Medicine – Gas Transfer Factor

Gas transfer factor refers to the ability of gases to move from the alveoli across the alveolar–capillary membrane into the pulmonary blood. It is usually measured using carbon monoxide because carbon monoxide binds avidly to haemoglobin and its transfer can therefore be used to estimate the efficiency of pulmonary gas exchange.

The measurement is commonly called TLCO in the UK or DLCO in other settings. A reduced transfer factor may result from destruction or thickening of the alveolar–capillary membrane, loss of pulmonary capillary blood volume, reduced haemoglobin concentration, or removal of functioning lung tissue.

1. Causes of Decreased Transfer Factor

A reduced TLCO/DLCO means that carbon monoxide crosses from the alveoli into the blood less efficiently than expected.

The causes can be divided into pulmonary, cardiovascular, and haematological disorders.

Emphysema

Emphysema is a classic cause of reduced gas transfer.

Destruction of alveolar walls reduces the total surface area available for diffusion and also destroys part of the associated pulmonary capillary bed.

Therefore, even though the lungs may be hyperinflated, their effective gas-exchange surface is substantially reduced.

Interstitial Lung Disease

Interstitial lung disease, including pulmonary fibrosis, reduces gas transfer because the alveolar–capillary membrane becomes thickened and fibrotic.

Oxygen and carbon monoxide must therefore diffuse across a greater distance.

TLCO/DLCO is often one of the earliest pulmonary function abnormalities in interstitial lung disease and may fall before marked spirometric restriction develops.

Pneumonia

Pneumonia can temporarily reduce gas transfer because inflammatory exudate fills alveoli and increases the effective distance between inspired gas and pulmonary capillary blood.

The associated V/Q mismatch and alveolar inflammation further impair oxygen transfer.

Pulmonary Embolism

A pulmonary embolism can reduce TLCO/DLCO because it obstructs part of the pulmonary circulation.

Although alveoli may remain ventilated, blood flow through the corresponding pulmonary capillary bed is reduced or absent.

Therefore, there is less pulmonary capillary blood available to take up carbon monoxide.

Pneumonectomy

Following a pneumonectomy, an entire lung has been removed.

This substantially reduces the total alveolar surface area and pulmonary capillary bed available for gas exchange.

Consequently, the absolute transfer factor is reduced.

2. Cardiovascular Causes of Decreased Transfer Factor

Cardiovascular disorders can reduce gas transfer when they decrease the amount of blood reaching the pulmonary capillary circulation or interfere with the alveolar–capillary interface.

Low Cardiac Output

A low cardiac output can reduce pulmonary capillary blood volume.

With less blood flowing through the pulmonary circulation, there is less haemoglobin available to bind the carbon monoxide used during the test.

This can contribute to a reduced measured TLCO/DLCO.

Pulmonary Oedema

Pulmonary oedema can reduce gas transfer because fluid accumulates within the pulmonary interstitium and sometimes within the alveoli.

This increases the diffusion distance between alveolar gas and the pulmonary capillary blood and therefore impairs gas transfer.

The effect may vary depending on the severity and timing of the oedema.

3. Haematological Cause of Decreased Transfer Factor

Anaemia

Anaemia causes a reduced measured TLCO/DLCO because there is less haemoglobin available to bind carbon monoxide.

The lungs themselves may be structurally normal, but the test reads lower because carbon monoxide uptake depends partly on haemoglobin concentration.

For this reason, modern interpretation of TLCO/DLCO should ideally include correction for haemoglobin concentration.

4. Causes of Increased Transfer Factor

An increased TLCO/DLCO occurs when carbon monoxide uptake by the lungs is greater than expected.

This can result from increased pulmonary blood volume, increased haemoglobin concentration, or the presence of blood within the alveoli.

Pulmonary Haemorrhage

Pulmonary haemorrhage can markedly increase TLCO/DLCO.

When blood enters the alveolar spaces, haemoglobin within that blood binds the inhaled carbon monoxide directly.

