Diffusing capacity of the lung for carbon dioxide (DLCO)

The diffusing capacity of the lungs for carbon monoxide (Dlco) measures the transfer of carbon monoxide from alveolar gas to hemoglobin in pulmonary capillary blood.

It is measured by having the patient fully inhale a low concentration of carbon monoxide and an inert tracer gas.

After a 10 second breath hold the carbon monoxide and tracer gas is measured in exhaled alveolar gas.

The Dlco is the product of the carbon monoxide transfer coefficient multiplied by alveolar gas volume.

The Dlco is determined by the surface area of the alveolar capillary interface, gas diffusivity across the interface, alveolar gas volume, and hemoglobin-carbon monoxide affinity and hemoglobin content in pulmonary capillaries.

Changes in the Dlco overtime of greater than 15 to 20% and 2 to 3 mL per minute per millimeter mercury are considered clinically important.

A low hemoglobin level and alveolar gas volume must be considered when interpreting  Dlco results.

A low Dlco is associated with conditions affecting the alveolar capillary interface including: interstitial lung disease, emphysema, pulmonary vascular disease, pulmonary hypertension, pulmonary emboli, chronic heart failure, drug toxicity from amiodarone, bleomycin and other chemotherapeutic agents.

Sensitive for detecting the presence of diffuse interstitial pulmonary fibrosis.

In patients with smoking abuse and evidence of airway obstruction a normal DLCO suggests chronic bronchitis and a decreased DLCO suggests emphysema.

Abnormal levels help in the differential diagnosis with restrictive lung disease.

Declines with restrictive lung disease because of reduced area allowing for gas exchange.

Restrictive patterns on pulmonary function tests and a normal DLCO level suggests the presence of extrapulmonary causes such associated pleural disease, neuromuscular weakness, or kyphoscoliosis.

Can be reduced by both pulmonary hypertension and emphysema.

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