4.0 Lung Diffusing Capacity
This test is used to evaluate how well oxygen moves into and out of the lungs. Certain diseases will lead to difficulties in getting oxygen from in the alveoli (air sac in the lung) into the blood where it is carried to the rest of the body.
In many ways, DLCO is a general measure of the complete ‘efficiency' of the lungs because it is influenced by three key components: The surface area of the lung with contact to diffusing alveoli (VA - Alveolar Volume), the thickness of the alveolar-capillary membrane (Dm - Membrane Diffusion) and the volume of blood available in the capillary bed of the lung (Vc - Capillary Blood Volume).
- DLCO may be abnormal in conjunction with obstructive, restrictive, or normal spirometry.
- Increased DLCO is rarely important but may occur with polycythemia or lung hemorrhage.
- DLCO can help distinguish emphysema from chronic bronchitis.
- Asthma produces a normal and sometimes increased DLCO.
- A reduced DLCO with a reduced TLC is indicative of parenchymal disease (eg, interstitial fibrosis), which impairs the diffusion of oxygen from the alveoli to the capillaries.
- Normal DLCO with reduced TLC: The lung parenchyma is not damaged and the restriction is extrapulmonary.
- Decreased DLCO with normal spirometry and lung volumes: decreased oxygen-carrying capacity.
- Reduced DLCO infers interstitial disease suspected by history and physical examination. Obtain chest CT.
- Pulmonary vascular disease may present with decreased DLCO with normal spirometry and lung volumes. Occlusion of part of the pulmonary vascular bed reduces blood flow and decreases diffusion capability.
In this test we use a special gas mixture containing 0.300% CO, 10.0% Helium, 21,0% O2 and balance N2. The CO is used to trace the diffusion in place of O2 because it is a one-way transfer across the alveolar-capillary membrane for combination with Hb. The helium in the mixture is used to obtain a measure of the alveolar volume.
The challenge of Single Breath Diffusion testing is to obtain a representative sample of gas from an area of the lungs where diffusion is taking place. The patient first breathes all the way out to residual volume and is then connected to the test gas. They breathe all the way in to TLC and are then instructed to hold their breath for approximately 10 seconds. After having held your breath for ten seconds, the first amount of gas that leaves your lips when you breathe out, has been resident in the physiological dead-space (mouth, trachea and two main bronchi) and must therefore be discarded before collecting a valid gas sample.
ComPAS Freedom™ looks at the patient's Vital Capacity and FEF25-75 to automatically determine the optimum settings for the DLCO test. At the conclusion of the test, the software automatically makes measures of the content of the inspiratory and expiratory bags.
- Parameters:
- DLCO Single Breath Diffusing Capacity
- VA Alveolar Volume
- DL/VA Diffusion per unit area of Lung Volume
