Pulm diffusing capacity
Pulmonary diffusing capacity is often measured by Diffusion capacity of the Lungs for carbon monoxide (DLCO). In essence, this measures how much CO can pass from the alveoli to the blood in the pulmonary capillaries, thus giving clinicians the broader idea of how much inhaled gas can pass into the blood through the lungs. The pulmonary diffusing capacity can be helpful for multiple reasons, for example helping to diagnose pulmonary disease and to help guide postoperative complications for lung volume reduction surgery. While some state the “DLCO correlates with the total functioning surface area of the alveolar-capillary interface (Butterworth, et al),” Dr. McCormack notes, “Older textbooks suggest that thickening of the alveolar-capillary membrane (in interstitial lung disease) and loss of alveolar membrane surface area (in emphysema) are the primary causes of a low DLCO. However, subsequent experimental data suggest these and most other diseases that influence the DLCO do so by reducing the volume of red blood cells in the pulmonary capillaries” (McCormack). Regardless of theory, whether the surface area or the alveolar surface itself is modified or the volume of the blood in the pulmonary capillaries is modified, the DLCO reflects how much gas can be transferred to the blood via the lungs.
To interpret the DLCO, one should compare a patient’s DLCO with the percent of the predicted value. A patient with a DLCO with <45% of predicted has “severe respiratory impairment” (McCormack). Alternatively, if the patient has had a prior DLCO test, the clinician should also compare their current DLCO with previous values to assess for disease progression. There are several conditions that can decrease the DLCO. These include cigarette smoking, emphysema, interstitial lung disease, anemia, decreased lung volume, heart failure, pulmonary vascular disease (pulmonary emboli and pulmonary hypertension), and others. For example, the DLCO may be falsely decreased if the patient has a severe restrictive or obstructive disease as they may not be able to inspire an adequate amount of CO. Therefore, the DLCO is often adjusted by the alveolar volume (VA), and listed as the DLCO/VA. A normal DLCO/VA is considered to be > 80%.
There are also conditions that can increase the DLCO. These include high altitude, lying supine, exercise, left-to-right cardiac shunt, polycythemia, pulmonary hemorrhage (there is blood exposed in the alveoli (Kaminsky); the DLCO test gives an inhaled breath of gas with a mixture of CO, then after a 10 second breath-hold, the patient exhales a sample to see how much CO remains), and even asthma and obesity.