This increases the measured uptake of carbon monoxide and therefore produces an artificially high transfer factor.

Anti-GBM Disease

Anti-glomerular basement membrane disease, historically called Goodpasture’s syndrome when pulmonary and renal involvement coexist, can cause diffuse alveolar haemorrhage.

The blood within the alveoli increases the apparent TLCO/DLCO because its haemoglobin absorbs carbon monoxide.

Therefore, an unexpectedly high transfer factor in a patient with haemoptysis, anaemia, and bilateral pulmonary infiltrates may support the possibility of alveolar haemorrhage.

5. Cardiovascular Cause of Increased Transfer Factor

Left-to-Right Shunt

A left-to-right cardiac shunt increases pulmonary blood flow.

The increased volume of blood within the pulmonary capillary bed provides more haemoglobin for carbon monoxide uptake.

Therefore, the measured transfer factor may be increased.

Examples of left-to-right shunts include significant atrial septal defects, ventricular septal defects, and patent ductus arteriosus before pulmonary vascular disease reverses the shunt.

6. Haematological Cause of Increased Transfer Factor

Polycythaemia

Polycythaemia increases the haemoglobin concentration of the blood.

Because more haemoglobin is available to bind inhaled carbon monoxide, TLCO/DLCO may be increased.

As with anaemia, haemoglobin concentration therefore needs to be considered when interpreting gas-transfer measurements.

7. Decreased Transfer Factor – Note Form

Emphysema: destruction of alveolar walls reduces gas-exchange surface area and pulmonary capillary bed.

Interstitial lung disease: thickening and fibrosis of the alveolar–capillary membrane impair diffusion.

Pneumonia: alveolar inflammation and exudate interfere with gas transfer.

Pulmonary embolism: reduced perfusion decreases the functioning pulmonary capillary bed.

Pneumonectomy: removal of lung tissue reduces total surface area available for diffusion.

Low cardiac output: reduced pulmonary capillary blood volume decreases carbon monoxide uptake.

Pulmonary oedema: interstitial/alveolar fluid increases diffusion distance.

Anaemia: reduced haemoglobin means less carbon monoxide can be taken up by blood.

8. Increased Transfer Factor – Note Form

Pulmonary haemorrhage: intra-alveolar blood contains haemoglobin that binds carbon monoxide, increasing the measured TLCO/DLCO.

Anti-GBM disease: diffuse alveolar haemorrhage may therefore produce an abnormally high transfer factor.

Left-to-right shunt: increased pulmonary blood flow increases pulmonary capillary blood volume and carbon monoxide uptake.

Polycythaemia: increased haemoglobin concentration increases carbon monoxide uptake.

9. Useful Interpretation with KCO

TLCO/DLCO is influenced by both the efficiency of gas transfer and the available alveolar volume.

Another measurement, KCO, represents transfer factor adjusted for alveolar volume and can sometimes help explain why the TLCO is reduced.

For example, after pneumonectomy, total TLCO is reduced because there is less lung available, but gas transfer per unit of remaining lung may be relatively preserved.

In emphysema, both destruction of alveolar surface and loss of capillaries mean gas-transfer efficiency itself is impaired, so the KCO is often reduced as well.

Key Clinical Pattern

Think of low TLCO/DLCO as occurring when there is:

Less alveolar surface area → emphysema or lung resection.

A thicker diffusion barrier → interstitial lung disease or pulmonary oedema.

Less pulmonary capillary blood flow → pulmonary embolism or low cardiac output.

Less haemoglobin → anaemia.

Think of high TLCO/DLCO as occurring when there is:

More haemoglobin → polycythaemia.

More pulmonary blood flow → left-to-right shunt.

Blood actually present inside the alveoli → pulmonary haemorrhage.

A particularly useful examination association is:

Haemoptysis + pulmonary infiltrates + unexpectedly raised TLCO/DLCO → consider diffuse alveolar haemorrhage.