Diffusing capacity for carbon monoxide

Diffusing Capacity for Carbon Monoxide (DLCO), also known as Carbon Monoxide Lung Diffusion Capacity (DLCO), is a clinical test that measures the ability of the lungs to transfer gas from inhaled air to the red blood cells in pulmonary capillaries. It is an important diagnostic tool used to assess the gas exchange function of the lungs. This test is particularly useful in diagnosing and monitoring diseases that affect lung tissue such as pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), and pulmonary hypertension.

Overview

The DLCO test involves measuring the extent to which carbon monoxide (CO) passes from the inhaled air in the lungs into the blood. Carbon monoxide is used because it binds to hemoglobin with much greater affinity than oxygen but does not participate in metabolic processes. The test is non-invasive and provides critical information about the condition of the lung's alveoli and the capillary blood volume.

Indications

DLCO testing is indicated for the evaluation of patients with unexplained dyspnea (difficulty breathing), and for the diagnosis and management of conditions affecting the lung parenchyma, such as:

• Pulmonary fibrosis
• Emphysema, a type of COPD
• Pulmonary hypertension
• Interstitial lung disease

Procedure

The DLCO test is performed using a spirometer in conjunction with a gas analyzer. The patient inhales a test gas mixture containing a small amount of carbon monoxide, usually less than 0.3%, a tracer gas (such as helium), to measure lung volume, and a balance of air or oxygen. After holding their breath for a specified period, typically 10 seconds, the patient exhales. The exhaled gas is analyzed to determine the amount of carbon monoxide absorbed by the blood.

Interpretation

The results of the DLCO test are usually given in milliliters per minute per millimeter of mercury (mL/min/mmHg). Normal values depend on age, sex, hemoglobin levels, and lung volume. Reduced DLCO values indicate impaired gas exchange which can be due to a variety of conditions including:

• Loss of alveolar surface area (as in emphysema or pulmonary fibrosis)
• Thickening of the alveolar-capillary membrane (as in interstitial lung disease)
• Reduced pulmonary capillary blood volume (as in pulmonary embolism or pulmonary hypertension)

Elevated DLCO values can be seen in conditions such as polycythemia or left-to-right intracardiac shunts.

Clinical Significance

The DLCO test is a critical component in the diagnostic workup of patients with suspected lung disease. It helps in distinguishing between different types of pulmonary pathology, particularly in the early stages of disease when other pulmonary function tests may be normal. Additionally, DLCO measurements can be used to monitor disease progression and response to treatment.

Limitations

While DLCO provides valuable information about lung function, it has limitations. It can be affected by factors such as the patient's effort, hemoglobin levels, and lung volume. Interpretation of DLCO results should always be done in conjunction with other pulmonary function tests and clinical findings.

See Also

• Pulmonary function test
• Spirometry
• Chronic obstructive pulmonary disease
• Pulmonary fibrosis