16th February 2025

Measures of gas exchange – TLᴄᴏ and Kᴄᴏ

To quantify alveolar gas exchange, the patient breathes a mix of an inert gas (helium) and carbon monoxide (CO), breath holding for 10 seconds, then the CO in exhaled gas is measured. Only the CO will be absorbed into the blood, so the drop in CO concentration is indicative of gas exchange, as recorded in two measures:

• The transfer factor of the lung for carbon monoxide (TL_ᴄᴏ), also called the diffusing capacity (DL_ᴄᴏ). This measures the total amount of CO transferred to the blood and therefore the total gas exchange capacity of the lungs
• The carbon monoxide transfer coefficient (K_ᴄᴏ) – this is the TL_ᴄᴏ divided by the total lung volume, giving a measure of the average gas exchange per unit volume of the lung. It is low where there is a problem with gas exchange at the level of the alveolar membrane, for example due to thickening

A low K_ᴄᴏ helps differentiate between causes of respiratory disease:

• In a restrictive spirometry pattern, a low K_ᴄᴏ suggests an intrapulmonary cause such as fibrosis. It will be normal in extrapulmonary causes such as kyphoscoliosis
• In an obstructive spirometry pattern, a low K_ᴄᴏ suggests COPD rather than asthma

A high K_ᴄᴏ is seen in two scenarios:

• In the few days after alveolar haemorrhage, due to red blood cells taking up CO. For example in SLE, Wegener’s and Goodpasture’s
• In poor lung expansion, such as due to neuromuscular disease or chest wall restrictive disease