Standardization of the measurement of transfer factor (diffusing capacity)

Abstract

### 1.1 What is being measured The lung is the organ of external respiration for the exchange of oxygen and carbon dioxide between the blood and the surrounding air. The stages in the process of gas transfer include: 1. Ventilation of the airways and some air spaces by bulk flow of gas; 2. Mixing and diffusion of gases in the alveolar ducts, air sacs and alveoli; 3. Transfer of gases across the gaseous to liquid interface of the alveolar membrane; 4. Mixing and diffusion in the lung parenchyma and alveolar capillary plasma; 5. Chemical reaction with constituents of blood; 6. Circulation of blood between the pulmonary and systemic vascular beds. The capacity of the lung to exchange gas is determined by the structural and functional dimensions of these processes. The structural dimensions include the lung volume, the path length for diffusion in the gas phase, the thickness and area of the alveolar capillary membrane including any effects of airway closure, and the volume of blood in capillaries supplying alveoli which are ventilated. The principal functional dimensions are the absolute levels of ventilation and perfusion and the uniformity of their distribution with respect to both each other and the diffusion characteristics of the membrane. Other functional dimensions are the quantity of haemoglobin in the alveolar capillaries, the composition of the alveolar gas, the gas tensions in blood entering the alveolar capillaries, the rates of chemical reaction with haemoglobin and of dissociation of the compound so formed, the transit time of blood through that part of the pulmonary vascular bed which exchanges gas with the alveoli and the slope of the relevant haemoglobin dissociation curve. The latter is a function of the temperature of the lung and the prevailing levels of oxygen, carbon dioxide, hydrogen ions and 2,3-diphosphoglycerate; many of these variables are dependent on the level of …