical test diffusing capacity of the lung most commonly uses carbon monoxide as the tracer gas for measurement because of its high affinity for binding to the hemoglobin molecule. This property allows a better measurement of pure diffusion, such that the movement of the carbon monoxide in essence only depends on the properties of the diffusion barrier and the amount of hemoglobin. The properties of oxygen and its relatively lower affinity for hemoglobin compared with carbon monoxide also make it more perfusion dependent; thus, cardiac output can influence actual measurement of oxygen diffusion measurements.

Diffusing capacity of the lung for carbon monoxide (DL_CO) is the measure of carbon monoxide transfer. In Europe, it is often called the transfer factor of carbon monoxide, which describes the process more accurately. DL_CO is a measure of the interaction of alveolar surface area, alveolar capillary perfusion, the physical properties of the alveolar capillary interface, capillary volume, hemoglobin concentration, and the reaction rate of carbon monoxide and hemoglobin. After a number of simplifications, the commonly used clinical tests to measure DL_CO are based on a ratio between the uptake of carbon monoxide in milliliters per minute divided by the average alveolar pressure of carbon monoxide. Overall, DL_CO is expressed as the uptake of carbon monoxide in milliliters of gas at standard temperature and pressure, dry, per minute, and per millimeter of mercury driving pressure of carbon monoxide. In principle, the total diffusing capacity of the whole lung is the sum of the diffusing capacity of the pulmonary membrane component and the capacity of the pulmonary capillary blood volume.

All methods for measuring diffusing capacity in clinical practice rely on measuring the rate of carbon monoxide uptake and estimating carbon monoxide driving pressure. The most widely used and standardized technique is the single-breath breath-holding technique. In this technique, a subject inhales a known volume of test gas that usually contains 10% helium, 0.3% carbon monoxide, 21% oxygen, and the remainder nitrogen. The patient inhales the test gas and holds his or her breath for 10 seconds. The patient exhales to wash out a conservative overestimate of mechanical and anatomic dead space. Subsequently, an alveolar sample is collected. DL_CO is calculated from the total volume of the lung, breath-hold time, and the initial and final alveolar concentrations of carbon monoxide. The exhaled helium concentration is used to calculate a single-breath estimate of total lung capacity and the initial alveolar concentration of carbon monoxide. The driving pressure is assumed to be the calculated initial alveolar pressure of carbon monoxide. The calculated DL_CO is a product of the patient's single-breath estimate of total lung capacity multiplied by the rate of carbon monoxide uptake during the 10-second breath hold.

Hemoglobin concentration is a very important measurement in interpreting reductions in DL_CO. Because the hemoglobin present in the alveolar capillaries serves as a carbon monoxide sink such that oxygen and carbon monoxide are removed from dissolved gases, the concentration gradient from alveolar to arterial blood remains relatively constant in favor of dissolved gas flow toward the arterial circulation. In this way, a DL_CO may be decreased when the patient is anemic. Because the level of hemoglobin present in the blood and diffusing capacity are directly related, a correction for anemic patients (DL_COc) is used to further delineate whether a DL_CO is decreased due to anemia or due to parenchymal or interface limitation. Recent work suggests strongly that the practice of dividing the calculated DL_CO by the single-breath estimate of total lung capacity (VA) to correct for low lung volumes (the DL/VA ratio) can yield a large number of false-negative results, and this practice should be used cautiously if at all.

A list of conditions associated with abnormal DL_CO is listed. Diseases such as interstitial pulmonary fibrosis or any interstitial lung disease can make the DL_CO abnormal long before spirometry or volume abnormalities are present. Low DL_CO is not only an abnormality of restrictive interstitial lung disease but also can occur in the presence of emphysema. In emphysema, the lung volumes may be normal or hyperinflated; therefore, the DL/VA is not useful. Additionally, the loss of alveolar surface area, the pathologic lesion of emphysema, is not proportionate to volume. Thus, one can understand that other obstructive entities that predominantly affect the airways can have similar spirometry, but a low DL_CO implies a loss of alveolar surface area consistent with emphysema. Unfortunately, it is not always this simple. Some forms of interstitial lung disease can have components of restrictive physiologies, such as low lung volume and clear evidence of decreased diffusion but also can have airway flow limitation. Sarcoidosis and Wegener's granulomatosis can produce an endobronchial component of airway webs or strictures, limiting flow before overt volume loss, and sufficient interstitial granulomatous inflammation to reduce the DL_CO.

Well, that was interesting. Are you still with me? That is just one test. There are a huge number of Pulmonary Function Tests to measure for everything even muscle tone around the lungs. Really rather fantastic. When I had my first PFT, I must have done at least 10-12 different tests. Now I do 5 or 6 regularly that track my specific disease.

Lungs are far more difficult to treat and understand than I ever would have guessed. These tests allow an effective way to gather data regarding the progress of the disease without surgery or high levels of radiation CT Scans. Thank goodness!