A preliminary review of physical concepts related with the solubility of inert gases in tissues and with the permeability of inert gases through the alveolar-capillary membrane in the context of this paper, is firstly presented. The solubility laws of gases in liquids and tissues, the concept of partial pressure of a gas in a tissue, the solubility and partition coefficients, the equilibrium of gases in the lungs, the gas diffusion through the capillary walls, etc., are brevely dealed with in this part of the work. The importance of a physiological method to study the alveolar-capillary barrier permeability with local information is emphasized. In the present paper the physiological and practical reasons which favour gas utilization, and particularly 133Xe, instead of aerosols, to evaluate the alveolar-capillary barrier permeability are considered. A method to evaluate the disappearance times of 133 Xe activity and obtain parametric images is described. The advantages to display 133 Xe permeability images concomitantly with the ventilation and perfusion images and its interest is shown with some examples. General aspects of the use of 99mTc-DTPA aerosols and CO classical spirometry with infra red analysis to study the alveolar-capiillary barrier permeability are considered.

In a group of pacients, parametric images of the alveolar-capillary barrier permeability were obtained both with 133Xe, using our technique and with the usual method of Nuclear Medicine using 99mTc-DTPA aerosols. The disapearence times obtained by the two techniques are in general terms correlated, however, a considerably higher specificity is obseved in the case of 133Xe.

In a second group of patients the global mean disapearence times of 133Xe, in the region of interest of the lungs, were compared with the CO diffusion times obtained with a convencional CO test. The agreement between the tests is very good in what concerns the ability to distinguish normal from abnormal, however, the local information offered by the 133Xe technique was felt to be an information of great value in many situations.

The results obtained, up to now, with 133Xe indicate that the information provided may be a further step in the study of the alveolar-capillary barrier compromises, even in the early stages.