Makindo Medical Notes

About

The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) is a pulmonary function test that measures how effectively gases are transferred from the air sacs (alveoli) in the lungs to the blood in the pulmonary capillaries. Carbon monoxide (CO) is used in this test because it binds readily to hemoglobin, and its uptake is diffusion-limited, making it an ideal gas for assessing the diffusing capacity of the lungs.

The test involves the patient inhaling a small amount of carbon monoxide mixed with an inert tracer gas (usually helium) and then holding their breath for about 10 seconds. The concentrations of CO and the tracer gas in the inspired and expired air are measured to calculate the DLCO.

Clinical Significance

DLCO is an important parameter in the evaluation of various lung diseases. It helps in differentiating between obstructive and restrictive lung diseases and in assessing the extent of pulmonary involvement in systemic diseases. It is particularly useful in detecting diseases that affect the alveolar-capillary membrane.

Increased DLCO

An increased DLCO indicates enhanced gas transfer across the alveolar-capillary membrane. Causes of increased DLCO include:

• Pulmonary Hemorrhage: The presence of blood in the alveoli increases CO uptake due to additional hemoglobin available for binding.
• Asthma: Although asthma is an obstructive airway disease, DLCO may be normal or increased due to increased pulmonary capillary blood volume.
• Left-to-Right Cardiac Shunts: Increased pulmonary blood flow enhances gas exchange.
• Exercise: Temporary increase due to increased cardiac output and pulmonary blood flow during exercise testing.
• Polycythemia: Elevated hemoglobin levels increase CO binding capacity.
• Obesity: Increased blood volume may slightly elevate DLCO.
• Congestive Heart Failure (Early stages): Increased pulmonary capillary blood volume can raise DLCO.
• Pregnancy: Increased blood volume and cardiac output can lead to higher DLCO values.

Decreased DLCO

A decreased DLCO indicates impaired gas transfer, which can result from conditions affecting the alveolar-capillary membrane, pulmonary vasculature, or lung parenchyma. Causes of decreased DLCO include:

• Emphysema: Destruction of alveolar walls reduces surface area for gas exchange.
• Interstitial Lung Diseases: Fibrosis thickens the alveolar-capillary membrane, hindering gas diffusion (e.g., idiopathic pulmonary fibrosis, asbestosis).
• Sarcoidosis: Granuloma formation disrupts normal lung architecture.
• Pulmonary Embolism: Obstruction of pulmonary arteries reduces perfusion to alveoli.
• Pneumocystis jirovecii Pneumonia (PCP): Infection causes interstitial inflammation and impaired diffusion.
• Anemia: Reduced hemoglobin levels decrease CO uptake capacity.
• Pulmonary Hypertension: Elevated pressures affect pulmonary vasculature and gas exchange.
• Resection or Loss of Lung Tissue: Pneumonectomy or lobectomy reduces the overall surface area (e.g., pneumonectomy).
• Connective Tissue Diseases: Conditions like systemic sclerosis can cause pulmonary fibrosis.

Interpretation Considerations

When interpreting DLCO results, it's important to consider factors that can affect measurements:

• Hemoglobin Levels: Anemia can falsely lower DLCO; conversely, polycythemia can increase it.
• Smoking: Recent smoking increases blood CO levels, affecting DLCO accuracy.
• Altitude: High altitudes can influence DLCO due to changes in atmospheric pressure.
• Body Position: Supine position can increase DLCO compared to upright position.
• Exercise: Recent physical activity can temporarily elevate DLCO.

Clinical Application

DLCO is used in conjunction with other pulmonary function tests (PFTs) to:

• Differentiate between emphysema (decreased DLCO) and chronic bronchitis (normal DLCO) in COPD patients.
• Assess severity and progression of interstitial lung diseases.
• Evaluate pulmonary involvement in systemic diseases (e.g., systemic lupus erythematosus).
• Monitor response to therapy in pulmonary hypertension.
• Preoperative assessment before lung resection surgeries.

References

• Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49(1):1600016.
• Ruppel GL, Enright PL. Pulmonary Function Testing. In: Broaddus VC, Mason RJ, Ernst JD, et al., editors. Murray & Nadel's Textbook of Respiratory Medicine. 6th ed. Elsevier; 2016. p. 407-428.
• MacIntyre N, Crapo RO, Viegi G, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J. 2005;26(4):720-735.