Measuring diffusing capacity is a great way to determine whether the lungs are effectively transporting gas in and out of the blood. By the end of this video, from our Pulmonary Function Testing Essentials course, you'll understand how diffusing capacity is measured and used for assessing lung disease.
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[00:00:00] Diffusing capacity is a very useful test to assess for the presence of lung disease and its effect on gas exchange. The test measures the efficiency with which the test gas moves across the alveolar capillary membrane, reflecting changes and diffusion of intrinsic alveolar gases, most notably oxygen. Interestingly, carbon monoxide has emerged as the most
[00:00:30] practical test gas. This is because of its great affinity for hemoglobin, which means changes in the measured value reflect the status of the alveolar capillary membrane, rather than the reaction rate of carbon monoxide with hemoglobin, in red blood cells. Diffusing capacity of carbon monoxide or DLCO, although nonspecific, is highly sensitive in assessing lung function in a wide variety of pulmonary
[00:01:00] and non-pulmonary disease states affecting gas exchange. Let's examine the underlying physiology of diffusion across the lung. As demonstrated by Fick's law, a number of factors influence the movement or flux of gas molecules across the alveolar capillary membrane. These include the surface area of the membrane, the thickness of the membrane, the solubility of the gas,
[00:01:30] the molecular weight of the gas, and the driving pressure or pressure gradient across the alveolar capillary membrane. Even though not included in Fick's law, these factors also influence flux—the resistance to diffusion across the red blood cell membranes, the reaction rate of the gas with hemoglobin, and the capillary blood volume.
[00:02:00] Conceptually, diffusing capacity (DL) represents the amount of test gas transferred per unit of time (V) relative to the driving pressure for the gas across the alveolar capillary membrane (PA – PC). How does this relate to diffusion of carbon monoxide? The diffusing capacity of carbon monoxide (DLCO) is the amount of carbon monoxide transferred across the
[00:02:30] lung per minute, represented by VCO, divided by the driving pressure (PACO – PCCO). PACO is the partial pressure of carbon monoxide in the alveolus, and PCCO is the partial pressure of carbon monoxide in the pulmonary capillaries. Let's examine the individual terms. VCO is calculated as the difference in carbon monoxide content between [00:03:00] inspired and expired gas samples. That leaves determination of alveolar, pulmonary, and capillary carbon monoxide concentrations to enable the calculation. In non-smokers, PCCO is zero. If VCO is calculated as the difference between inspiratory and expiratory values, DLCO can be calculated once the alveolar concentration of carbon monoxide
[00:03:30] (PACO) is determined.There are two general methods of measuring diffusing capacity: the single-breath method and the steady-state method. We'll focus on the single-breath method. In the single-breath method, the patient takes a few tidal volume breaths followed by expiration to residual volume. The patient is then asked to take a full inspiration to TLC of a gas
[00:04:00] mixture, which contains 0.3% carbon monoxide. The patient holds his or her breath for ten seconds and then rapidly exhales. The concentration of carbon monoxide is measured using a carbon monoxide meter, then a rather complicated equation is used to calculate the diffusing capacity. It includes measurement of lung or alveolar volume based on plethysmography or helium delusion techniques
[00:04:30] and measurements of alveolar carbon monoxide concentration at the beginning of the breath hold and the final concentration of carbon monoxide at the end of the breath hold. A variety of conditions may affect diffusing capacity and these can be understood by considering Fick's law discussed earlier. For example, interstitial lung disease characterized by a thickened alveolar capillary membrane
[00:05:00] increases the diffusion pathway for oxygen and reduces the DLCL. Emphysema, characterized by loss of alveolar walls and loss of surface area for diffusion, also produces a reduction in DLCL. On the other hand, polycythemia and alveolar hemorrhage increase diffusing capacity, as the increase in hemoglobin within the lungs provides a sink for carbon monoxide.