Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 86

At one time, the rate at which gases diffused across the alveolar–capillary membrane was thought to be the principal factor limiting gas exchange. The term diffusing capacity was thus coined, defined as ‘The quantity of gas transported across in each minute for every unit of pressure gradient’. Although the measurement proved to be very useful clinically, it was later realised that it was affected by many other factors in addition to diffusion, particularly V/Q matching. It was therefore renamed transfer factor.

Clearly, it is the transfer of oxygen that is of most interest to clinicians. This is very difficult to measure in practice, however, as transfer of oxygen into the blood quickly becomes limited by the saturation of haemoglobin. Carbon monoxide is thus used as a surrogate for oxygen in this measurement. Very low concentrations are used so that haemoglobin remains avid for the gas as it passes through the alveolar capillary (and of course high concentrations would be dangerous).

The term ‘diffusion capacity’ (D _L CO) can still be found in some texts; this is synonymous with ‘transfer factor’ (T _L CO).

To measure T_LCO, we need to know:

1 the amount of CO transferred per minute, and

2 the pressure gradient across the alveolar membrane (in effect, the alveolar partial pressure, as the partial pressure in blood is essentially zero).