Makindo Medical Notes

About

• The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) 🫁 is a pulmonary function test that measures how effectively gases move from the alveoli into the pulmonary capillaries.
• Carbon monoxide (CO) is used because it binds readily to haemoglobin and is diffusion-limited, making it an ideal gas to assess alveolar-capillary transfer.
• Test method: the patient inhales a tiny amount of CO mixed with an inert tracer (usually helium), holds their breath for ~10 seconds, then exhales. The difference in inspired vs expired concentrations allows calculation of DLCO.

Clinical Significance

DLCO helps differentiate obstructive vs restrictive disease, and assess the extent of parenchymal or vascular involvement. 📌 It is especially useful for detecting diseases that affect the alveolar-capillary membrane (e.g. ILD, emphysema).

Causes of Increased DLCO 📈

• Pulmonary haemorrhage: Extra Hb in alveoli increases uptake.
• Asthma: Often normal or raised due to ↑ capillary blood volume.
• Left-to-right shunts: Increased pulmonary blood flow.
• Polycythaemia: More Hb available to bind CO.
• Exercise & Pregnancy: Both raise cardiac output → ↑ DLCO.
• Obesity / Early CHF: Increased pulmonary capillary blood volume.

Causes of Decreased DLCO 📉

• Emphysema: Loss of alveolar walls reduces surface area.
• Interstitial Lung Diseases: Fibrosis thickens the membrane (e.g. IPF, asbestosis).
• Sarcoidosis: Granulomas distort lung architecture.
• Pulmonary Embolism: Loss of perfused vascular bed.
• Pulmonary Hypertension: Impaired vascular gas exchange.
• Pneumocystis jirovecii pneumonia (PCP): Interstitial inflammation.
• Anaemia: Fewer Hb molecules for CO binding.
• Loss of Lung Tissue: Post-lobectomy or pneumonectomy.
• Connective Tissue Diseases: e.g. systemic sclerosis → fibrosis.

Interpretation Considerations ⚖️

• Hb level affects DLCO (↓ in anaemia, ↑ in polycythaemia).
• Smoking ⛔ raises COHb → artificially lowers DLCO.
• Altitude 🏔️ reduces DLCO due to atmospheric pressure changes.
• Body position: supine values ↑ vs upright.
• Exercise temporarily raises DLCO.

Clinical Applications

• Differentiate COPD phenotypes: emphysema (↓ DLCO) vs chronic bronchitis (normal DLCO).
• Monitor progression of ILD and pulmonary fibrosis.
• Assess pulmonary involvement in systemic disease (e.g. lupus, scleroderma).
• Follow pulmonary hypertension treatment response.
• Pre-op assessment before lung resection / transplant.

Key UK Teaching Points 🇬🇧

• BTS/NICE guidance: DLCO is often combined with FVC and TLC to stage ILD severity.
• Always interpret DLCO in the context of spirometry + lung volumes + clinical picture.
• Falling DLCO may precede radiological changes in progressive fibrotic lung disease.

References

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