A decreased diffusion capacity of the lung, measured by a test known as DLCO, signifies a problem with how efficiently oxygen moves from the air you breathe into your bloodstream. DLCO stands for “Diffusing Capacity of the Lung for Carbon Monoxide,” a standard pulmonary function test. This measurement provides direct insight into the integrity and functionality of the lung’s gas exchange surface. A reduced capacity indicates that the transfer process is impaired, which can be an early indicator of various underlying health conditions affecting the lungs or the circulation.
The Mechanism of Healthy Gas Exchange
Gas exchange, the process by which the body oxygenates blood, occurs in the deepest parts of the lungs. Inhaled air travels through airways until it reaches millions of tiny air sacs called alveoli. These alveoli are the primary sites where oxygen enters the blood and carbon dioxide is removed.
Each alveolus is wrapped in a dense network of capillaries. The alveolar-capillary membrane, the physical barrier separating the air and blood, is exceptionally thin, often less than one micrometer thick, facilitating rapid gas movement. Oxygen diffuses from the alveolar air into the capillary blood, while carbon dioxide diffuses out to be exhaled. This vast, thin surface area, estimated at 70 square meters in a healthy adult, is necessary for effective gas transfer.
Understanding the DLCO Measurement
The DLCO test is a specialized pulmonary function measurement that quantifies the efficiency of this gas transfer process. Instead of directly measuring oxygen, the test uses a small, harmless amount of carbon monoxide (CO) as a proxy gas because CO binds readily to hemoglobin. The test measures how quickly CO moves across the alveolar-capillary membrane into the red blood cells during a single, short breath hold.
A patient inhales a gas mixture containing trace amounts of CO and an inert tracer gas, holds their breath for about 10 seconds, and then exhales. By analyzing the CO concentration in the exhaled air, a machine calculates the amount absorbed into the bloodstream. The result is expressed as a percentage of the predicted value, calculated based on the person’s age, height, and sex.
A decreased DLCO is typically defined as a result below 75% or 80% of the predicted value. This low result indicates that the lung’s ability to diffuse gas is compromised. The interpretation of a low DLCO result suggests either a reduced surface area for gas exchange or a thickening of the barrier separating the air and blood.
Primary Categories of Reduced Capacity
A reduction in DLCO can be physiologically categorized into three main problems: damage to the exchange surface, thickening of the barrier, and issues with blood flow or content. The destruction of the alveolar walls, such as occurs in emphysema, directly reduces the total surface area available for gas exchange. This loss of alveolar units is a common cause of reduced DLCO.
The alveolar-capillary membrane can also become scarred or inflamed, causing it to thicken. Conditions like interstitial lung diseases, including pulmonary fibrosis, increase the distance gases must travel to reach the blood. This increased distance significantly slows the rate of diffusion, resulting in a lower DLCO reading.
Problems with blood flow through the lungs also reduce the measured capacity because the process relies on the blood to absorb the gas. Pulmonary vascular diseases, such as pulmonary hypertension or pulmonary emboli, restrict the flow of blood through the pulmonary capillaries. This restriction reduces the number of capillaries available to pick up the diffusing gas, mimicking a reduction in the functional exchange surface.
The DLCO value can be influenced by the blood’s capacity to carry the gas. Since the test relies on carbon monoxide binding to hemoglobin in red blood cells, a low hemoglobin concentration, such as in anemia, will artificially lower the DLCO result. In these cases, the lung structure itself may be healthy, but the reduced number of carriers lessens the measured gas uptake.
Implications for Daily Life and Further Steps
The primary consequence of a decreased diffusion capacity is reduced efficiency in transferring oxygen to the body. Even a mild reduction in DLCO can lead to noticeable symptoms, especially during physical activity. The most common implication is shortness of breath (dyspnea), particularly when a person exerts themselves.
This reduced oxygen transfer can also manifest as generalized fatigue and a diminished capacity for exercise, leading to early exhaustion. Even though the lungs may move air in and out normally, the final, crucial step of gas transfer is compromised.
Following an abnormal DLCO result, further diagnostic steps are necessary to identify the specific underlying cause. Follow-up tests may include high-resolution computed tomography (HRCT) scans of the chest to look for structural changes like scarring or emphysema. Blood gas tests may also be performed to directly measure the oxygen and carbon dioxide levels in the blood, confirming the extent of the gas exchange impairment.