DLCO

Diffusing Capacity of the Lung for Carbon Monoxide (DLCO)

The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) is a clinical measurement used to assess the ability of the lungs to transfer gas from inhaled air to the red blood cells in pulmonary capillaries. It is an important test in the evaluation of patients with suspected pulmonary disease, particularly those with interstitial lung disease, emphysema, and pulmonary vascular diseases.

Physiology

The DLCO test measures how effectively carbon monoxide (CO) is transferred from the alveoli of the lungs to the blood. Carbon monoxide is used in this test because it binds to hemoglobin in a similar manner to oxygen, but is not normally present in the blood in significant amounts. The test involves the patient inhaling a small amount of CO, holding their breath for a short period, and then exhaling. The concentration of CO in the exhaled air is measured to determine how much was absorbed by the blood.

The transfer of gas from the alveoli to the blood is influenced by several factors:

• Alveolar Surface Area: A larger surface area allows for more gas exchange.
• Membrane Thickness: Thicker membranes can impede gas transfer.
• Hemoglobin Concentration: More hemoglobin allows for more CO to be absorbed.
• Ventilation-Perfusion Matching: Efficient matching of air flow and blood flow in the lungs is crucial for optimal gas exchange.

Clinical Significance

DLCO is particularly useful in differentiating between various types of lung diseases:

• Obstructive Lung Diseases: In conditions like emphysema, DLCO is often reduced due to the destruction of alveolar walls, leading to decreased surface area for gas exchange.
• Restrictive Lung Diseases: In diseases such as pulmonary fibrosis, DLCO is reduced due to thickening of the alveolar membrane and decreased lung volumes.
• Pulmonary Vascular Diseases: Conditions like pulmonary hypertension can lead to a reduced DLCO due to impaired blood flow through the lungs.

Interpretation of Results

DLCO results are typically reported as a percentage of the predicted value based on age, sex, height, and ethnicity. A DLCO value less than 80% of the predicted value is generally considered abnormal. However, interpretation must consider the clinical context and other pulmonary function test results.

Factors Affecting DLCO

Several factors can affect DLCO measurements, including:

• Anemia: Reduced hemoglobin levels can lead to a falsely low DLCO.
• Smoking: Carbon monoxide from smoking can saturate hemoglobin, affecting the test results.
• Body Position: The test is usually performed in a seated position, but changes in posture can affect lung volumes and DLCO.

Also see

• Pulmonary Function Test
• Spirometry
• Lung Volumes
• Gas Exchange